CSS Forums

CSS Forums (http://www.cssforum.com.pk/)
-   General Science Notes (http://www.cssforum.com.pk/css-compulsory-subjects/general-science-ability/general-science-notes/)
-   -   How things work? (http://www.cssforum.com.pk/css-compulsory-subjects/general-science-ability/general-science-notes/26735-how-things-work.html)

dr.janxeb Tuesday, September 08, 2009 10:55 PM

How things work?

Can you imagine a world without cameras? There would be no photographs in newspapers, books, and magazines, or even on your computer. There would be no school pictures, no snapshots of your summer vacation, no television, and no movies.

It’s hard to imagine, but that’s what the world was like until the mid-1800s. That’s when the first cameras were made.


A basic camera works a lot like your eyes. Try this: First, close your eyes. Now quickly open and shut them. What did you see? You saw an image, or “picture,” from your surroundings.

A camera does the same thing, but it has a shutter instead of eyelids. When you take a picture, the shutter quickly opens and shuts. While the shutter is open, the camera “sees” an image, much like your eyes. The camera captures this picture.

A film camera catches the picture using chemicals on film. A digital camera captures the image electronically and stores it in memory or on a computer disk. The first popular photographs, called daguerreotypes, were captured on copper plates in the 1840s. Later, pictures were recorded on glass plates. Flexible film, much like we still use today, replaced glass plates in the late 1800s.

Like your eyes, a camera has a lens. A lens is a piece of glass shaped to focus light so the picture will be clear. Some cameras even have automatic focus, just like healthy eyes. If a camera lens is out of focus, the picture will be blurry.


The camera changed the world. Before the camera was invented, people created pictures by painting or drawing. That took time and could be inaccurate.

Around 1840, that all changed. The camera allowed people to keep a visual record of their lives and important events. Suddenly, people could see pictures of faraway places. The camera brought the whole world into people’s homes. Photographs began to influence people’s opinions about the world.

Cameras brought big changes to family life as well. Before the camera, only wealthy people could afford to pay painters to make portraits. Suddenly, ordinary people could afford to have snapshots of themselves and their children or grandchildren.

Later, the motion-picture camera was invented. Thanks to that, we have television and movies.


Today, many people have cameras. Most people use point-and-shoot cameras. A point-and-shoot camera automatically focuses the lens and controls how quickly the shutter opens and closes.

Many banks, stores, and schools use security cameras to watch what people are doing. Cameras on highways show traffic patterns. There are even tiny cameras on some computers and cell phones.

Cameras are important tools for scientists. Doctors use tiny cameras to look inside the human body. Cameras on satellites orbit Earth, taking pictures of weather patterns. Cameras bring us pictures from the deepest oceans, the insides of volcanoes, and even of distant galaxies in space! Cameras are just about everywhere.

dr.janxeb Tuesday, September 08, 2009 10:57 PM


Airplanes are a relatively recent invention. The first one flew just over 100 years ago. As little as 50 years ago, only small numbers of people had ridden in an airplane. Today, air travel is one of the most common means of transportation.

Hundreds of thousands of people fly on airplanes each year. You buy your ticket. You pack your suitcase. Then, off you go to the airport.

The airport is where planes take off and land. An agent takes your suitcase. You go to a gate (loading and unloading area) and get on your plane. There are rows and rows of seats. You sit down next to a window. You fasten your seat belt. You are ready to take off.


The place where you sit is called the cabin. The cabin is in a long tube called the body, or fuselage, of the airplane. The front of the fuselage is called the nose. The pilot and copilot sit in the cockpit right behind the nose. The pilot steers the plane in the cockpit. Your suitcase is stowed in the cargo hold under the cabin.

Two big wings stick out from the fuselage. In back of the wings are moveable parts called flaps and ailerons. These parts help control the plane. A big tail sticks up from the end of the fuselage. A rudder, located on the back of the tail, helps the plane turn left and right.

Sets of wheels sit underneath the airplane. The airplane rolls on the wheels before it takes off and after it lands. The wheels on big planes go up into the fuselage when the plane is in the air. They come down before the plane lands.


There are different kinds of airplane engines. Propeller engines turn propellers on the nose or on the wings. Propellers pull an airplane through the air.

Jet engines suck air in. They heat the air and shoot it out of the back of the engine. Jet engines push the plane through the air. Turboprops are a combination, using the power of a jet engine to turn a propeller.


Airplanes are heavier than air. They need to go fast in order to fly. Engines and wings make a plane fly.

An airplane builds up speed on a runway. Runways at airports are long concrete strips. Runways in some faraway places can be level places made of dirt or grass. Some planes can even take off on water. When the plane is going fast enough, the pilot takes it up into the air.


The pilot uses many controls in the cockpit to fly a plane. The pilot pulls a wheel or stick back to make the plane go up. Air rushing over and under the wings lifts the plane into the sky.

Dials on a control panel in the cockpit tell the pilot how high the plane is, how much fuel it has, and which direction it is heading. A radar screen tells the pilot if other planes are nearby. The pilot uses the rudder on the tail and the ailerons on the wings to make the plane turn.


It’s time to land. The pilot pushes the wheel or stick forward to make the plane go down. The pilot lowers the wheels and landing gear. The plane touches down on the runway. The pilot uses brakes to slow and stop the plane.


Planes do different kinds of work. Passenger planes carry people in the cabin. Cargo planes carry packages, boxes, and other things. Cargo planes do not have seats.

Military cargo planes can carry soldiers, tanks, and cannons. Some military planes are fighter jets. Some are bombers. Some military jets take off and land on aircraft carriers at sea. Certain military planes can take off straight up like a helicopter, then fly ahead like a plane.

Crop-duster planes spray farm fields with chemicals that kill bugs or fertilizer that helps crops grow. Firefighting planes drop water or chemicals on forest fires. Seaplanes have skis instead of wheels. They can land on lakes in faraway places to deliver passengers, supplies, and mail.


The smallest airplanes are called ultralights. They weigh about 100 pounds (about 46 kilograms) and carry only a pilot. The biggest planes are jumbo jets. They can carry several hundred people and several hundred tons of cargo. Jumbo jets fly long trips over oceans.

In between, there are planes of many sizes. There are two-seater and four-seater propeller planes. There are commuter planes that can carry about 20 passengers on short trips. Airlines also fly many jets that hold from 80 to over 400 passengers.


Long ago, people dreamed of flying like the birds. They tried to build machines that would fly. The first people to succeed were two American brothers, Orville and Wilbur Wright. They made a heavier-than-air machine of wood and cloth. It had an engine that turned a propeller. The brothers made their first flight near Kitty Hawk, North Carolina, on December 17, 1903.

The Wright brothers and other inventors experimented with different designs. They made better and better planes. The first warplanes flew during World War I (1914-1918). Then, pilots started taking passengers on trips. Jet engines in the 1950s made air travel faster and made passenger planes very popular. Now, millions of people travel everywhere on airplanes.


Today, there are planes that can fly as fast as the speed of sound. Inventors hope to make planes that can fly five times faster than sound. They want these planes to fly up to the edge of space. Then the planes will come back down and land. They call these planes hypersonic planes. Today, it would take you more than 12 hours to fly from Chicago, Illinois, to Tokyo, Japan. In a hypersonic plane, you could make that trip in two to three hours.

dr.janxeb Tuesday, September 08, 2009 10:58 PM


There is music in the air all around you. There are sounds of people talking in the air all around you. The sounds of music and talking are carried by radio waves. There are radio waves everywhere indoors and outdoors.

Radio waves are invisible. You cannot see or feel them. You can only hear radio waves if you turn on a radio. Radios turn radio waves into sound.


Radios need electricity in order to work. Your portable radio gets electricity from batteries. Your clock radio gets electricity from a cord that you plug into an electrical outlet in a wall.

Radios have a power switch or button that lets you turn the radio on or off. Radios have a volume control that lets you play the sounds loudly or softly. Radios also have a dial or button that lets you tune in your favorite radio stations. Each station has a special number on the dial. When you tune in a station, your radio turns radio waves from that station into sound.

Radios have a special wire called an antenna that can pick up radio waves in the air. Radios first turn the radio waves into electrical signals. Then they turn the electrical signals into the sounds of music, traffic and weather reports, or news about your hometown sports teams.


A radio station sends electrical signals through wires to a tall tower called a broadcast antenna. Electrical signals get changed into radio waves at the antenna. The antenna sends the radio waves out in all directions.

Some radio stations broadcast on AM radio waves. Some programs are broadcast on FM waves. AM radio waves travel farther than FM waves, but FM waves make clearer sounds. Most radios can pick up both AM and FM radio waves.


Radio broadcasts only go one way, from the station to your radio. You can listen to radio, but you cannot talk back. Two-way radio lets people talk to each other on radio waves.

Police officers and firefighters use two-way radio. Firefighters at a big blaze can call for more help on their two-way radios. Soldiers use two-way radios on battlefields.


Cell phones use radio waves. Your cell phone sends your phone calls on radio waves to an antenna. The antenna passes your call along. You can talk on a cell phone in a car, on a bus, or just when you are walking around.

Some computers hook up to the Internet with radio waves. These computers have special antennas that can find wireless “hot spots.” These computers do not need to be plugged into a telephone line to surf the Internet.


The radar that lets airplanes and ships “see” things in fog or things far away uses radio waves. Radar systems send out radio waves. The radio waves bounce back from any large object they hit and make images on a radar screen.

Radio waves help us explore deep space. Radio telescopes listen for radio waves from far away in the universe. Astronauts in spacecraft talk to control centers on Earth using radio waves. Radio waves beam pictures to Earth from cameras on space probes visiting other planets.

Doctors use radio waves to see inside the body. They use radio waves from MRI machines to make pictures of people’s insides.


During the 1800s, several scientists made discoveries that led to the invention of radio. An Italian inventor named Guglielmo Marconi sent the first sounds on radio waves in 1895. The sounds he sent were just clicks. The clicks were a kind of code that carried telegraph messages. People already knew how to send telegraph messages over wires on land. Telegraph messages sent on radio waves helped ships at sea where there were no wires. Sinking ships could send messages calling for help.

Other inventors learned how to send music and voices over radio waves. Radio stations began broadcasting programs in the 1920s. Families used to gather around the radio to listen to band music, soap operas, or other radio programs.

Inventors have found more and more uses for radio waves. Radio waves have become very important for helping you stay in touch with family and friends.

dr.janxeb Tuesday, September 08, 2009 11:00 PM


People in the 1800s didn’t know what to think about a new invention called the automobile. No one was sure it would catch on. In those days, people often traveled in carriages pulled by horses. So when the first automobiles appeared, people nicknamed them “horseless carriages.”

The first automobiles looked a lot like horse carriages. That was the style people knew. But the automobile soon took on a look that was all its own. The modern automobile has a hood and fenders. It has a roof, sides, and four wheels. It has seats where the driver and passengers sit. Modern automobiles are commonly called cars or autos.

