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Old Tuesday, May 09, 2006
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Default Composition and Types of Plastic

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plastic, any organic material with the ability to flow into a desired shape when heat and pressure are applied to it and to retain the shape when they are withdrawn.

Composition and Types of Plastic

A plastic is made up principally of a binder together with plasticizers, fillers, pigments, and other additives. The binder gives a plastic its main characteristics and usually its name. Thus, polyvinyl chloride is both the name of a binder and the name of a plastic into which it is made. Binders may be natural materials, e.g., cellulose derivatives, casein, or milk protein, but are more commonly synthetic resins. In either case, the binder materials consist of very long chainlike molecules called polymers. Cellulose derivatives are made from cellulose, a naturally occurring polymer; casein is also a naturally occurring polymer. Synthetic resins are polymerized, or built up, from small simple molecules called monomers. Plasticizers are added to a binder to increase flexibility and toughness. Fillers are added to improve particular properties, e.g., hardness or resistance to shock. Pigments are used to impart various colors. Virtually any desired color or shape and many combinations of the properties of hardness, durability, elasticity, and resistance to heat, cold, and acid can be obtained in a plastic.There are two basic types of plastic: thermosetting, which cannot be resoftened after being subjected to heat and pressure; and thermoplastic, which can be repeatedly softened and remolded by heat and pressure. When heat and pressure are applied to a thermoplastic binder, the chainlike polymers slide past each other, giving the material “plasticity.” However, when heat and pressure are initially applied to a thermosetting binder, the molecular chains become cross-linked, thus preventing any slippage if heat and pressure are reapplied.

Molding of Plastic

Plastics are available in the form of bars, tubes, sheets, coils, and blocks, and these can be fabricated to specification. However, plastic articles are commonly manufactured from plastic powders in which desired shapes are fashioned by compression, transfer, injection, or extrusion molding. In compression molding, materials are generally placed immediately in mold cavities, where the application of heat and pressure makes them first plastic, then hard. The transfer method, in which the compound is plasticized by outside heating and then poured into a mold to harden, is used for designs with intricate shapes and great variations in wall thickness. Injection-molding machinery dissolves the plastic powder in a heating chamber and by plunger action forces it into cold molds, where the product sets. The operations take place at rigidly controlled temperatures and intervals. Extrusion molding employs a heating cylinder, pressure, and an extrusion die through which the molten plastic is sent and from which it exits in continuous form to be cut in lengths or coiled.

Development of Plastics

The first important plastic, celluloid, was discovered (c.1869) by the American inventor John W. Hyatt and manufactured by him in 1872; it is a mixture of cellulose nitrate, camphor, and alcohol and is thermoplastic. However, plastics did not come into modern industrial use until after the production (1909) of Bakelite by the American chemist L. H. Baekeland. Bakelite, made by the polymerization of phenol and formaldehyde, is thermosetting. New uses for plastics are continually being discovered. Following World War II optical lenses, artificial eyes, and dentures of acrylic plastics, splints that X rays may pierce, nylon fibers, machine gears, fabric coatings, wall surfacing, and plastic lamination were developed. More recently a hydrophilic, or water-attracting, plastic suitable for use in non-irritating contact lenses has been developed. Among the trade names by which many plastic products are widely known are Plexiglas, Lucite, Polaroid, Cellophane, Vinylite, and Koroseal. Plastics reinforced with fiberglass are used for boats, automobile bodies, furniture, and building panels.Another important plastic, "polyethylene" (PE), sometimes known as "polythene", was discovered in 1933 by Reginald Gibson and Eric Fawcett at the British industrial giant Imperial Chemical Industries (ICI). This material evolved into two forms, "low density polyethylene" (LDPE), and "high density polyethylene" (HDPE).

H H H H H
\ / | | |
C == C -> -- C -- C -- C --
/ \ | | |
H H H H H

ethylene monomer polyethylene polymer

PEs are cheap, flexible, durable, and chemically resistant. LDPE is used to make films and packaging materials, while HDPE is used for containers, plumbing, and automotive fittings. While PE has low resistance to chemical attack, it was found later that a PE container could be made much more robust by exposing it to fluorine gas, which modified the surface layer of the container into the much tougher "polyfluoroethylene".

