Man's physical universe

xanabras

482 MAGNETISM AND ELECTRICITY

The molecular theory does not explain magnetism but merely indicates

that magnetism is a molecular phenomenon.

Magnetism Is Now Explained by the Electron Theory.

Undoubtedly you have already wondered what iron has that copper

or lead do not have as far as magnetism is concerned. According to

the Bohr theory of atomic structure, the atom is a miniature solar

system in which electrons (negative charges of electricity) revolve

about a positively charged nucleus. (The meaning of negative and

positive charges will be discussed in the next Section.) These electrons

also spin about their own axes.

Later we shall learn that electrons in

motion are surrounded by magnetic fields, i.e., that electrons in motion

behave like magnets. The electrons in an atom create two magnetic

fields, one for their orbital motion and one for their spin. It is the spin

that causes each electron to behave like a small bar magnet. In many

atoms the electrons spinning in a clockwise direction cancel the magnetic

effects of the electrons spinning in a counterclockwise direction.

In iron and other magnetic substances the electrons spinning in one

direction exceed the number of electrons spinning in the other direction,

and magnetism results. The above theory is based upon considerable

experimental evidence, but it has not been proven to be correct.

Magnetism Tells the " Lay of the Land."

The petroleum geologist is aided in petroleum exploration and

development by a knowledge of the direction and inclination of underground

rock layers. An ingenious method of obtaining this information

consists in

slowly revolving near suspended magnetic needles a core

obtained in drilling the well. When the needles are attracted or repelled,

their movement is photographed. The permanent magnetism in

the core's minerals resulting from lying in

the earth's magnetic field

for millions of years shows which part of the core had faced north.

STUDY QUESTIONS

1. Describe the circumstances that seem to have led to the discovery of magnetism.

2. Describe a simple compass.

3. How can the magnetism of a magnet be destroyed?

4. Why can the magnetic compass not be used in some regions of the earth?

5. What makes a freely moving magnet point north and south?

6. How may an iron bar be magnetized?

7. If an iron bar is easy to magnetize, will it retain its magnetism for a long

time?

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