Few machines are as important as cars. You can ride to school in one. Adults can drive one to work. You can drive in a car to shopping malls. You can take long vacations traveling in an automobile.


The typical passenger car can carry up to six people. Larger vehicles called minivans are like big cars. They can usually carry up to eight people. Pickups or trucks are built to carry cargo.

Sport-utility vehicles (SUVs) are made for driving in all types of conditions, including mud or snow. Sports cars are built for power and good road handling. Many sports cars have room for just two passengers.

Racing cars are specially designed to compete on tracks and courses. Most racing cars are built to be lightweight and very fast. Because they are made for racing, they usually are not suited for driving on public streets.


A car gets power from its engine. Most auto engines burn gasoline. Gasoline goes through fuel lines from a gas tank to the engine.

When it burns fuel, the engine makes exhaust gases. These gases go out through pipes called the exhaust system.

Moving parts hooked up to the engine are called the drivetrain. The drivetrain carries mechanical energy from the engine to the wheels. The turning wheels make an automobile go.

Springs and shock absorbers give passengers a smoother ride on bumpy roads. Electrical parts make the headlights, turn signals, horn, radio, and windshield wipers work. The electrical parts also help start the car. Brake parts rub against the wheels to slow the car down. Seat belts and air bags help protect you in an accident.


People don’t just jump into cars and start driving. First, they must get their learner’s permit. Local auto bureaus can tell you how. Driver-education classes teach people how to drive a car. Students learn how a car works and the rules of safe driving.

To start a car, you sit in the driver’s seat. Turning a key in the ignition starts the engine. Moving the car’s gearshift connects the engine to the drivetrain. Pressing the gas pedal on the floor sends fuel to the engine. The harder you press, the faster the car goes.

To make the car turn left or right, you turn the steering wheel in the direction you want to go. To make the car move forward or backward, you use the gearshift.

What about stopping? Press your foot down on the brake pedal. The brakes will press against the wheels, making them slow down and then stop turning.


The first cars were built in the 1700s. They were powered by steam engines. In England, steam-powered cars weren’t allowed on the roads. They were run like trains on private railroad tracks!

Auto racing became popular in the late 1800s and early 1900s. Some early racing cars had steam engines. These included the American-made Stanley Steamer. In 1906, a Stanley Steamer hit a speed of more than 121 miles per hour (195 kilometers per hour), setting a new land speed record.

Other cars made at the time ran on electricity from batteries. People liked them because they were quiet and less likely to scare horses and people. Still other cars had gasoline engines. The first gasoline-powered cars were loud, slow, and unreliable. But over time, the cars were improved, and more people wanted to drive them.


In the United States, a businessman named Henry Ford started the Ford Motor Company in 1903. His company made two famous kinds of cars: the Model A and the Model T.

Ford invented the factory assembly line for making cars. Workers in one place along the assembly line worked on just one part of the car. Other workers, in another area of the assembly line, worked on another part of the car.

Automobiles made this way were not very expensive. Ordinary people could afford them. The Model T became one of the biggest-selling automobiles of all time. Henry Ford sold more than 15 million Model T cars before his company stopped making them in 1927.


Modern cars are much better than earlier models. They are easier to drive and have advanced safety features such as air bags. Engines are more efficient and powerful. Cars are quieter and more comfortable inside.

Today, cars are more popular than ever. They are the main form of transportation for many people in the United States and Canada. Many people own more than one car.


The exhaust gases that come from burning gasoline can pollute the air. These gases contain chemicals that cause a smoky pollution called smog. The worst smog forms in cities. Exhaust gases also contain a gas called carbon dioxide. Scientists think carbon dioxide pollution is making Earth’s climate warmer.

Scientists and engineers are working to reduce pollution from cars. They have made cars that burn less gasoline. They have designed exhaust systems that give off less pollution. They have also developed efficient hybrid cars.


Many people believe hybrid cars could be a big help in reducing pollution. Hybrid cars are automobiles that run partly on gasoline and partly on some other fuel. Most hybrid cars use electricity from batteries. Scientists are also experimenting with hybrids that run on energy from sunlight and other sources.

dr.janxeb Tuesday, September 08, 2009 11:01 PM


Helicopters can fly straight up. They can fly forward, sideways, and backward. They can even hover in one place. An airplane must speed down a long runway to take off and land. Wings hold an airplane in the air. Helicopters do not need runways, and they do not have wings.


Big blades on top of a helicopter keep it in the air. The blades are a little like fan blades. The blades spin very fast. Wind blowing down from the whirling blades holds a helicopter up. The blades also control the direction in which the helicopter flies.

The blades make a loud chop-chop-chop noise as they turn. The noise caused people to nickname helicopters “choppers.”

Helicopters cannot fly as fast as an airplane. The fastest helicopters go about 200 miles per hour (320 kilometers per hour). They also cannot go as far as an airplane. Helicopters burn a lot of fuel.


The blades of a helicopter are called the rotor. Some rotors have two blades. Some rotors have three or four blades. Some big helicopters have rotors with eight blades. Big helicopters sometimes have two rotors on top.

A long metal tail sticks out from the back of most helicopters. These helicopters have a small rotor on the tail. The tail rotor blows air sideways instead of down. It helps the helicopter steer.


A helicopter has a cockpit just like an airplane. The controls are in the cockpit. A helicopter has two control sticks, or levers. It has two pedals on the floor.

When you’re flying a helicopter, you push or pull on the stick on your left side to make the blades tilt. Tilting the blades makes the helicopter go up or down. A grip on this stick controls the speed. You twist the grip to make the helicopter go faster or slower.

The other lever is between your knees. You move this stick around to make the helicopter fly forward, backward, or sideways.

The pedals on the floor control the tail rotor. You step on the pedals to turn the helicopter. Pushing the left pedal makes the helicopter turn left. Pushing the right pedal makes the helicopter turn right.


Helicopters can go places that are hard to reach. They can go places where airplanes cannot land, or where there are no roads for cars or trucks.

Helicopters can rush injured people from a car accident to a hospital. They can rescue people from the tops of burning buildings. They can pluck them from trees in the middle of raging floods. They can lift people from the decks of sinking ships at sea.

Military helicopters are important in war. They carry troops to battle. They carry wounded soldiers to hospitals. They can even shoot missiles.

Helicopters do other kinds of work. Reporters can fly in helicopters to cover news stories. Police use helicopters to chase suspected criminals. Farmers can use helicopters to spray their fields. Sometimes helicopters work on construction. The carry heavy parts to the tops of buildings.


People imagined machines like helicopters hundreds of years ago. The ancient Chinese made a spinning top that could rise up into the air. The Italian artist and inventor Leonardo da Vinci drew a design for a machine like a helicopter in 1480.

The first helicopters flew in France in 1907. They were hard to control. People kept trying to build better helicopters. Finally, a Russian-born American engineer named Igor Sikorsky made a workable helicopter. It was the first helicopter with a tail rotor. He flew this helicopter in 1939. Most helicopters today are like the helicopter that Sikorsky built. Inventors and engineers are still working to make bigger and better helicopters.

dr.janxeb Tuesday, September 08, 2009 11:05 PM


What if you want to talk right now to a friend who lives far away? The answer is simple. You pick up your telephone and press some buttons. Next, you hear a ringing sound—one, two, three rings. Then you hear your friend’s voice say, “Hello.” Making a phone call seems so easy. But did you ever think about what makes it possible?

When you pick up your phone, it instantly hooks up with a vast, worldwide telephone network. The network has millions of miles of wire. It has cables that run under the oceans. It has optical (glass) fibers as thin as a hair. It has satellites that orbit high above Earth. It has powerful computers that keep track of everything on the network, including the call to your friend. The word “hello” might have zipped through wires, shot up to a satellite, or zoomed through a cable under the sea before it got to your ear.


Take a close look at your telephone. It has several parts. The part that you speak into is called the transmitter. The part that you put to your ear for listening is called the receiver. The buttons you press are called the dial.

Most phones have a handset and a base. The handset contains the transmitter and receiver and sometimes the dial. We often call the entire handset the receiver, even though only the part we place to our ear receives sound. Many phones have a wire that connects the handset to the base. Wires connect the base to the telephone network.

Some phones use radio signals instead of wires to send messages. Radio signals connect the handset of a cordless phone to the base. Cordless phones let you walk around while talking, but you can’t go too far. Radio signals from a cordless phone only work over a short distance.

Radio signals from a cell phone go much farther. A cell phone does not need a base to connect to the telephone network. You can take your cell phone almost anywhere. Radio signals from a cell phone go to an antenna, a tower that picks up radio signals. The area around an antenna is called a cell. The antenna in one cell can send signals to an antenna in another cell. An antenna can also send radio signals to wires in the telephone network, enabling you to call anyone you want.


Your voice does not really go through the telephone wires. Instead, a copy of your voice goes to the telephone network. The copy can travel through wires. It can also go through the air as a radio signal.

The telephone’s transmitter makes the copy of your voice. When you say “hello,” sound waves go into the transmitter. The sound waves hit a thin sheet of metal or plastic inside the transmitter. The sound waves make the sheet vibrate. Some vibrations are big and some are small. The transmitter turns the vibrations into electric signals. These signals are a copy of your voice saying “hello.”

The copy of your voice goes through the telephone network and into the receiver in your friend’s telephone. The way the receiver works is the opposite of the way the transmitter works. The receiver turns the electric signals into sound waves. Your friend hears the copy of your voice say “hello.” It sounds just like you!


Millions and millions of phones are connected to the telephone network. Computers and other equipment in the network can tell which phone belongs to your friend by the telephone number you dial. A telephone number is a kind of code. All phones in the United States have a code that is ten numbers long. The numbers are also called digits.

The first three digits of a phone number are called the area code. An area code tells the network what part of a state or city the phone is in. The next three digits are the exchange. They tell what neighborhood or other small area the phone is in. The last four digits tell the network exactly which phone in that area and exchange belongs to your friend.

Phone numbers in other countries have different numbers of digits. You also have to use extra numbers, called a country code, to call a different country.


Before the telephone, it was hard for people to communicate over long distances. They wrote letters to each other. It could take days or even weeks for letters to be delivered.

Then people learned how to send telegraph messages. The messages traveled as electric signals that represented a code of dots and dashes. An operator on the other end converted the dots and dashes into a regular message. As the telephone became more and more popular, it largely replaced the telegraph.


Today our huge telephone network does many things besides carrying telephone calls. It sends copies of letters and pictures from one machine to another, called a fax machine. It connects computers all over the world into another vast network called the Internet. This network lets you send e-mail messages from your computer to your friends’ computers. It is hard to imagine what life would be like without the telephone.

Saqib Riaz Tuesday, September 08, 2009 11:42 PM

When you use computers, entertainment systems or telephones, the various pieces and parts of the systems make up a community of electronic devices. These devices communicate with each other using a variety of wires, cables, radio signals and infrared light beams, and an even greater variety of connectors, plugs and protocols.

