Electric charge in motion

Chapter Electric charge in motion

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • of ferromagnetic material, the atoms in the material become lined up, so that the metal istemporarily magnetized. This produces a magnetic force between the atoms of the fer-romagnetic substance and those in the magnet.If a magnet is brought near another magnet, the force is even stronger. Not onlyis it more powerful, but it can be repulsive or attractive, depending on the way themagnets are turned. The force gets stronger as the magnets are brought near eachother.Some magnets are so strong that no human being can ever pull them apart if they get“stuck” together, and no person can bring them all the way together against their mutualrepulsive force. This is especially true of electromagnets, discussed later in this chapter.The tremendous forces available are of use in industry. A huge electromagnet can be usedto carry heavy pieces of scrap iron from place to place. Other electromagnets can providesufficient repulsion to suspend one object above another. This is called magnetic levita-tion and is the basis for some low-friction, high-speed trains now being developed.Electric charge in motionWhenever the atoms in a ferromagnetic material are aligned, a magnetic field exists. Amagnetic field can also be caused by the motion of electric charge carriers, either in awire or in free space.The magnetic field around a permanent magnet arises from the same cause as thefield around a wire that carries an electric current. The responsible factor in either caseis the motion of electrically charged particles. In a wire, the electrons move along theconductor, being passed from atom to atom. In a permanent magnet, the movement oforbiting electrons occurs in such a manner that a sort of current is produced just by theway they move within individual atoms.Magnetic fields can be produced by the motion of charged particles through space.The sun is constantly ejecting protons and helium nuclei. These particles carry a posi-tive electric charge. Because of this, they have magnetic fields. When these fields inter-act with the geomagnetic field, the particles are forced to change direction. Chargedparticles from the sun are accelerated toward the geomagnetic poles. If there is a solarflare, the sun ejects far more charged particles than normal. When these arrive at thegeomagnetic poles, the result can actually disrupt the geomagnetic field. Then there isa geomagnetic storm. This causes changes in the earth’s ionosphere, affectinglong-distance radio communications at certain frequencies. If the fluctuations are in-tense enough, even wire communications and electric power transmission can be inter-fered with. Microwave transmissions are generally immune to the effects of ageomagnetic storm, although the wire links can be affected. Aurora (northern or south-ern lights) are frequently observed at night during these events.Flux linesPerhaps you have seen the experiment in which iron filings are placed on a sheet of pa-per, and then a magnet is placed underneath the paper. The filings arrange themselvesin a pattern that shows, roughly, the “shape” of the magnetic field in the vicinity of themagnet. A bar magnet has a field with a characteristic form (Fig. 8-2).136 Magnetism