Magnetic Induction
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-314
2025-11-29
15
We have seen that when a piece of soft iron is put in contact with a bar magnet, it becomes a magnet itself and will attract iron. When a magnet is dipped in iron filings or nails and lifts a number of them attached to one another, it is because each. has become a distinct magnet. This influence of a magnet over pieces of iron or steel, by which they are made magnets, is called magnetic induction, and extends to a considerable distance from the magnet.
Demonstration. - Place a bar magnet upon a table, and at one end, in line with it and about an inch away, put a large nail, as in Fig. 1. Bring smaller nails near the end of the large one, and observe that it attracts them. Test the end of the nail with the magnetized sewing needle, to find which kind of polarity it has.
This property possessed by a magnet - of inducing polarity in magnetic substances that are near it -is the basis of a great many magnetic phenomena. By the application of this principle, and that of the mutual action of magnets, we see why a magnet attracts ordinary pieces of iron - an iron ball, for instance. Suppose the ball to be at a distance from the magnet, as in (a), Fig. 2. The ball is practically beyond the influence of the magnet and remains in a neutral condition. When, however, it is brought near the magnet, as in (b), induction takes place, and the ball becomes polarized, the side nearest the magnet being S and the other side N. Now between the N of the magnet and the S of the ball there is attraction, while between the N of the magnet and the N of the ball there is repulsion; but as the S of the ball is much nearer the N of the magnet than the N of the ball is, the attraction is much greater than the repulsion.
We see also why the iron indicate the lines of force. Each particle of iron becomes a magnet by induction, and, like the magnetized needle, turns so that its two poles are in a line of force. Its poles also are attracted by the adjacent unlike poles of other particles, and thus the particles cling together, with their + poles turned toward the - end of the magnet, in lines of force.
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