There are lots of different ways that electronic devices can connect to one another. For example:

•Component cables
•Electrical wires
•Ethernet cables
•Infrared signals

**The art of connecting things is becoming more and more complex every day. In this article, we will look at a method of connecting devices, called Bluetooth, that can streamline the process. A Bluetooth connection is wireless and automatic, and it has a number of interesting features that can simplify our daily lives.

[B]**The Problem[/B]
When any two devices need to talk to each other, they have to agree on a number of points before the conversation can begin. The first point of agreement is physical: Will they talk over wires, or through some form of wireless signals? If they use wires, how many are required -- one, two, eight, 25? Once the physical attributes are decided, several more questions arise:

•How much data will be sent at a time? For instance, serial ports send data 1 bit at a time, whil*e parallel ports send several bits at once.

•How will they speak to each other? All of the parties in an electronic discussion need to know what the bits mean and whether the message they receive is the same message that was sent. This means developing a set of commands and responses known as a protocol.

[B]How Bluetooth Creates a Connection[/B]
*Bluetooth takes small-area networking to the next level by removing the need for user intervention and keeping transmission power extremely low to save battery power. Picture this: You're on your Bluetooth-enabled cell phone, standing outside the door to your house. You tell the person on the other end of the line to call you back in five minutes so you can get in the house and put your stuff away. As soon as you walk in the house, the map you received on your cell phone from your car's Bluetooth-enabled GPS system is automatically sent to your Bluetooth-enabled computer, because your cell phone picked up a Bluetooth signal from your PC and automatically sent the data you designated for transfer. Five minutes later, when your friend calls you back, your Bluetooth-enabled home phone rings instead of your cell phone. The person called the same number, but your home phone picked up the Bluetooth signal from your cell phone and automatically re-routed the call because it realized you were home. And each transmission signal to and from your cell phone consumes just 1 milliwatt of power, so your cell phone charge is virtually unaffected by all of this activity.

Bluetooth is essentially a networking standard that works at two levels:

•It provides agreement at the physical level -- Bluetooth is a radio-frequency standard.

•It provides agreement at the protocol level, where products have to agree on when bits are sent, how many will be sent at a time, and how the parties in a conversation can be sure that the message received is the same as the message sent.

The big draws of Bluetooth are that it is wireless, inexpensive and automatic. There are other ways to get around using wires, including infrared communication. Infrared (IR) refers to light waves of a lower frequency than human eyes can receive and interpret. Infrared is used in most television remote control systems. Infrared communications are fairly reliable and don't cost very much to build into a device, but there are a couple of drawbacks. First, infrared is a "line of sight" technology. For example, you have to point the remote control at the television or DVD player to make things happen. The second drawback is that infrared is almost always a "one to one" technology. You can send data between your desktop computer and your laptop computer, but not your laptop computer and your PDA at the same time. (See How Remote Controls Work to learn more about infrared communication.)

These two qualities of infrared are actually advantageous in some regards. Because infrared transmitters and receivers have to be lined up with each other, interference between devices is uncommon. The one-to-one nature of infrared communications is useful in that you can make sure a message goes only to the intended recipient, even in a room full of infrared receivers.

Bluetooth is intended to get around the problems that come with infrared systems. The older Bluetooth 1.0 standard has a maximum transfer speed of 1 megabit per second (Mbps), while Bluetooth 2.0 can manage up to 3 Mbps. Bluetooth 2.0 is backward-compatible with 1.0 devices

[B]How Bluetooth Operates[/B]
Bluetooth networking transmits data via low-power radio waves. It communicates on a frequency of 2.45 gigahertz (actually between 2.402 GHz and 2.480 GHz, to be exact). This frequency band has been set aside by international agreement for the use of industrial, scientific and medical devices (ISM).

A number of devices that you may already use take advantage of this same radio-frequency band. Baby monitors, garage-door openers and the newest generation of cordless phones all make use of frequencies in the ISM band. Making sure that Bluetooth and these other devices don't interfere with one another has been a crucial part of the design process.

One of the ways Bluetooth devices avoid interfering with other systems is by sending out very weak signals of about 1 milliwatt. By comparison, the most powerful cell phones can transmit a signal of 3 watts. The low power limits the range of a Bluetooth device to about 10 meters (32 feet), cutting the chances of interference between your computer system and your portable telephone or television. Even with the low power, Bluetooth doesn't require line of sight between communicating devices. The walls in your house won't stop a Bluetooth signal, making the standard useful for controlling several devices in different rooms.

Bluetooth can connect up to eight devices simultaneously. With all of those devices in the same 10-meter (32-foot) radius, you might think they'd interfere with one another, but it's unlikely. Bluetooth uses a technique called spread-spectrum frequency hopping that makes it rare for more than one device to be transmitting on the same frequency at the same time. In this technique, a device will use 79 individual, randomly chosen frequencies within a designated range, changing from one to another on a regular basis. In the case of Bluetooth, the transmitters change frequencies 1,600 times every second, meaning that more devices can make full use of a limited slice of the radio spectrum. Since every Bluetooth transmitter uses spread-spectrum transmitting automatically, it’s unlikely that two transmitters will be on the same frequency at the same time. This same technique minimizes the risk that portable phones or baby monitors will disrupt Bluetooth devices, since any interference on a particular frequency will last only a tiny fraction of a second.

When Bluetooth-capable devices come within range of one another, an electronic conversation takes place to determine whether they have data to share or whether one needs to control the other. The user doesn't have to press a button or give a command -- the electronic conversation happens automatically. Once the conversation has occurred, the devices -- whether they're part of a computer system or a stereo -- form a network. Bluetooth systems create a personal-area network (PAN), or piconet, that may fill a room or may encompass no more distance than that between the cell phone on a belt-clip and the headset on your head. Once a piconet is established, the members randomly hop frequencies in unison so they stay in touch with one another and avoid other piconets that may be operating in the same room. Let's check out an example of a Bluetooth-connected system.


dr.janxeb Wednesday, September 09, 2009 07:40 PM


You probably have a favorite television show. Maybe you have more than one favorite TV show. Did you ever wonder where your favorite shows come from? Did you ever wonder how they get to the TV set in your home?


Some TV shows are made in TV studios. Some of these shows are broadcast live—that is, as they are being made. Some shows are taped in the studio. The tape gets played on TV later on.

Other TV shows are made outside of studios. Baseball and football games and other sports events come from stadiums. Some parts of news programs are broadcast “on the scene.” TV reporters go to the scenes of accidents, floods, and fires and describe what is happening.

Shows in studios are made on sets. Sets for plays or soap operas can look like living rooms or kitchens. Sets for talk shows might have a desk for the host and chairs for the guests. Bright lights shine down on the sets.


A TV picture starts with a TV camera. Some TV cameras are big and some are small. The cameras in TV studios are big. Camera operators roll the big cameras around on wheels. There are usually several big cameras in a TV studio. Cameras used outside a TV studio are smaller. TV camera crews take the smaller cameras to news and sports events.

Some cameras send out live pictures to your TV set. Some cameras make videotapes that get played later on a television program.

All TV cameras need electricity to work. A camera operator points the camera at a scene. The camera picks up light from the scene. It changes this light into an electric signal called the video signal. A microphone changes the sound of people talking or music playing into an electric signal called the audio signal.

TV cameras do not snap pictures the way an ordinary camera does. Parts inside a TV camera scan, or sweep over, the scene and trace a series of thin, horizontal lines, one below the other. A TV camera scans a whole scene much faster than you can blink. Lines from the scans go together to make a picture.


The pictures and sound from the TV cameras and microphones go to a control room. Every television station has one or more control rooms. TV cameras in a studio can send live pictures to the control room. The control room is full of dials, switches, and small TV screens. There are screens that show pictures from each TV camera in the studio.

Producers and directors work in the control room. They make sure that the best pictures with the best views go to your TV screen at home.

People who work in control rooms also use taped pictures to make programs. They use computers to put together the best taped scenes.


The picture and sound signals go from the control room to a transmitter. The transmitter makes the signals stronger and sends them to a transmitting antenna. This antenna is very tall. It changes the electric signals into invisible television signals that go through the air. The television signals go out from the antenna in all directions.

TV signals can get to the TV set at your home in several ways. They can go through the air to an antenna on your roof. The antenna picks up the signals and sends them through wires to your TV set. The signals could go to a cable TV company. The company sends the signals through a cable to your home. The TV signals could come right to your house from a satellite circling high above Earth. A satellite dish outside your home can pick up the TV signals and send them over wires to your TV set indoors.


Your TV set changes the television signals back into pictures and sound. Your set picks up the thin lines that the TV camera scanned. Your set uses parts called electron guns to “paint” a picture on the TV screen one thin line at a time. The lines get painted from top to bottom.

A color TV set uses three electron guns to beam out three colors—red, green, and blue. These three colors make all the colors you see on your TV screen. The beams scan fast enough to paint a picture on your screen 30 times a second.


Television can do many things. TV cameras can be sent to places that are difficult or dangerous for people. They can travel to outer space. Spacecraft carry TV cameras to other planets. The cameras send back pictures that let us see what other planets look like.

TV cameras on robot submarines can go deep down in the sea. Doctors use tiny TV cameras to see inside the human body.


Inventors made the first TV pictures in the 1920s. Television stations started broadcasting the first regular TV shows in the 1940s. The first TV sets had small screens. The first TV sets showed black-and-white pictures.

Television sets have gotten better and better. Most TVs sold today show color pictures. TV screens have gotten bigger and bigger. TV sets have gotten thinner. Plasma TV sets are so thin that you can hang them on a wall.

dr.janxeb Wednesday, September 09, 2009 07:42 PM


What if you want to talk right now to a friend who lives far away? The answer is simple. You pick up your telephone and press some buttons. Next, you hear a ringing sound—one, two, three rings. Then you hear your friend’s voice say, “Hello.” Making a phone call seems so easy. But did you ever think about what makes it possible?

When you pick up your phone, it instantly hooks up with a vast, worldwide telephone network. The network has millions of miles of wire. It has cables that run under the oceans. It has optical (glass) fibers as thin as a hair. It has satellites that orbit high above Earth. It has powerful computers that keep track of everything on the network, including the call to your friend. The word “hello” might have zipped through wires, shot up to a satellite, or zoomed through a cable under the sea before it got to your ear.


Take a close look at your telephone. It has several parts. The part that you speak into is called the transmitter. The part that you put to your ear for listening is called the receiver. The buttons you press are called the dial.

Most phones have a handset and a base. The handset contains the transmitter and receiver and sometimes the dial. We often call the entire handset the receiver, even though only the part we place to our ear receives sound. Many phones have a wire that connects the handset to the base. Wires connect the base to the telephone network.

Some phones use radio signals instead of wires to send messages. Radio signals connect the handset of a cordless phone to the base. Cordless phones let you walk around while talking, but you can’t go too far. Radio signals from a cordless phone only work over a short distance.

Radio signals from a cell phone go much farther. A cell phone does not need a base to connect to the telephone network. You can take your cell phone almost anywhere. Radio signals from a cell phone go to an antenna, a tower that picks up radio signals. The area around an antenna is called a cell. The antenna in one cell can send signals to an antenna in another cell. An antenna can also send radio signals to wires in the telephone network, enabling you to call anyone you want.


