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Old Wednesday, September 09, 2009
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Default Nuclear Weapons

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.

WHY DO WE HAVE NUCLEAR WEAPONS?

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.

WHY WAS THERE A NUCLEAR ARMS RACE?

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.

HOW DO NUCLEAR WEAPONS WORK?

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.

THE FUTURE OF NUCLEAR WEAPONS

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.
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Default Ships

Ships

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.

THE PARTS OF 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.

HOW SHIPS MOVE THROUGH WATER

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.

THE AGE OF SAILING SHIPS

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.

MODERN SHIPS

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.

CARGO SHIPS

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.

PASSENGER SHIPS

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.

NAVY 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.

THE NEWEST SHIPS

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.
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Default Train

TRAIN

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.

WHAT MOVES A TRAIN?

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.

KINDS OF FREIGHT CARS

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.

KINDS OF PASSENGER CARS

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.

HOW DO TRAINS STAY ON THE TRACKS?

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.

WHAT WERE EARLY TRAINS LIKE?

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.

HOW HAS TRAIN TRAVEL CHANGED?

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.
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  #24  
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Default

Solar Cells


Introduction
*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.

Photovoltaic Cells: Converting Photons to Electrons
**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.

How Silicon Makes a Solar Cell
*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.

Anatomy of a Solar Cell
*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.

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Default Vaccines

VACCINES

“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.

VIRUSES

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.

WHAT ARE VACCINES?

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.

BECOMING IMMUNE

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.

WHICH DISEASES?

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.

PROBLEMS WITH VACCINES

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.

CHANGING VIRUSES

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.
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