Common plastics and their typical uses

polyethylene (PE)
wide range of uses, very inexpensive

polypropylene (PP)
food containers, appliances

polystyrene (PS)
packaging foam, food containers, disposable cups, plates and cutlery

polyethylene terephthalate (PETE)
beverage containers

Polyamide (PA) (Nylon)
fibers, toothbrush bristles, fishing line

polyester
fibres, textiles

polyvinyl chloride (PVC)
plumbing pipes, flooring, erotic clothing

polycarbonate (PC)
compact discs, eyeglasses acrylonitrile butadiene styrene (ABS)
electronic equipment cases (e.g., computer monitors, printers, keyboards)

polyvinylidene chloride (PVDC) (Saran) food packaging

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Old Wednesday, May 10, 2006
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Default computer

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A computer is a device capable of processing data according to a program — a list of instructions. The data to be processed may represent many types of information including numbers, text, pictures, or sound.
A general-purpose machine that processes data according to a set of instructions that are stored internally either temporarily or permanently. The computer and all equipment attached to it are called hardware. The instructions that tell it what to do are called "software." A set of instructions that perform a particular task is called a "program" or "software program."
digital computer is designed to process data in numerical form (see digital circuit); its circuits perform directly the mathematical operations of addition, subtraction, multiplication, and division. The numbers operated on by a digital computer are expressed in the binary system; binary digits, or bits, are 0 and 1, so that 0, 1, 10, 11, 100, 101, etc., correspond to 0, 1, 2, 3, 4, 5, etc. Binary digits are easily expressed in the computer circuitry by the presence (1) or absence (0) of a current or voltage. A series of eight consecutive bits is called a “byte”; the eight-bit byte permits 256 different “on-off” combinations. Each byte can thus represent one of up to 256 alphanumeric characters, and such an arrangement is called a “single-byte character set” (SBCS); the de facto standard for this representation is the extended ASCII character set. Some languages, such as Japanese, Chinese, and Korean, require more than 256 unique symbols. The use of two bytes, or 16 bits, for each symbol, however, permits the representation of up to 65,536 characters or ideographs. Such an arrangement is called a “double-byte character set” (DBCS); Unicode is the international standard for such a character set. One or more bytes, depending on the computer's architecture, is sometimes called a digital word; it may specify not only the magnitude of the number in question, but also its sign (positive or negative), and may also contain redundant bits that allow automatic detection, and in some cases correction, of certain errors (see code; information theory). A digital computer can store the results of its calculations for later use, can compare results with other data, and on the basis of such comparisons can change the series of operations it performs. Digital computers are used for reservations systems, scientific investigation, data-processing and word-processing applications, desktop publishing, electronic games, and many other purposes.

Processing of Data

The operations of a digital computer are carried out by logic circuits, which are digital circuits whose single output is determined by the conditions of the inputs, usually two or more. The various circuits processing data in the computer's interior must operate in a highly synchronized manner; this is accomplished by controlling them with a very stable oscillator, which acts as the computer's “clock.” Typical computer clock rates range from several million cycles per second to several hundred million, with some of the fastest computers having clock rates of about a billion cycles per second. Operating at these speeds, digital computer circuits are capable of performing thousands to trillions of arithmetic or logic operations per second, thus permitting the rapid solution of problems that would be impossible for a human to solve by hand. In addition to the arithmetic and logic circuitry and a small number of registers (storage locations that can be accessed faster than main storage and are used to hold the intermediate results of calculations), the heart of the computer—called the central processing unit, or CPU—contains the circuitry that decodes the set of instructions, or program, and causes it to be executed.