Your voice does not really go through the telephone wires. Instead, a copy of your voice goes to the telephone network. The copy can travel through wires. It can also go through the air as a radio signal.

The telephone’s transmitter makes the copy of your voice. When you say “hello,” sound waves go into the transmitter. The sound waves hit a thin sheet of metal or plastic inside the transmitter. The sound waves make the sheet vibrate. Some vibrations are big and some are small. The transmitter turns the vibrations into electric signals. These signals are a copy of your voice saying “hello.”

The copy of your voice goes through the telephone network and into the receiver in your friend’s telephone. The way the receiver works is the opposite of the way the transmitter works. The receiver turns the electric signals into sound waves. Your friend hears the copy of your voice say “hello.” It sounds just like you!


Millions and millions of phones are connected to the telephone network. Computers and other equipment in the network can tell which phone belongs to your friend by the telephone number you dial. A telephone number is a kind of code. All phones in the United States have a code that is ten numbers long. The numbers are also called digits.

The first three digits of a phone number are called the area code. An area code tells the network what part of a state or city the phone is in. The next three digits are the exchange. They tell what neighborhood or other small area the phone is in. The last four digits tell the network exactly which phone in that area and exchange belongs to your friend.

Phone numbers in other countries have different numbers of digits. You also have to use extra numbers, called a country code, to call a different country.


Before the telephone, it was hard for people to communicate over long distances. They wrote letters to each other. It could take days or even weeks for letters to be delivered.

Then people learned how to send telegraph messages. The messages traveled as electric signals that represented a code of dots and dashes. An operator on the other end converted the dots and dashes into a regular message. As the telephone became more and more popular, it largely replaced the telegraph.


Today our huge telephone network does many things besides carrying telephone calls. It sends copies of letters and pictures from one machine to another, called a fax machine. It connects computers all over the world into another vast network called the Internet. This network lets you send e-mail messages from your computer to your friends’ computers. It is hard to imagine what life would be like without the telephone.

dr.janxeb Wednesday, September 09, 2009 07:44 PM


Telescopes help us see things that are far away. They make distant objects look bigger. Using telescopes, astronomers have discovered thousands of stars, planets, moons, and many other extraordinary objects, such as black holes.

The most common type of telescope is the optical telescope. This kind of telescope gathers light from distant objects.


Imagine having eyes as big as your fist. You’d look funny, but more light would enter your eyes. You would be able to see better. Telescopes bring extra light to our eyes. They effectively make our eyes bigger. Distant objects appear larger when you look through a telescope, and you can see more detail.

A refracting telescope is the simplest type of optical telescope. It is made up of two lenses. These lenses are similar to the lens in a magnifying glass. A reflecting telescope has a lens and a dish-shaped mirror. The mirror collects and focuses (concentrates) light.

A telescope’s eyepiece can be replaced by a camera. Then the image from the telescope is recorded on film or as a digital image.


The bigger a telescope’s main lens or mirror, the more light the telescope gathers. The more light the telescope gathers, the more detail it shows, and the more distant the objects that you can see through it.

Astronomers use huge telescopes housed inside buildings called observatories. These telescopes have mirrors as large as 26 feet (8 meters) across. They gather enormous amounts of light.


Gigantic telescope mirrors are hard to build because they bend under their own weight. When a mirror bends, it makes a blurry image. One way to keep a giant mirror from bending is to divide the mirror into smaller sections. Another way to avoid huge mirrors is to use computers to combine images from several telescopes. The Very Large Telescope in Chile, for example, has four telescopes with 26-foot (8-meter) mirrors. Together they gather the same amount of light as a telescope with a 52-foot (16-meter) mirror.

Air causes another problem for telescopes. The air low in Earth’s atmosphere swirls about. This movement bends the light coming down from space just a bit, making the images we see through telescopes appear slightly blurry. To reduce this effect, large telescopes are often built on high mountains. This puts them above much of the air in the atmosphere. Many modern telescopes also have flexible mirrors. The shape of their mirrors can be automatically adjusted hundreds of times a second to adjust for the swirling atmosphere and keep the image sharp.


Optical telescopes are only one type of telescope. Astronomers also use telescopes that detect other kinds of electric and magnetic rays from space, such as X rays and radio waves. Our eyes cannot see these rays. Some objects in space aren’t bright enough to be seen with visible light. We wouldn’t know they exist without telescopes that can detect other types of radiation.

A radio telescope, for example, detects radio waves given off by planets, stars, and other objects in space. It has a huge dish that collects the radio waves and focuses them on to an antenna in the center of the dish. The dish can be turned to point at any part of the sky. The antenna turns the radio waves into electrical signals that astronomers record and study.


Several space telescopes are in orbit around Earth, beyond the atmosphere. From there, they have a perfectly clear view into space. This means they can see much more detail on distant objects.

Some types of radiation, such as ultraviolet light, X rays, and gamma rays cannot pass through Earth’s atmosphere. Telescopes that detect these types of radiation must be launched into space.


We do not know exactly who invented the telescope, but we do know it was invented in Holland at the beginning of the 17th century. The first person to look into space through a telescope was the Italian scientist Galileo. He was the first to see moons orbiting Jupiter, Saturn’s rings, and mountains on the Moon.

dr.janxeb Wednesday, September 09, 2009 07:46 PM

X Rays
[B][SIZE="5"]X RAYS[/SIZE][/B]

Imagine that you could see right through your own skin. You could see the bones inside your body. You could watch food go down your throat when you swallow it. Imagine looking inside someone’s suitcase to see what’s inside. Does that sound impossible? Not when you know about X rays!


X rays are very powerful light rays that your eyes can’t detect. These light rays can slip through objects that visible light bounces off. We use X rays as a powerful tool to detect and discover things our eyes can’t see.


X rays were discovered by accident. In 1895, a man named Wilhelm Roentgen was experimenting with electricity in vacuum tubes in a black cardboard box. He noticed that a special screen he had nearby glowed when electricity went through the tubes. He experimented more and determined that invisible light rays from the tubes caused the screen to glow. These rays went right through the cardboard box! He named the invisible light rays he had found X rays.


Just a few years after X rays were discovered, doctors were already using them to find bullets inside people who had been shot. Doctors later began to use X rays to find out if people are sick or have broken bones. Dentists use X rays to check up on people’s teeth.

An X-ray device called a CAT scan rotates around a person and creates a 3-D picture of the person’s insides on a screen. This device gives doctors clear views inside any part of the person’s body.

Scientists who study matter and energy often use X rays in their research. X rays help them see what things are made of. Many chemical elements were discovered using X rays.

Industries use X rays to test products and materials for flaws such as cracks in an airplane wing. X rays are also used to tell whether gems and works of art are real or fake. Border guards use X rays to look inside cars and containers. The X rays can find goods that are being smuggled from one country to another. Airports use low energy X rays to see inside luggage and check for dangerous items.


When a doctor takes an X ray of you, the X-ray machine shoots X rays at you. Most of the rays go through you and into a special film, which catches them. Some of the X rays that hit your bones, however, don’t make it through you. Bones absorb X rays more than other parts of your body. Because X rays absorbed by your bones never make it to the film, lighter areas appear on the film where your bones are! These lighter areas provide a picture of the bones.

X rays can be harmful. Doctors use X rays to kill cells that are harmful to people, such as cancer tumors. Because too many X rays can be harmful, doctors warn that X rays should be used only when necessary.

dr.janxeb Wednesday, September 09, 2009 07:48 PM

Solar Energy

Imagine a source of energy more powerful than a million electric power plants. And imagine that this energy source will never run out—at least not for a few billion years. This energy source is not imaginary. It’s the Sun! Solar energy shines down on us every day.

Solar energy is produced inside the Sun. It is the source of nearly all energy on Earth. This energy is stored in the ground, the oceans, and the wind. Even fossil fuels, such as oil and natural gas, come from ancient plant life that once soaked up sunlight. Today we use solar energy to heat buildings and produce electricity.


You may have seen solar collecting plates on top of buildings. They are thin, flat boxes. The solar collectors capture the Sun’s energy. Sunlight heats air or water flowing through tubes in the boxes. The tubes carry the heat into the building.

Most of the Sun’s energy does not reach Earth’s surface. It is scattered and absorbed by the atmosphere, especially by clouds. That’s why you usually find solar-heated houses in areas that get lots of sunlight. Even in sunny places, it takes a lot of collecting plates to heat a house. Sometimes, not enough solar energy can be stored for use at night or on cloudy days. So the house needs an ordinary water heater and furnace, too.

There are different kinds of solar collectors. Concentrating collectors are much more powerful than flat-plate collectors. Concentrating collectors use curved mirrors to focus the Sun’s energy. They follow the Sun as it moves through the sky. They can produce temperatures high enough to boil water. They can be used to produce electricity.


We use small amounts of electricity from solar energy today. A photovoltaic cell is a kind of battery. It produces an electric current from solar energy. Tiny photovoltaic cells power watches and calculators. They provide electricity to satellites in space. Many photovoltaic cells linked together can produce enough electricity for an entire house.

Generating large amounts of solar power is more difficult. Power plants that burn oil or coal can produce electricity more cheaply than a solar power plant can. There are very few solar-energy power plants operating today.


It will become cheaper to produce electricity from solar energy as technology advances. Fossil fuels will become more expensive as they begin to run out. Solar-energy plants could become more common, once they can produce energy more cheaply than other types of power plants.

Photovoltaic cells can be used to power cars. So far, such cars are only experimental. But in 2003, a car was driven nearly 2,500 miles (about 4,000 kilometers) across Australia using only solar power.

Some scientists have proposed building solar-energy stations in space. These stations would collect energy from sunlight almost 24 hours a day. Then the energy could be beamed to Earth. But for now, such a system would be far too expensive to be useful.

dr.janxeb Wednesday, September 09, 2009 07:50 PM


Do you play computer games over the Internet? Do you surf the Web? Do you send e-mail messages to your friends? You can get all kinds of information on the Internet. People use the Internet to work at home. Scientists use the Internet to help them do research. The Internet has made big changes in the way many people live and work.


The Internet is a system that connects computer networks. The Internet links millions of computers all over the world. It allows your computer to get information stored on other computers far away. Some networks have only a few computers. Some networks have thousands of computers. Computers connect to the Internet through telephone and cable systems.

Many governments, big companies, and other organizations have intranets. The computers on an intranet are hooked up to the Internet. But only people who work for the organization that owns the intranet can use it. Other people on the Internet cannot see what is on the intranet computers.


The Internet grew out of a computer network called ARPANET. The United States military created ARPANET in the 1960s. From the 1970s until the late 1980s, the U.S. government only let a few scientists and people in the military use it. In the 1980s, the government let networks at universities join with ARPANET to create the Internet. The Internet grew quickly. Schools, libraries, local and state governments, companies, and families were on the Internet by the mid-1990s.

