Реферат: Electricity Essay Research Paper WHAT IS ELECTRICITYElectricity
The Capacitor
Another device capable of electrical work is the capacitor, a descendant of the Leyden jar, which is used to store charge. If a charge Q is placed on the metal plates the voltage rises to amount V. The measure of a capacitor’s ability to store charge is the capacitance C, where C = Q/V. Charge flows from a capacitor just as it flows from a battery, but with one significant difference. When the charge leaves a capacitor’s plates, no more can be obtained without recharging. This happens because the electrical force is conservative. The energy released cannot exceed the energy stored. This ability to do work is called electric potential.
A type of conservation of energy is also associated with emf. The electrical energy obtainable from a battery is limited by the energy stored in chemical molecular bonds. Both emf and electric potential are measured in volts, and, unfortunately, the terms voltage, potential, and emf are used rather loosely. For example, the term battery potential is often used instead of emf.
Voltage
Whether as an emf or an electric potential, voltage is a measure of the ability of a system to do work on a unit amount of charge by electrical means. Voltage is a better-known quantity than electric field. For instance, voltages measured in an electrocardiogram peak at 5 millivolts; many are familiar with the 115-volt potential of a house. The potential between a cloud and the ground just before a typical lightning bolt is a minimum of 10,000 volts.
Sometimes high voltages are needed. For instance, the electron beams in television tubes require more than 30,000 volts. Electrons “falling” through such a potential reach velocities as high as one-third the speed of light and have sufficient energy to cause a spot of light on the screen. Such high potentials may be developed from lower alternating potentials by using a transformer.
By scuffing shoes on a carpet on a dry day, an electric potential of more than 20,000 volts can be developed, resulting in a spark.
Electric Current
An electric charge in motion is called electric current. The strength of a current is the amount of charge passing a given point (as in a wire) per second, or I = Q/t, where Q coulombs of charge pass in t seconds. The unit for measuring current is the ampere or amp, which equals 1 coulomb/sec.
Because it is the source of magnetism as well, current is the link between electricity and magnetism. In 1819 the Danish physicist Hans Christian Oersted found that a compass needle was affected by a current-carrying wire. Almost immediately, Andre Ampere in France discovered the magnetic force law. Michael Faraday in England and Joseph Henry in the United States added the idea of magnetic induction, whereby a changing magnetic field produces an electric field. The stage was then set for the encompassing electromagnetic theory of James Clerk Maxwell.
The variation of actual currents is enormous. A modern electrometer can detect currents as low as 1/100,000,000,000,000,000 amp, which is a mere 63 electrons per second. The current in a nerve impulse is approximately 1/100,000 amp; a 100-watt light bulb carries 1 amp; a lightning bolt peaks at about 20,000 amps; and a 1,200-megawatt nuclear power plant can deliver 10,000,000 amps at 115 V.
Most materials are insulators. In them, all electrons are bound in individual atoms and do not permit a flow of charge unless the electric field acting on the material is so high that breakdown occurs. Then, in a process called ionisation, the most loosely bound electrons are torn from the atoms, allowing current flow. This condition exists during a lightning storm. The separation of charge between the clouds and the ground creates a large electric field that ionises the air atoms, thereby forming a conducting path from cloud to ground.
Resistance
Although a conductor permits the flow of charge, it is not without a cost in energy. The electrons are accelerated by the electric field. Before they move far, however, they collide with one of the atoms of the conductor, slowing them down or even reversing their direction. As a result, they lose energy to the atoms. This energy appears as heat, and the scattering is a resistance to the current.
In 1827 a German teacher named George Ohm demonstrated that the current in a wire increases in direct proportion to the voltage V and the cross-sectional areas of the wire A, and in inverse proportion to the length I. Because the current also depends on the particular material, Ohm’s law is written in two steps, I = V/R, and R = pI/A X the resistivity. The quantity R is called the resistance. The resistivity depends only on the type of material. The unit of resistance is the ohm, where 1 ohm is equal to 1 volt/amp.
Certain materials, such as lead, lose their resistance almost entirely when cooled to within a few degrees of absolute zero. Such materials are called superconductors. Substances have recently been found that become super conductive at much higher temperatures.
The resistive heating caused by electron scattering is a significant effect and is used in electric stoves and heaters as well as in incandescent light bulbs. In a resistor the power P, or energy per second, is given by P = (I squared) R.
Speed of Electricity
As electrons bounce along through the wire, the general charge drift constitutes the current. The average, or drift, speed is defined as the speed the electrons would have if all were moving with constant velocity parallel to the field. The drift speed is actually small even in good conductors. In a 1.0-mm-diameter copper wire carrying a current of 10 amps at room temperature, the drift speed of the electrons is 0.2 mm per second. In copper, the electrons rarely drift faster than one hundred-billionth the speed of light.
On the other hand, the speed of the electric signal is the speed of light. This means that, at the speed of light, the removal of one electron from one end of a long wire would affect electrons elsewhere. For example, consider a long, motionless freight train, with the cars representing electrons in a wire. Because the couplings between cars have play in them, the caboose is affected a short while after the engine begins moving.
During this time the engine moves forward a short distance. The signal telling the caboose to start moves backward quickly, travelling the length of the train in the same time it takes the engine to go forward a meter or so. Similarly, the electron drift speed in a conductor is low, but the signal moves at the speed of light in the opposite direction.
Electrical Theory of Matter
The possibility that electricity does not consist of a smooth, continuous fluid probably occurred to many scientists. Even Franklin once wrote that the “fluid” consists of “particles extremely sub tile.”
Nevertheless, a great deal of evidence had to be accumulated before the view was accepted that electricity comes in tiny, discrete amounts, looking not at all like a fluid when viewed microscopically. James Clerk Maxwell opposed this particle theory. Toward the end of the 1800s, however, the work of Sir Joseph John Thomson (1856-1940) and others proved the existence of the electron.
The Electron
Thomson had measured the ratio of the electron’s charge to its mass. Then in 1899 he inferred a value for the electronic charge itself by observing the behavior of a cloud of tiny charged water droplets in an electric field. This observation led to Millikan’s Oil-Drop Experiment.
Robert Millikan, a physicist at the University of Chicago, with the assistance of his student Harvey Fletcher, sought to measure the charge of a single electron, an ambitious goal in 1906. A tiny droplet of oil with an excess of a few electrons was formed by forcing the liquid through a device similar to a perfume atomizer. The drop was then, in effect, suspended, with an electric field attracting it up and the force of gravity pulling it down. By determining the mass of the oil drop and the value of the electric field, the charge on the drop was calculated. The result: the electron charge e is negative and has the value e = 1.60/10,000,000,000,000,000,000 coulombs. This charge is so small that a single copper penny contains more than 10,000,000,000,000,000,000,000 electrons.
Robert Millikan (1868-1953) won the 1923 Nobel Prize in physics for his work on the elementary electric charge and on the photoelectric effect. He also did much work on cosmic rays, which he named. He is seen here (right) in his basement with his assistant and his self-recording electroscope. Under Millikan’s leadership the California Institute of Technology quickly developed into one of the foremost scientific centers in the world. (The Bettmann Archive)
Millikan also found that a charge always appears to be in exact integer multiples of plus or minus e; in other words, the charge is quantized. Other elementary particles discovered later were also found to have a charge of plus or minus e. For example, the positron, discovered in 1932 by Carl David Anderson of the California Institute of Technology, is exactly the same as the electron, except that it has a charge of +e.