Storage and Retrieval of Data

Associated with the central processing unit is the storage unit, or memory, where results or other data are stored for periods of time ranging from a small fraction of a second to days or weeks before being retrieved for further processing. Once made up of vacuum tubes and later of small doughnut-shaped ferromagnetic cores strung on a wire matrix, main storage now consists of integrated circuits, each of which contains thousands of semiconductor devices. Where each vacuum tube or core represented one bit and the total memory of the computer was measured in thousands of bytes (or kilobytes, KB), each semiconductor device now represents millions of bytes (or megabytes, MB) and the total memory of mainframe computers is measured in billions of bytes (or gigabytes, GB). Random-access memory (RAM), which both can be read from and written to, is lost each time the computer is turned off. Read-only memory (ROM), which cannot be written to, maintains its content at all times and is used to store the computer's control information.
Programs and data that are not currently being used in main storage can be saved on auxiliary storage, or external storage. Although punched paper tape and punched cards once served this purpose, the major materials used today are magnetic tape and magnetic disks, which can be read from and written to, and two types of optical disks, the compact disc (CD) and its successor the digital versatile disc (DVD). DVD is an improved optical storage technology capable of storing vastly greater amounts of data than the CD technology. CD–Read-Only Memory (CD-ROM) and DVD–Read-Only Memory (DVD-ROM) disks can only be read—the disks are impressed with data at the factory but once written cannot be erased and rewritten with new data. The latter part of the 1990s saw the introduction of new optical storage technologies: CD-Recordable (CD-R) and DVD-Recordable (DVD-R), optical disks that can be written to by the computer to create a CD-ROM or DVD-ROM, but can be written to only once; and CD-ReWritable (CD-RW), DVD-ReWritable (DVD-RW and DVD+RW), and DVD–Random Access Memory (DVD-RAM), disks that can be written to multiple times.
When compared to semiconductor memory, magnetic and optical storage is less expensive, is not volatile (i.e., data is not lost when the power to the computer is shut off), and provides a convenient way to transfer data from one computer to another. Thus operating instructions or data output from one computer can be stored away from the computer and then retrieved either by the same computer or another. In a system using magnetic tape the information is stored by a specially designed tape recorder somewhat similar to one used for recording sound. In magnetic and optical disk systems the principle is the same except that the magnetic or optical medium lies in a path, or track, on the surface of a disk. The disk drive also contains a motor to spin the disk and a magnetic or optical head or heads to read and write the data to the disk. Drives take several forms, the most significant difference being whether the disk can be removed from the drive assembly.
Removable magnetic disks are most commonly made of mylar enclosed in a paper or plastic holder. These floppy disks have varying capacities, with very high density disks holding 250 MB—more than enough to contain a dozen books the size of Tolstoy's Anna Karenina. Compact discs can hold many hundreds of megabytes, and are used, for example, to store the information contained in an entire multivolume encyclopedia or set of reference works, and DVD disks can hold ten times as much as that. Nonremovable disks are made of metal and arranged in spaced layers. They can hold more data and can read and write data much faster than floppies.
Data are entered into the computer and the processed data made available via input/output devices. All auxiliary storage devices are used as input/output devices. For many years, the most popular input/output medium was the punched card. Although this is still used, the most popular input device is now the computer terminal and the most popular output device is the high-speed printer. Human beings can directly communicate with the computer through computer terminals, entering instructions and data by means of keyboards much like the ones on typewriters, by using a pointing device such as a mouse, trackball, or touchpad, or by speaking into a microphone that is connected to computer running voice-recognition software. Responses may be displayed on a cathode-ray tube, liquid-crystal display, or printer. The CPU, main storage, auxiliary storage, and input/output devices collectively make up a system.

Sharing the Computer's Resources

Generally, the slowest operations that a computer must perform are those of transferring data, particularly when data is received from or delivered to a human being. The computer's central processor is idle for much of this period, and so two similar techniques are used to use its power more fully.
Time sharing, used on large computers, allows several users at different terminals to use a single computer at the same time. The computer performs part of a task for one user, then suspends that task to do part of another for another user, and so on. Each user only has the computer's use for a fraction of the time, but the task switching is so rapid that most users are not aware of it. Most of the tens of millions of computers in the world are stand-alone, single-user devices known variously as personal computers or workstations. For them, multitasking involves the same type of switching, but for a single user. This permits a user, for example, to have one file printed and another sorted while editing a third in a word-processing session. Such personal computers can also be linked together in a network, where each computer is connected to others, usually by wires or coaxial cables, permitting all to share resources such as printers, modems, and hard-disk storage devices.

Computer Programs and Programming Languages

Before a computer can be used to solve a given problem, it must first be programmed, that is, prepared for solving the problem by being given a set of instructions, or program. The various programs by which a computer controls aspects of its operations, such as those for translating data from one form to another, are known as software, as contrasted with hardware, which is the physical equipment comprising the installation. In most computers the moment-to-moment control of the machine resides in a special software program called an operating system, or supervisor. Other forms of software include assemblers and compilers for programming languages and applications for business and home use (see computer program). Software is of great importance; the usefulness of a highly sophisticated array of hardware can be severely compromised by the lack of adequate software.
Each instruction in the program may be a simple, single step, telling the computer to perform some arithmetic operation, to read the data from some given location in the memory, to compare two numbers, or to take some other action. The program is entered into the computer's memory exactly as if it were data, and on activation, the machine is directed to treat this material in the memory as instructions. Other data may then be read in and the computer can carry out the program to solve the particular problem.
Since computers are designed to operate with binary numbers, all data and instructions must be represented in this form; the machine language, in which the computer operates internally, consists of the various binary codes that define instructions together with the formats in which the instructions are written. Since it is time-consuming and tedious for a programmer to work in actual machine language, a programming language, or high-level language, designed for the programmer's convenience, is used for the writing of most programs. The computer is programmed to translate this high-level language into machine language and then solve the original problem for which the program was written. Certain high-level programming languages are universal, varying little from machine to machine.

