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?
MAGNETISM 483 8. Explain the magnetizing and demagnetizing of an iron bar in terms of the molecular theory. 9. Why does a dipping needle not dip to the same degree at different points on the earth's surface? 10. What is the difference between an unmagnetized and a magnetized iron bar according to the molecular theory? 11. Explain how magnetism may be increased by rhythmical pounding. 12. Give the facts of magnetism, and show how the molecular theory explains each fact. 13. Give the laws of magnetism. 14. How can you prove that the earth is a magnet, i.e., (a) that it has two poles, and (b) that it is surrounded by a magnetic field? 15. Try to explain the laws of magnetism in terms of the molecular theory. 16. What examples of the inverse square law have we studied up to date? Look up the inverse square law as it applies to sound energy, and on the basis of the explanation given, try to work out an explanation for the inverse square law as it applies to magnetism. 17. What metals compose the alloys which are used to make powerful permanent magnets? 18. Give an example of induced magnetism and explain it. 19. Give the facts that support the molecular theory of magnetism. 20. Distinguish between a magnet and a magnetic substance.