At first, it was hard to get information from the Internet. You could only see words and numbers on your computer screen. Then a British computer scientist named Timothy Berners-Lee created the World Wide Web in the 1980s.


The difference between the Internet and the Web is sort of like the difference between highways and a delivery service. Delivery service trucks use highways to move packages from one place to another. The Web is like the delivery service. The Internet is like the highways. Information traffic from the Web travels over the Internet.

The Web is made of places called sites. People use special computer programs to make the sites. The sites are stored on computers called Web servers. Each site is made up of documents called Web pages. These Web pages can have text, pictures, sounds, and videos.

You need computer software called a Web browser to find and see Web pages. Each Web page has a URL (Uniform Resource Locator). The URL is like an address that the browser looks for. An example of a URL is: [url]http://www.encarta.com/[/url].

Many computer experts think that the Internet became so popular because of the Web. The Web is easier to use than the Internet by itself. By the end of 2000, more than 80 percent of all traffic on the Internet highway came from the Web.


Millions of people use the Internet every day. In 1981, only 213 computers were connected to the Internet. By 2003, more than 216 million computers were connected to the Internet.

No one knows for sure exactly how many people use the Internet. Computer experts thought that there were 61 million Internet users worldwide at the end of 1996. There may have been from 700 million to 900 million users by the end of 2003.


You get on the Internet by joining a computer network. The network that you join is called an Internet service provider (ISP). America Online (AOL), Earthlink, and Microsoft’s MSN are popular ISPs. You pay a fee to the ISP just as you pay a phone company to use their telephone system.

The company that owns your ISP sends you software to install on your computer. The software lets you use the ISP’s network to get on the Internet. The ISP also gives you an e-mail address.

There are different ways to connect your computer to the ISP. You can hook up your computer with a modem and your home telephone line. This is called dial-up access. You can hook up to the ISP with a digital subscriber line (DSL) or a cable modem. A DSL uses the same wires as your telephone. A cable modem uses the same wiring that cable television uses. DSLs and cable modems bring Web pages to your computer screen much faster than a dial-up connection.

DSLs and cable modems are called broadband connections. Many computer experts think more people must get broadband connections in order for the Internet to continue growing.

dr.janxeb Wednesday, September 09, 2009 07:53 PM


How does a television signal get to the other side of the world in seconds? What tells ships exactly where they are in the middle of the ocean? How do we get warning that storms are coming? Satellites do all these things and more.


Satellites are objects in outer space that fly around planets in circular paths called orbits. Artificial satellites are made by people. Thousands of satellites are zooming around our planet right now.

The Soviet Union launched the first artificial satellite, Sputnik 1, in 1957. Sputnik 1 broadcasted a steady signal of beeps. It circled Earth for three months and then fell back into the atmosphere and burned up. The atmosphere is the air that surrounds Earth.


Satellites need to reach a height of at least 120 miles (200 kilometers) to orbit. They also need to travel faster than 18,000 miles per hour (29,000 kilometers per hour). A satellite any lower or slower would soon fall back down to Earth. It takes a rocket to bring satellites up to that height and speed.

Most satellites are launched from the ground. Some small satellites can be launched from high-flying planes. This uses less fuel.

Other satellites are launched using a space shuttle or other piloted rocket. This way, astronauts on the space shuttle can make sure the satellite is working and gets into the right orbit.


Satellites are used for a great many things. Communications satellites beam TV, radio, and telephone signals all around the world. Navigational satellites help people know where they are and get where they are going. Weather satellites take pictures of clouds and storms from above to help make weather forecasts. Spy satellites look down and snoop on other countries. Other satellites help scientists to study Earth and other planets.


Space is a difficult place to be. You can’t plug in a cord in outer space, so satellites need to take a power source with them. It’s hard to get satellites pointed in the right direction because there’s nothing to turn them with. Satellites need to work in the freezing cold of Earth’s shadow as well as in the blazing heat of the Sun’s rays. They also need to be tough enough to survive collisions with tiny asteroids (space rocks)!

Most satellites use both power from the Sun and batteries to work. They catch the Sun’s energy using large flat solar panels. Satellites keep these panels pointed at the Sun. They use batteries when the Sun doesn’t shine on them.

Satellites can stay pointed in the right direction using small rockets called attitude thrusters. They can also use instruments called gyroscopes. Sometimes magnets on board the satellite can push against the magnetic field of Earth to aim the satellite correctly.

No air flows past satellites to cool them. To keep from getting too hot in the Sun, satellites have panels that open and close. This lets heat escape. Satellites often spin so the Sun doesn’t make one side so hot that it melts.

Satellites also need to be made from strong materials in case tiny asteroids hit them. They need materials that don’t become brittle in the cold and the harsh radiation of space.


When satellites stop working they are often left in orbit as so much space junk. Others drift too low to keep orbiting and burn up as they fall. Still others are brought back to Earth for repairs.

Nonworking satellites are sometimes sent down from orbit into the atmosphere to burn up on purpose. Space is very large, but still scientists need to be careful that satellites don’t crash into each other. They try to get rid of the broken ones.


Since Sputnik 1, more than 5,000 satellites from many countries have been launched. Artificial satellites now orbit the Sun, Mars, Venus, and other planets and their moons. Most satellites, however, orbit Earth. High above your head thousands of satellites circle the planet every day.

dr.janxeb Wednesday, September 09, 2009 07:55 PM


Lasers are powerful enough to cut through steel. Lasers are delicate enough to use in eye surgery. Lasers “read” the information coded on compact discs (CDs). These are just a few things that lasers do.


A laser is a device that produces a beam of light and makes the beam more intense. A laser beam is very exact. It can travel a long distance without spreading out and losing its power.

Laser light is unlike sunlight or light from a light bulb. To understand the difference, think of a crowded city street. Thousands of people are walking along the sidewalk. Their clothes are of many different colors. They are walking in many different directions. Now think of a marching band in a parade. Everyone is wearing a uniform of the same color. They are all walking in the same direction, in step with one another.

Sunlight and light from lamps are like the crowd on the sidewalk. They are made up of many colors—all the colors of the rainbow. They spread out in all directions from their source. Laser light is like the marching band. It is light of a single color. It travels in a beam. It spreads out very little, even when traveling through outer space.


Because lasers produce such exact beams of light, they are very useful tools. Some lasers are so powerful they can drill holes in diamonds. These lasers can cut a piece of steel to an exact shape needed for a machine part. Powerful lasers can produce temperatures of 10,000° Fahrenheit (5500° Celsius) and higher. This ability makes them useful in factories for joining together large pieces of metal.

Much smaller lasers “read” price tags on products. At the supermarket, you’ve probably seen the checkout clerk run foods over the laser scanner. Small lasers in compact disc (CD) players read the information coded on CDs. This information is then played back as music. Lasers even carry telephone conversations. Laser beams send thousands of telephone calls through thin glass threads called optical fibers.

In hospitals, doctors use lasers for delicate operations such as repairing damaged eyes. The military uses lasers to guide airplanes and missiles. And laser beams are used to create colorful light shows. These are just a few of the ways we use lasers.


Famous scientist Albert Einstein first suggested the idea of a laser in 1917. In the 1950s, scientists began making the idea work. American scientist Gordon Gould suggested the name laser in 1957. It was short for Light Amplification by Stimulated Emission of Radiation. The first working laser was built in 1960. It was built by another American scientist, Theodore Maiman. The development of lasers advanced rapidly during the 1970s and 1980s.

Scientists today are using a huge and powerful laser to study how atoms join together in the Sun to release energy. This laser was built at Lawrence Livermore National Laboratory in California. It takes up a building the size of a football stadium.

dr.janxeb Wednesday, September 09, 2009 07:57 PM


Radar was once a secret military weapon. In 1940 and 1941, radar helped Britain defend itself against German bombers during World War II. Today, radar has many uses. It prevents airplanes from colliding in midair. Radar is even found in many kitchens. Microwave ovens use radar technology to heat food.


Radar uses radio waves to locate airplanes, ships, storm clouds, and other large objects. A radar system sends out radio waves. These invisible waves move at the speed of light—about 186,000 miles (300,000 kilometers) a second. When the waves strike an airplane or other object, some of the waves bounce back. The waves that come back provide information to the radar operator. They tell how far away the object is, how fast it is moving, and what direction it is moving in. The word radar comes from RAdio Detection And Ranging.


Radar was developed during the 1920s. Scientists worked on military uses of radar in top-secret laboratories during the late 1930s and early 1940s. Radar became an important weapon during World War II. Radar stations on England’s coast located German bombers before the planes reached England. British fighter planes then attacked the bombers. Radar also helped locate German submarines before they could attack American and British ships.


Airports use radar to keep track of planes in the air. In the control tower, air traffic controllers study radar displays that look like computer screens. Tiny dots on the screens show where planes are. The dots move as the planes move. Radar prevents collisions in the air and helps pilots land planes safely at airports. Airplanes carry radar to warn pilots if their plane is too close to another plane.

Have you seen a weather map on television? Did the weather reporter say it was a radar map? Radar locates rain, storm clouds, and moving masses of air. It can help predict thunderstorms and tornadoes.

Satellites orbiting Earth produce radar maps for predicting weather. Scientists have made pictures of Earth’s surface by using radar in satellites. During the 1990s, the Magellan spacecraft used radar to map the surface of the planet Venus. Magellan’s radar penetrated the thick clouds that surround Venus.

Police officers use radar to detect cars and trucks that are going faster than the speed limit. Stopping people from speeding improves traffic safety. But some drivers buy devices that detect radar waves. The devices warn drivers to slow down before they are caught speeding.

The military builds stealth aircraft and stealth missiles that can dodge enemy radar. Other aircraft and missiles are guided by radar systems.

Radar guns in baseball parks track the speed of pitches. That’s how the announcer knows the pitcher has thrown a 98-mile-per-hour fastball.

dr.janxeb Wednesday, September 09, 2009 08:00 PM


What do a dollar bill, a cardboard box, and a book have in common? They’re all made from paper, of course!

Paper is one of the world’s most important and useful products. Without it, there would be no newspapers, magazines, writing paper, or greeting cards. There would be no paper bags or boxes, paper money, gift-wrapping, or toilet paper. Take a look around you. How many things can you see that are made from paper?


Paper is made from tiny fibers from plants. You can see the fibers at the edge of a torn piece of paper.

You can make paper from many types of plant fibers. Papermakers use fibers in straw, leaves, bamboo, sugar cane, and bark. Long ago, most papermakers used the fibers in cotton and linen rags. Today, most paper is made from wood fibers.

The most important trees used for making paper are softwood trees. Softwoods include pine, fir, hemlock, and spruce. The long fibers in softwoods are ideal for making many kinds of paper.

After paper is used, it can be reused, or recycled, to make new paper.


Paper is made in two stages. The first stage is to remove the fibers from the wood. This is done by grinding the wood or cutting it into chips that are softened with chemicals. The wood fibers are then mixed with water to make a souplike substance called pulp.