Development of Computers

Although the development of digital computers is rooted in the abacus and early mechanical calculating devices, Charles Babbage is credited with the design of the first modern computer, the “analytical engine,” during the 1830s. American scientist Vannevar Bush built a mechanically operated device, called a differential analyzer, in 1930; it was the first general-purpose analog computer. John Atanassoff constructed the first semielectronic digital computing device in 1939.
The first fully automatic calculator was the Mark I, or Automatic Sequence Controlled Calculator, begun in 1939 at Harvard by Howard Aiken, while the first all-purpose electronic digital computer, ENIAC (Electronic Numerical Integrator And Calculator), which used thousands of vacuum tubes, was completed in 1946 at the Univ. of Pennsylvania. UNIVAC (UNIVersal Automatic Computer) became (1951) the first computer to handle both numeric and alphabetic data with equal facility; this was the first commercially available computer.
First-generation computers were supplanted by the transistorized computers (see transistor) of the late 1950s and early 60s, second-generation machines that were smaller, used less power, and could perform a million operations per second. They, in turn, were replaced by the third-generation integrated-circuit machines of the mid-1960s and 1970s that were even smaller and were far more reliable. The 1980s and 90s were characterized by the development of the microprocessor and the evolution of increasingly smaller but powerful computers, such as the personal computer and personal digital assistant, which ushered in a period of rapid growth in the computer industry.

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Old Friday, May 12, 2006
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Question Questions and answers on science

Q1: Who discovered the electron?

Q2: Which is the longest bone in the human body?

Q3: At which temperature do both the Centigrade and Fahrenheit show the same reading?

Q4: Who invented the bifocal lens?

Q5: Why we do not feel pains to cut our nails?

Q6: Who invented the jet engine?

Q7: What is Mach Number?

Q8: What is the scientific name for study of handwriting?

Q9: What causes an object to look black?

Q10: Who discovered insulin?

Q11: Which element has the highest boiling point?

Q12: Why does a tennis ball bounce higher on hills than on plains?

Q13: Which planet has the biggest satellite?

Q14: How do bats manage to avoid obstacles while flying?

Q15: Which is the most common of all the metals in the earth's crust?

Q16: What is the source of caviar?

Q17: Why does an iron ball sink and not an iron ship?

Q18: What is herpetology?

Q19: What is constellation?

Q20: What is an aardvark?

Q21: From which plant is morphine obtained?

Q22: Who discovered the cause of malaria?
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Old Friday, May 12, 2006
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Lightbulb Answers

A1: Sir J J Thomson.

A2: The femur or the thigh bone.

A3: -40C equals -40F.

A4: Benjamin Franklin.

A5: Nails are not connected with cartilages or blood vessels. While cutting, therefore, the nervous system is not affected.

A6: Sir Frank Whittle of England in 1937.

A7: Ratio of speed of the body to the speed of sound under similar conditions.

A8: Graphology.

A9: When radiation falling on an object is fully absorbed, it appears black.

A10: Sir Frederick Grant Banting & J J R Macleod.

A11: Tantalum, about 6000C.

A12: The gravitational pull in the hills is less than that at plains.

A13: Saturn's seventh, known as Titan (diameter 5680 km).

A14: Bats while flying, emit a set of short squeaks which are ultrasonic waves. These, after reflection from trees and buildings reach the ear of the bats and make them judge, by hearing the echo of the ultrasonic sound, the distance and direction of the obstacle in the air.

A15: Aluminum, 8.13% of the earth's crust by weight.

A16: It is made by salting the roe of sturgeon.

A17: The weight of the iron ball is more than the water it displaces, in case of the ship, it is opposite.

A18: Study of reptiles.

A19: A cluster of fixed stars in the heaven like Great Bear, Orion.

A20: An ant-eating animal.

A21: Opium poppy.

A22: Ronald Ross
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Old Monday, May 15, 2006
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ghyas bhai has copy-pasted the answers from the following link:

http://www.101kidz.com/book/science1.html

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Old Monday, May 15, 2006
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questions bhi wahin se hein ............
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