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ТАГАНРОГ учебное пособие (2 курс).doc
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What is an electron?

What is an electron? We can think of the electron as a very small, indivisible, fundamental particle—a major constituent of all 'matter. All electrons appear to be iden­tical and to have properties that do not change with time. Two essential characteristics of the electron are its mass and its charge. Qualitatively, we can think of an electron as a "piece of matter" that has weight and is affected by gravity. Just as the mass of any object is defined, we can define the mass of the electron by applying a force and measuring the resulting rate of change in the velocity of the electron, that is, the rapidity with which its velocity changes. This rate of change is called acceleration, and the electron mass is then defined as the ratio of the applied force to the resulting acceleration. The mass of the electron is found to be about 9.11 X 10-28 grams.1 Not only the elec­tron but all matter appears to have positive mass, which is equivalent to saying that a force applied to any object re­sults in an acceleration 2 in the same direction as the force.

How does the other aspect, the charge of the electron, arise? If we investigate further, we find that all electrons have an electric charge, and the amount of charge, like the mass, is identical for all electrons. No one has ever succeed­ed in isolating an amount of charge smaller than that of the electron. The sign of the charge of the electron fs con­ventionally defined as negative; the electron thus represents the fundamental unit of a negative charge.

No experiment has yet succeeded in removing the charge from the electron, leaving only its mass. Therefore, instead of considering the electron a "massive" body that has some-how acquired a charge, it seems more realistic to think that the charge and the mass are two inseparable aspects of a single unity.

The motion of an electron, like that of any other body, results from a force acting on it. How can force be applied to an electron? One way is by gravity. Another is by bring­ing a second charge near the electron, thus exerting an at­tractive or a repulsive force on it. In this case we may say that the second charge sets up an electric field which ap­plies a force to the first charge. Finally, we find that an electric current flow will affect the motion of a nearby charge, but only if that charge is already in motion. In this case, we say that the current sets up a magnetic field which applies a force to the moving charge. These three are the only known ways of applying force to an electron. The relationship between these fields, the charges pro­ducing them and the resulting effects on other charges are the Jaws or electron motion.

Notes

1. 9.1 IX 10-28 grams—nine point eleven multiplied by ten to the minus twenty-eighth power

2. to result in an acceleration — вызывать ускорение

Gravitation

Gravitation is a very important force in the universe. Every object has a gravitational pull which is like magnet­ism. But, unlike magnetism, gravitation is not only in iron and steel. It is in every object large or small; but large objects, such as earth, have a stronger pull than small ones.

Isaac Newton, the great scientist of the seventeenth century, first studied gravitation. When he was a boy, he often saw how apples fell to the ground. He wondered why they fell towards the earth and why they did not fly up into the sky.

According to l the law which he later produced every­thing in the universe attracts everything else towards it­self. The sun attracts the earth and the earth attracts the sun. The earth attracts the moon and the moon attracts the sun. Although the bigger object has the stronger attraction, all objects, in fact,2 have some attraction too but we do not notice the gravitational pull of a book be­cause the pull of the earth is very much greater.

Why does the earth always move round the sun, and not fly off into cold space? The sun's gravitation gives the answer. The earth always tries to move away in a straight line, but the sun always pulls it back. So it con­tinues on its journey round and round the sun.

The sun is one of the stars in the galaxy, in which there are about 100,000 million stars. It is not in the middle of the galaxy, but rather 3 near one edge.

There are millions of galaxies in the universe and so there are thousands of millions of millions of suns. Many astronomers believe that some of these suns have planets as our sun does.

Gravitation is the force which holds all the atoms of a star together. It holds the sun together and it holds the atoms of the earth together. It holds us on the earth.

Einstein produced a new law of gravitation. Its main results are the same as the results of Newton's law; but in very small and fine matters Einstein's law gives differ­ent results. One of these is that gravitation bends light a little; but according to Newton's law gravitation has very little effect on light. Einstein showed this fact by means of mathematics and not by experiment. And astron­omers later proved by experiments that Einstein was right.

Notes

1. according to—в соответствии с

2. in fact — на самом деле, фактически

3. but rather — а скорее