The second stage is to spread out the pulp, press it flat, and dry it. This makes the fibers stick together in thin sheets. Some paper is still made by hand. But most paper is made by machines at factories called paper mills.


Different kinds of pulp make different kinds of paper. Pulp made by grinding is called groundwood pulp. It’s inexpensive to make, but the grinding breaks the wood fibers into very short pieces. Groundwood pulp is used to make cheap papers, such as newsprint.

Pulp made using chemicals is called chemical pulp. The chemicals separate the fibers from each other but do not break them. Chemical pulp is used to make stronger, longer-lasting paper for use in fine books and magazines.

The best writing paper and stationary comes from cotton and linen rag fibers. Thin rag fibers are long, strong, and make very durable paper.

Fiber from recycled paper is used to make paper towels, napkins, and tissue. Paper for printing is treated with special chemicals so the paper won’t absorb ink and cause fuzzy lines that are hard to read.


The main part of a papermaking machine is a wide belt made of tightly woven wire mesh. The belt moves in a loop, and it keeps moving all the time.

Pulp is poured evenly onto the belt at one end of the machine. As the belt moves along, water drains from the pulp. The fibers remain, leaving a mat of wet paper. The belt goes through metal rollers that squeeze out more water.

Now the paper is strong enough to be lifted off the belt. It passes between heated rollers that dry it completely. Finally, the paper is pressed tightly between cold metal rollers that make it smooth. The finished paper is wound onto large rolls or cut into standard sizes.


The ancient Chinese invented paper about 2,000 years ago. Chinese papermakers used fibers from tree bark and old rags. The art of paper-making spread out from China about 500 years later. It finally arrived in Europe about 900 years ago.

The invention of the printing press in the 1400s made books popular, and the demand for paper increased. All paper was handmade until 1798. That’s when a Frenchman named Nicholas Robert invented a papermaking machine that could make paper in continuous rolls.

Before the invention of paper, ancient people used many different surfaces for writing. They wrote on clay, wood, stone, and metals. More than 4,500 years ago, the ancient Egyptians made a paperlike material from a plant called papyrus. Papyrus reeds were cut into flat slices, layered, moistened with water, and pressed into sheets. The English word paper comes from the word papyrus.

dr.janxeb Wednesday, September 09, 2009 08:02 PM


Beavers build them from sticks. Landslides create them from trees, mud, and debris. Humans make them from earth and concrete. These structures are dams. Dams hamper the flow of water in a river or stream.

Landslides don’t mean to create dams. They do so by dumping a lot of earth and other stuff in a river. Scientists think beavers build dams for protection. Beaver dams capture water in front of the lodges in which beavers live. Beavers can hide from their enemies in this deeper water. A dam also protects the beaver lodge by slowing the river’s speed.


We build dams to control water. A dam built across a river or stream stops the water’s flow. Water collects in a lake behind the dam. The lake stores water for people to use later. The lake, or water storage area, is called a reservoir.

The water in reservoirs travels in pipes to people’s homes for drinking water. It can flow through canals for farmers to use in watering their crops. People also sail boats and swim in reservoirs.

Many dams use reservoir water to produce electricity. Water flows into large machines called turbines inside the dams. The turbines power other machines that generate electricity. Electricity produced in this way is called hydroelectric power.

Some dams are built to prevent flooding. During the rainy season, the reservoir stores the river’s extra water. During the dry season, the dam sends the reservoir water back into the river.


If you’ve ever visited a large dam, you know it is an amazing sight. Dams are some of the biggest structures ever built.

The Hoover Dam on the border of Nevada and Arizona is as tall as a 72-story building. The Grand Coulee Dam in the state of Washington contains enough concrete to build a sidewalk all the way around the Earth.

Few dams are this big, however. Most dams are small structures less than 10 feet (3 meters) tall.


Many large dams are made of concrete. Some are made of packed earth or rocks. Because these materials are not as strong as concrete, dams made of earth or rocks must be very thick. The Tarbela Dam in Pakistan is made of earth and rock. It contains more than 15 times as much material as the Grand Coulee Dam.

Dams must be strong enough to withstand the pressure of water against them. Dams also must be cared for and repaired. A dam that breaks can cause disaster. In 1889, a dam in Pennsylvania broke and let loose a wall of water. The water submerged the town of Johnstown, knocking down houses and killing more than 2,000 people.

dr.janxeb Wednesday, September 09, 2009 08:04 PM


You hear a rumble and a roar. A blast of fire shoots out of a big rocket. The rocket heads up into the air. Maybe the rocket is carrying a satellite into orbit around Earth. Maybe the rocket is carrying a space probe to another planet!

Only a big rocket can make it into outer space. No other machine is as powerful as a rocket.


A rocket looks like a long tube. Most rockets have fins on the back end to help them fly straight.

Rockets that carry fireworks can be short, only a few inches long. They are usually made of cardboard.

Rockets that go into space are huge. They are made mostly of metal.


Rockets burn fuel. Many different chemicals can be used as rocket fuel. The burning fuel makes hot gases. The gases blow out of the bottom end of the rocket. The hot gases shooting downward make the rocket go upward.

You can see how a rocket moves by blowing up a balloon. Hold the end of the balloon tightly so the air cannot get out. Then let go. The air rushes out of the opening in the balloon. The air rushing out makes the balloon fly around.

A rocket, like a balloon, has a small opening. The opening in a rocket is called a nozzle. Hot gases blasting out of the nozzle make the rocket go.


People use rockets to carry things through air and space. Different kinds of rockets carry different things.

Sounding rockets carry instruments to measure air pollution, rays from space, and weather. Lifesaving rockets carry ropes to ships stranded offshore. Distress rockets signal for help.

The most powerful rockets carry satellites and spacecraft into space. Many spacecraft use smaller rockets called thrusters to move around once they’re in space.

Rockets can also be used as weapons. The rocket weapons are called missiles. Most of the rockets made are missiles.


Missiles are rockets that carry bombs. The British used rockets carrying bombs against the United States in the War of 1812. The national anthem of the United States even has a line about the rockets: “And the rockets’ red glare ….”

Guided missiles have steering systems that guide them to destroy their targets. The smallest guided missiles can be carried by soldiers. The biggest guided missiles are huge. They can carry nuclear bombs around the world.


Missiles can be launched (fired into the air) from the ground, from airplanes, from ships, and even from submarines. Missiles can also be launched from bombproof underground tubes called silos.

Soldiers on battlefields launch small missiles out of tubes that they can carry on their shoulders. Special trucks carry ground-to-air missiles that aim at airplanes. Special racks underneath fighter planes carry air-to-air missiles.


Big rockets are launched from launch pads. A rocket stands on the pad next to a tall tower. The towers have elevators to take workers up and down. Gigantic tractors called crawler transporters bring big rockets or the space shuttle to a launch pad.

Controllers count the seconds before launch as they finish checking everything. “Five, four, three, two, one ….” Bridges that connect the tower to the rocket swing away. “Ignition!” The rocket engines fire. The spacecraft lifts off into the sky.

Firing one rocket does not always provide enough power to send a spacecraft far from Earth. The most powerful rockets often have different stages. Stages are separate rockets stacked on top of each other.


Rockets headed for space must go really fast, about 25,000 miles per hour (40,000 kilometers per hour). They must go fast enough to overcome Earth’s gravity. Gravity is the force that pulls you back down to the ground when you jump up. Using more than one rocket stage is the best way to go really fast.

The bottom stage fires, uses up its fuel, and drops off. Then the next stage fires. Each stage takes the spacecraft faster and higher. The huge Saturn V rocket that sent Apollo astronauts to the Moon had four stages.


Chinese people probably invented rockets more than 1,000 years ago. By the end of the 13th century people in Asia and Europe also knew how to make rockets.

American physicist Robert H. Goddard researched new, more powerful kinds of rockets during the early 1900s. A German inventor named Wernher von Braun helped Germany make missiles during World War II (1939-1945). After World War II, von Braun helped the Americans make rockets. The Soviet Union also made rockets. Soviet scientists launched the first satellite into space in 1957.

The United States, Russia, and other countries made bigger and more powerful rockets. Rockets have launched spacecraft to the Moon and most of the planets.

Space engineers are working on better rockets. They are testing rockets that use nuclear power. They are trying to build rockets that get their power from beams of light.

dr.janxeb Wednesday, September 09, 2009 08:05 PM

Simple Machines

You ride your bicycle to school. You run the flag up the flagpole. You use a shovel to dig. In all of these actions, you are using simple machines.

When you think of a machine, you probably imagine something that is made of metal and has moving parts, such as a car or washing machine. However, anything that changes the force, or effort, needed to do work is a machine.

There are four kinds of simple machines: levers, pulleys, wheels and axles, and inclined planes. No matter how complicated any machine is, it is made up of these four simple machines.


Simple machines help us by reducing the force necessary to move something. They reduce the force by increasing the distance over which the force is applied. If you want to open the lid of a paint can, for example, you wedge a screwdriver into the edge and pry the lid up. To lift the edge of the lid just a tiny bit, you must move the handle of the screwdriver a much greater distance.


How easy do you think it would be to lift a friend 3 feet (1 meter) off the ground? Pretty difficult? But there is a simple way. Put your friend on a seesaw.

A seesaw is a type of lever. It is a rigid bar or plank with a central point, called a fulcrum. If you push down on one end of the lever, you get a force pushing up at the other end. The closer the load is to the fulcrum, the easier it is to lift the load. So if your friend weighs more than you, he or she should sit nearer to the middle of the seesaw.

Other examples of levers are nutcrackers, scissors, shovels, and tweezers. Even your arm is a lever—its fulcrum is your elbow.


A pulley is a wheel with a groove in the edge to guide a rope or cable. A single pulley does not decrease the force needed to lift something. It only changes the direction of the force. The pulley on a flagpole, for instance, lets you raise the flag by pulling down instead of up.

Two pulleys combined can decrease the force necessary to move something. Builders use a system of pulleys to lift heavy items such as bricks to the tops of buildings.


A bicycle wheel turns around a rod at its center, called an axle. Imagine trying to spin the front wheel of a bike by twisting the axle. It would be much easier to spin the wheel by grabbing its outside edge. By applying force on a wheel, you move the load at its axle. It takes less force (over more distance) to turn a wheel than it does to turn its axle directly.

Wheels and axles are found in many everyday objects, such as doorknobs, bathroom taps, and ceiling fans.


If you wanted to raise a car 3 feet (1 meter), how would you do it? Lifting the car straight up would be extremely difficult. Driving it up a ramp would be easy. A ramp is a slanted surface called an inclined plane.

Inclined planes decrease the effort needed to lift a load by increasing the distance over which the load is moved. Instead of moving the car 3 feet straight up, the car moves several feet forward as it goes up the ramp—farther, but easier.

A wedge is a double inclined plane. When you split wood, you apply downward force. The wedge-shaped axe head changes redirects the force sideways against the wood.


