Материал: Андрианова English Thesaurus of Student-Physicist 2013

P A R T IV

Electricity and Magnetism

UNIT 15

ELECTRIC FORCES AND ELECTRIC FIELDS

Read the following texts. Study the thesaurus on fig. 15, answer the questions after the texts

Electricity is the lifeblood of technological civilization and modern society. Without it, we revert to the mid-nineteenth century: no telephones, no television, none of the household appliances that we take for granted. Modern medicine would be a fantasy, and due to the lack of sophisticated experimental equipment and fast computers - and especially the slow dissemination of information - science and technology would grow at a glacial pace.

Instead, with the discovery and harnessing of electric forces and fields, we can view arrangements of atoms, probe the inner workings of the cell, and send spacecraft beyond the limits of the solar system. All this has become possible in just the last few generations of human life, a blink of the eye compared to the million years our kind spent foraging the savannahs of Africa.

Around 700 B.C. the ancient Greeks conducted the earliest known study of electricity. It all began when someone noticed that a fossil material called amber would attract small objects after being rubbed with wool. Since then we have learned that this phenomenon is not restricted to amber and wool, but occurs (to some degree) when almost any two nonconducting substances are rubbed together.

We use the effect of charging by friction to begin an investigation of electric forces. Coulomb's law is the fundamental law of force between any two stationary charged particles. The concept of an electric field associated with charges is introduced and its effects on other charged particles described.

Properties of electric charges

After running a plastic comb through your hair, you will find that the comb attracts bits of paper. The attractive force is often strong enough to suspend the paper from the comb, defying the gravitational

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pull of the entire Earth. The same effect occurs with other rubbed materials, such as glass and hard rubber.

Another simple experiment is to rub an inflated balloon against wool (or across your hair). On a dry day, the rubbed balloon will then stick to the wall of a room often for hours. These materials have become electrically charged. Experiments also demonstrate that there are two kinds of electric charge, which Benjamin Franklin (1706-1790) named positive and negative.

Objects usually contain equal amounts of positive and negative charge - electrical forces between objects arise when those objects have net negative or positive charges. Charge transfers readily from one type of material to another. Rubbing the two materials together serves to increase the area of contact, facilitating the transfer process.

An important characteristic of charge is that electric charge is always conserved. Charge isn't created when two neutral objects are rubbed together; rather, the objects become charged because negative charge is transferred from one object to the other. One object gains a negative charge while the other loses an equal amount of negative charge and hence is left with a net positive charge.

Insulators and conductors

Substances can be classified in terms of their ability to conduct electric charge. In conductors, electric charges move freely in response to an electric force. All other materials are called insulators.

Glass and rubber are insulators. When such materials are charged by rubbing, only the rubbed area becomes charged, and there is no tendency for the charge to move into other regions of the material. In contrast, materials such as copper, aluminum, and silver are good conductors. When such materials are charged in some small region, the charge readily distributes itself over the entire surface of the material. If you hold a copper rod in your hand and rub the rod with wool or fur, it will not attract a piece of paper. This might suggest that a metal can't be charged. However, if you hold the copper rod with an insulator and then rub it with wool or fur, the rod remains charged and attracts the paper. In the first case, the electric charges produced by rubbing readily move from the copper through your body and finally to ground. In the second case, the insulating handle prevents the flow of charge to ground.

Semiconductors are a third class of materials, and their electrical properties are somewhere between those of insulators and those of conductors. Silicon and germanium are well-known semiconductors that are widely used in the fabrication of a variety of electronic devices.

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Coulomb's law

In 1785 Charles Coulomb (1736-1806) experimentally established the fundamental law of electric force between two stationary charged particles. An electric force has the following properties:

1.It is directed along a line joining the two particles and is inversely proportional to the square of the separation distance between them.

2.It is proportional to the product of the magnitudes of the charges of the two particles.

3.It is attractive if the charges are of opposite sign and repulsive if the charges have the same sign.

The electric field

The gravitational force and the electrostatic force are both capable of acting through space, producing an effect even when there isn't any physical contact between the objects involved. Field forces can be discussed in a variety of ways, but an approach developed by Michael Faraday (1791-1867) is the most practical. In this approach, an electric field is said to exist in the region of space around a charged object. The electric field exerts an electric force on any other charged object within the field. This differs from the Coulomb's law concept of a force exerted at a distance, in that the force is now exerted by something - the field - that is in the same location as the charged object.

Conductors in electrostatic equilibrium

A good electric conductor like copper, though electrically neutral, contains charges (electrons) that aren't bound to any atom and are free to move about within the material. When no net motion of charge occurs within a conductor, the conductor is said to be in electrostatic equilibrium.

In 1929 Robert J.Van de Graaff (1901-1967) designed and built an electrostatic generator that has been used extensively in nuclear physics research. The principles of its operation can be understood with knowledge of the properties of electric fields and charges already presented in this Unit.

Electric flux and Gauss's law

Gauss's law is essentially a technique for calculating the average electric field on a closed surface, developed by Karl Friedrich Gauss (17771855). When the electric field, because of its symmetry, is constant everywhere on that surface and perpendicular to it, the exact electric field can be

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found. In such special cases, Gauss's law is far easier to apply than Coulomb's law.

Gauss's law relates the electric flux through a closed surface and the total charge inside that surface. A closed surface has an inside and an outside: an example is a sphere. Electric flux is a measure of how much the electric field vectors penetrate through a given surface. If the electric field vectors are tangent to the surface at all points, for example, then they don't penetrate the surface and the electric flux through the surface is zero.

1.Why electricity is the lifeblood of technological civilization and modern society?

2.What law is the fundamental law of force between any two stationary charged particles?

3.What properties of electric charges do you know?

4.What’s the difference between insulators and conductors?

5.What does the Coulomb’s law state?

6.For what purpose do we use Gauss’s law?

Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

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