All complex machines are made of combinations of simple machines. A car includes hundreds of simple machines. Even a can opener contains three. The hinged handle is a lever. The turning knob is a wheel and axle. And the sharpened cutting disk is a wedge.

dr.janxeb Wednesday, September 09, 2009 08:06 PM

Nuclear Weapons

First you see a blinding flash of light, brighter than the Sun. Moments later, a huge ball of fire appears, brilliant orange. The fireball begins to rise into the sky. Soon it widens at the top and is shaped like a mushroom. A thundering sound and blast of heat reach you 15 miles (24 kilometers) away. You are seeing the explosion of the world’s first nuclear weapon, on July 16, 1945, in a New Mexico desert.


Nuclear weapons are the most destructive weapons ever made. Building a nuclear weapon was a top-secret project during World War II. Scientists had been working on this weapon—the atomic bomb—for three years by 1945. Almost nobody else, except the president of the United States, knew about this work. The secret effort to build a nuclear weapon was called the Manhattan Project.

By 1942, when the Manhattan Project began, Germany had conquered much of Europe and was out to conquer the rest. The United States had just joined the war. The United States and its allies were afraid that Germany would develop an atomic bomb first. Then Germany would win the war. The United States and its allies had to beat Germany to the bomb.

Germany had already surrendered by the time the atomic bomb was ready. But Japan was still fighting the war. To end the war quickly, the United States dropped two atomic bombs on Japan. The bombs killed at least 100,000 people and destroyed the cities of Hiroshima and Nagasaki. Japan surrendered soon afterward. The nuclear age had begun.


The nuclear arms race was a buildup of nuclear weapons after World War II. When the war ended, scientists knew that it was possible to build nuclear bombs far more powerful and destructive than the first atomic bomb. Some people, including scientists, thought it was wrong to build these weapons of mass destruction. Others feared that the Soviet Union would make them first.

By the late 1940s, the Cold War pitted the United States and its allies against the Soviet Union and its allies. Each side feared an attack from the other side, though their armies did not actually fight during the Cold War. Everyone knew that a war using nuclear weapons would be a terrible disaster. A nuclear war would kill millions of people and possibly end life on Earth.

Each side believed that having a large supply of nuclear weapons would frighten the other side and stop it from starting a nuclear war. If one side attacked, the other side would strike back with even more nuclear bombs. And so began a race to have more nuclear weapons than the other side. Luckily, no nuclear attacks happened after World War II.


A nuclear weapon gets its name and its explosive power from the nucleus (core) of an atom. Atoms are tiny building blocks of matter much too small to see. An atomic bomb works by fissioning (splitting) the nuclei of atoms of the metals uranium or plutonium. It is sometimes called a fission weapon. A hydrogen bomb works by fusing (joining together) the nuclei of atoms of the gas hydrogen.

Atomic bombs and hydrogen bombs are the two main kinds of nuclear weapons. The hydrogen bomb is far more powerful and destructive than the atomic bomb. The hydrogen bomb is like a tiny star. It works by the same process—the fusion of hydrogen atoms—that makes the Sun and other stars shine.

A nuclear weapon destroys by the power and heat of its blast. The atomic bomb dropped on Japan flattened buildings within 3 miles (5 kilometers) of the blast. Heat from the bomb caused fires and burned everything near the place it exploded. People’s skin was burned as far as 11 miles (18 kilometers) from the blast site.

A nuclear weapon also releases harmful radiation. People near the blast can die of radiation sickness even if the bomb doesn’t kill them. People farther from the blast may develop cancer and other illnesses from radiation months and years after the bomb explodes.


No one has used a nuclear weapon in war since the United States dropped atomic bombs on Japan in 1945. For some years, countries tested their bombs underground or in remote places. However, test-ban treaties have halted the testing of nuclear weapons.

The Cold War ended in the 1990s. It left the United States and Soviet Union with huge numbers of nuclear weapons. Other countries also have built nuclear weapons. The large number of nuclear weapons has produced new fears. What if a terrorist or an unstable government gets hold of a nuclear weapon? This possibility continues to frighten people.

dr.janxeb Wednesday, September 09, 2009 08:08 PM


Every day, huge ships made of steel cross the oceans and travel the world’s great rivers and lakes. Powerful engines turn propellers that make the ships go. Ships transport people and goods to all parts of the world.

Ships are very important to the way we live. Ships carry oil that is made into gasoline for our cars. They bring in much of the food we eat and the clothes we wear. They carry computers, furniture, and televisions for our homes. Look around you. Many of the things you see traveled to where you are on a ship.


Ships may look very different from each other, but they all have the same basic parts. All ships float in water. The part that floats is called the hull. Inside the hull there are decks. Decks are like the floors in a building. You can go up and down from one deck to another.


The front of a ship is called the bow. The back is called the stern. Attached to the stern is a wooden or metal plate called the rudder. A steering wheel or a stick called a tiller makes the rudder swing back and forth. Moving the rudder makes the ship turn.

Some ships use sails to move. Sails are big sheets of fabric. The sails hang from a long pole called a mast. Ships with sails use the energy of blowing wind to move through the water.

Most modern ships have engines that burn fuel. Engines make power to turn propellers at the stern. Propellers make ships go through the water.


By about 5,000 years ago, the Egyptians were building some of the first sailing ships. They made them by tying bundles of reeds to a wooden frame. The ships carried cargo and had one or two square sails.

The best ancient shipbuilders were the Phoenicians. They made cargo ships and warships called galleys. Galleys had sails and many oars.

The ancient Greeks fought with the Phoenicians. The Greeks added a big spike to the front of their galleys. They used the spike to ram into Phoenician ships.

In China and other parts of Asia, builders made cargo ships called junks. Junks had a flat bottom, a square bow, and a rudder. The sails had pieces of bamboo in them to make them stiffer.

Arab builders began to use triangular sails called lateens. A ship with lateen sails could sail almost directly into the wind.

In the 1200s, Europeans began building ships with three masts and many square and triangular sails. These ships were called full-rigged ships, or square-riggers. Starting in the 1400s, European explorers set off on voyages in these ships to faraway parts of the world. Christopher Columbus, Vasco da Gama, and other explorers used square-rigged ships.

In the 1600s, the Spanish built huge ships called galleons. In the 1700s and 1800s, the British built big sailing ships that they used to fight sea battles.

The fastest sailing cargo ships were the clipper ships of the mid-1800s. They had sleek, narrow hulls and as many as six sails on each tall mast.


During the 1800s, iron and steel hulls replaced wooden hulls. New types of engines were also developed. For the first time, ships could move without wind or human-powered oars. Steam engines fueled by coal replaced sails.

Later, engines that used oil as a fuel replaced steam engines. Today, most ships have steel hulls and are driven by powerful motors that turn big propellers.


There are many kinds of cargo ships. Container ships carry cargo in huge boxes the size of railroad cars. Oil tankers and supertankers carry oil in their hulls. Freighters transport tons of coal, grain, and ore.


There were no passenger ships in ancient times. Travelers had to look for space on a cargo ship. Most passengers slept wherever they could find a spot on the deck. After Europeans learned about the Americas and Australia, settlers wanted to move to these new lands. Full-rigged ships carried passengers along with cargo. It was not very comfortable traveling on those wooden sailing ships.

By the mid-1800s, shipping companies began to offer regular passenger service. Companies competed with each other for passengers. They built luxurious ocean liners that could cross the Atlantic Ocean in just a few days.

In the 1950s, airplanes became more popular than ships for traveling over oceans. Today, most passenger ships are cruise ships. You can take a vacation aboard big cruise ships.


For many years, battleships were the biggest warships. They were used in World War I and World War II. Today, aircraft carriers are the biggest warships. The largest carriers can hold 85 airplanes. They have crews of more than 5,500 people.

Modern navies have many other kinds of ships. Submarines are ships that can dive underwater. Some submarines carry missiles to attack enemy ships. Cruisers escort and defend aircraft carriers from attack by planes and submarines. Destroyers defend carriers and merchant ships from air and submarine attacks. Frigates escort and defend ships from submarines.


Shipbuilders are looking for ways to build big ships that go faster and carry more cargo. They are looking for new hull shapes that go faster in the water. They are also looking for better engines. Water jet engines may replace propellers. A jet boat engine works by shooting out water, just as a jet plane engine shoots out air.

dr.janxeb Wednesday, September 09, 2009 08:09 PM


Have you ever stopped at a railroad crossing when a freight train rumbled by? Did you try to count the cars? Have you ever seen a high-speed passenger train whiz past? Trains are very important to transportation. Trains carry freight and people in places all over the world.

A train is made up of railroad cars hooked together and pulled by a locomotive. Locomotives are sometimes called engines. All trains run on tracks. Freight trains haul goods. Passenger trains carry people.


Locomotives push or pull railroad cars. They have powerful motors. The motors turn locomotive wheels that run on railroad tracks. Sometimes you will see three or four locomotives hooked together to pull a long freight train up a steep mountain.

Some locomotives get their power from electricity. The electricity comes from wires above the track or from a special third rail next to the track. Other locomotives get their power from diesel fuel, which is similar to the gasoline that most cars use. The kind of locomotive engines most used today are diesel-electrics. Engines that burn diesel fuel drive generators that make electricity. Powerful electric motors turn the wheels of a diesel-electric locomotive.


A freight train can have as many as 200 cars hooked together. There are special railroad cars for different kinds of freight.

The boxcar has four sides, a floor, and a roof. It looks like a box on wheels. Boxcars carry freight that has to be kept clean and dry, such as radios, television sets, and boxes of cereal.

Refrigerator cars work like your home refrigerator. They are boxcars that are cool inside. Refrigerator cars carry meat, fruit, frozen dinners, and other food that must be kept cold.

The hopper car is open on the top. Hopper cars carry coal, sand, gravel, and ore (rocks that contain metals). Hopper cars are easy to unload because they have doors on the bottom. The doors open and the coal, sand, or gravel pours out.

A flatcar has no top or sides. It has a floor on wheels. Flat cars carry lumber, steel beams, huge pieces of machinery, and other big items. Lifting machines called cranes load cargo onto flat cars. Special flatcars carry cars, boats, and trucks.

A tank car carries liquids or gases in a big, round tank that is lying on its side. Tank cars can carry milk, gasoline, or oil. Some tank cars carry dangerous chemicals.


Passenger cars have seats in rows along each side. Passengers can place small bags in a rack above the seat. Some passenger cars are made for long trips. They have seats that can be made into beds at night. Trains that carry passengers over long distances have special baggage cars to carry suitcases. They have dining cars where people can sit down and eat.


The track has two long rails made of steel. Pieces of wood or concrete called ties hold the rails in place and keep them from moving. Spikes hold the ties to the rails.

Locomotives, freight cars, and passenger cars have wheels that hold the train on the track. The wheels have a flange, a special shape that fits over the rails and keeps the train from slipping off the rails.

Railroad tracks are laid on a roadbed made of tightly packed dirt, gravel, or other material. When tracks have to go over rivers, the railroad company builds bridges. Sometimes railroad companies dig tunnels through mountains.


The first trains were wagons hooked together and pulled by horses, oxen, or other animals. The wagon wheels rolled over two strips made of wooden planks. Trains with wooden tracks were used as early as the 1500s to haul coal and stone. In the 1760s, iron rails replaced wooden ones.

Inventors made the first locomotives in the early 1800s. Early locomotive engines burned coal to heat water and make steam. The steam drove big pistons that turned the wheels. Inventors made bigger and better steam-engine locomotives. Steam engines drove most locomotives until the 1940s.

The first passenger cars were stagecoaches set on four railroad wheels. Then came larger cars with six wheels. In 1830, the Baltimore & Ohio became the first railroad in the United States to offer passenger service. The train was pulled by horses.

Passenger trains got better and better. In the late 1800s, a U.S. company called the Pullman Palace Car Company began making a comfortable sleeping car. Other companies made luxurious parlor cars for passengers to sit in. Train travel became very popular.


Many people traveled by train until the 1950s. Jet planes then began to replace trains as the most popular form of passenger travel. Today, most passenger trains in the United States and Canada are commuter trains. Passengers ride commuter trains twice a day between homes in the suburbs and jobs in the city. Trains continue to carry passengers between cities in Europe and in other parts of the world.

Some countries have high-speed trains. The first high-speed trains were in France and Japan. These trains can go about 260 kilometers per hour (160 miles per hour).

Engineers are working on a train that floats above its track. This type of train is called a maglev. Powerful magnets push the train a short distance above the rails as it moves along. Engineers are designing maglev trains that can travel much faster than trains on rails can.

Saqib Riaz Wednesday, September 09, 2009 09:54 PM

[B][SIZE="6"]Solar Cells [/SIZE][/B]

*You've probably seen calculators that have solar cells -- calculators that never need batteries, and in some cases don't even have an off button. As long as you have enough light, they seem to work forever. You may have seen larger solar panels -- on emergency road signs or call boxes, on buoys, even in parking lots to power lights.

Although these larger panels aren't as common as solar powered calculators, they're out there, and not that hard to spot if you know where to look. There are solar cell arrays on satellites, where they are used to power the electrical systems.

Yo*u have probably also been hearing about the "solar revolution" for the last 20 years -- the idea that one day we will all use free electricity fro*m the sun. This is a seductive promise: On a bright, sunny day, the sun shines approximately 1,000 watts of energy per square meter of the planet's surface, and if we could collect all of that energy we could easily power our homes and offices for free.

*In this article*, we will examine solar cells to learn how they convert the sun's energy directly into electricity. In the process, you will learn why we are getting closer to using the sun's energy on a daily basis, and why we still have more research to *do before the process becomes cost effective.

[B]Photovoltaic Cells: Converting Photons to Electrons[/B]
**T*he solar cells that you see on calculators and satellites are photovoltaic cells or modules (modules are simply a group of cells electrically connected and packaged in one frame). Photovoltaics, as the word implies (photo = light, voltaic = electricity), convert sunlight directly into electricity. Once used almost exclusively in space, photovoltaics are used more and more in less exotic ways. They could even power your house. How do these devices work?
*Photovoltaic (PV) cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Ba*sically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. PV cells also all have one or more electric fields that act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons i*s a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

That's the basic process, but there's really much more to it. Let's take a deeper look into one example of a PV cell: the single-crystal silicon cell.

[B]How Silicon Makes a Solar Cell[/B]
*Silicon has some special chemical properties, especially in its crystalline form. An atom of sili*con has 14 electrons, arranged in three different shells. The first two shells, those closest to the center, are completely full. The outer shell, however, is only half full, having only four electrons. A silicon atom will always look for ways to fill up its last shell (which would like to have eight electrons). To do this, it will share electrons with four of its neighbor silicon atoms. It's like every atom holds hands with its neighbors, except that in this case, each atom has four hands joined to four neighbors. That's what forms the crystalline structure, and that structure turns out to be important to this type of PV cell.

We've now described pure, crystalline silicon. Pure silicon is a poor conductor of electricity because none of its electrons are free to move about, as electrons are in good conductors such as copper. Instead, the electrons are all locked in the crystalline structure. The silicon in a solar cell is modified slightly so that it will work as a solar cell.

A solar cell has silicon with impurities -- other atoms mixed in with the silicon atoms, changing the way things work a bit. We usually think of impurities as something undesirable, but in our case, our cell wouldn't work without them. These impurities are actually put there on purpose. Consider silicon with an atom of phosphorous here and there, maybe one for every million silicon atoms. Phosphorous has five electrons in its outer shell, not four. It still bonds with its silicon neighbor atoms, but in a sense, the phosphorous has one electron that doesn't have anyone to hold hands with. It doesn't form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place.

When energy is added to pure silicon, for example in the form of heat, it can cause a few electrons to break free of their bonds and leave their atoms. A hole is left behind in each case. These electrons then wander randomly around the crystalline lattice looking for another hole to fall into. These electrons are called free carriers, and can carry electrical current. There are so few of them in pure silicon, however, that they aren't very useful. Our impure silicon with phosphorous atoms mixed in is a different story. It turns out that it takes a lot less energy to knock loose one of our "extra" phosphorous electrons because they aren't tied up in a bond -- their neighbors aren't holding them back. As a result, most of these electrons do break free, and we have a lot more free carriers than we would have in pure silicon. The process of adding impurities on purpose is called doping, and when doped with phosphorous, the resulting silicon is called N-type ("n" for negative) because of the prevalence of free electrons. N-type doped silicon is a much better conductor than pure silicon is.

Actually, only part of our solar cell is N-type. The other part is doped with boron, which has only three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free electrons, P-type silicon ("p" for positive) has free holes. Holes really are just the absence of electrons, so they carry the opposite (positive) charge. They move around just like electrons do.

The interesting part starts when you put N-type silicon together with P-type silicon. Remember that every PV cell has at least one electric field. Without an electric field, the cell wouldn't work, and this field forms when the N-type and P-type silicon are in contact. Suddenly, the free electrons in the N side, which have been looking all over for holes to fall into, see all the free holes on the P side, and there's a mad rush to fill them in.

[B]Anatomy of a Solar Cell[/B]
*B*efore now, our silicon was all electrically neutral. Our extra electrons were balanced out by the extra protons in the phosphorous. Our missing electrons (holes) were balanced out by the missing protons in the boron. When the holes and electrons mix at the junction between N-type and P-type silicon, however, that neutrality* is disrupted. Do all the free electrons fill all the free holes? No. If they did, then the whole arrangement wouldn't be very useful. Right at the junction, however, they do mix and form a barrier, making it harder and harder for electrons on the N side to cross to the P side. Eventually, equilibrium is reached, and we have an electric field separating the two sides.


This electric field acts as a diode, allowing (and even pushing) electrons to flow from the P side to the N side, but not the other way around. It's like a hill -- electrons can easily go down the hill (to the N side), but can't climb it (to the P side).

So we've got an electric field acting as a diode in which electrons can only move in one direction.

When light, in the form of photons, hits our solar cell, its energy frees electron-hole pairs.

Each photon with enough energy will normally free exactly one electron, and result in a free hole as well. If this happens close enough to the electric field, or if free electron and free hole happen to wander into its range of influence, the field will send the electron to the N side and the hole to the P side. This causes further disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the path to their original side (the P side) to unite with holes that the electric field sent there, doing work for us alo*ng the way. The electron flow provides the current, and the cell's electric field causes a voltage. With both current and voltage, we have power, which is the product of the two.

*There are a few more steps left before we can really use our cell. Silicon happens to be a very shiny material, which means that it is very reflective. Photons that are reflected can't be used by the cell. For that reason, an antireflective coating is applied to the top of the cell to reduce reflection losses to less than 5 percent.

The final step is the glass cover plate that protects the cell from the elements. PV modules are made by connecting several cells (usually 36) in series and parallel to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with a glass cover and positive and negative terminals on the back.


dr.janxeb Wednesday, September 09, 2009 11:07 PM


“You’ll just feel a little jab.” Ouch! That wasn’t too bad, and it could save your life. Most of us have had “shots” from a needle. These are usually vaccinations, and they are extremely valuable. They help protect us against diseases.


Most vaccinations are given to protect against diseases caused by viruses. Viruses are germs, and they are extremely tiny. They infect (get into) your body, multiply, and make you feel sick. Chicken pox is one example of a disease caused by a virus. The chicken pox virus gets inside the body and multiplies. It causes a fever followed by a rash of itchy red spots.


A vaccine is usually a small amount of liquid that contains dead or weakened versions of a virus or other type of germ. Weakened viruses can still multiply within the body but cannot cause disease. Vaccines can also contain tiny amounts of harmful substances, called toxins, which are made by the viruses. But they don’t have enough toxin to make you sick.

Some vaccines are oral, which means you can eat them or drink them. However, the powerful digestive juices in your stomach would destroy most vaccines. So vaccines are usually given by needle.


Your body attacks and destroys the weakened virus or toxin in the vaccine before it can make you sick. In this way, you become immune to (protected from) the disease the virus causes. The vaccine enables the body’s defenses, or immune system, to recognize and destroy the virus.

To destroy the virus, your immune system produces special substances, called antibodies, in the blood. Antibodies are able to fight and destroy particular viruses. If the real virus later invades, the immune system can kill it very quickly, before it starts to multiply.


Vaccination is carried out in many countries as a regular part of healthcare. Vaccines are usually given to babies and young children so that they are protected from diseases as soon as possible. Vaccines exist for many diseases, including chicken pox, measles, mumps, rubella, polio, diphtheria, tetanus, and whooping cough.

Some of these vaccines are given to almost everyone. Others are given only to people who are considered likely to get the disease, perhaps because of where they live or their age. Several vaccines may be given at the same time as a combined vaccine. For example, the MMR vaccine protects against measles, mumps, and rubella.

Some vaccines only make you immune for a few months or years. They include vaccines against typhoid fever, cholera, tetanus, and yellow fever. They may need a “booster” dose later to keep up the immunity.


Sometimes giving a vaccine to a person may cause health problems. These problems are known as side effects, and they can be serious. People who have certain illnesses and conditions are more likely to have side effects. So medical workers ask questions about health before giving vaccines. Once in a while, they advise people not to get vaccinated. Medical workers must balance the risks of catching the disease with the risks of possible side effects of vaccination.


Viruses can change, or mutate, over time. A vaccine against one strain of a mutated virus may not work against another strain. The flu (influenza) is one of these viruses that mutate into different strains. A new vaccine for the flu has to be developed every year.

Every now and then a new kind of germ appears. One example is HIV (human immunodeficiency virus), which causes a disease called AIDS (acquired immune deficiency syndrome). No one knew about HIV until the 1980s. When new viruses appear, medical scientists try to develop new vaccines against them. It is a long and difficult process. But vaccination is one of our most powerful medical weapons in the battle against diseases.

03:42 PM (GMT +5)

vBulletin, Copyright ©2000 - 2020, Jelsoft Enterprises Ltd.