Lines of Force
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-304
2025-11-26
42
A north magnetic pole placed near the+ end of a bar magnet is at the same time repelled by the + end and attracted by the - end, and each force varies inversely as the square of the distance. Consequently, if the pole were free to move under the sole influence of the bar magnet, it would follow a curved path called a line of magnetic force, and would come to rest in contact with the - end of the magnet. By starting the pole at different points, an infinite number of such lines of force could be described. The direction of a line of force means the direction in which a free north pole would move. Though a free north pole cannot exist (there is no north pole without its accompanying south pole), this direction at any point will be that along which the north pole of a short piece of magnetized needle suspended by a thread will tend to move. This needle will turn so as to lie as nearly as possible in the line of force, because in any other position the forces acting on the two poles of the needle, not being in line with the needle, would form a couple and cause rotation. A few lines of force are shown in Fig. 1, some complete and some in part.
FIG. 1. -Lines of Force
Demonstration. - Lay a bar magnet upon the table and cover it with a sheet of white paper. Put over this a thin glass plate and sift iron filings evenly over it. The sieve for this purpose can be made by melting the solder that holds on the bottom of a tin can, removing the bottom, and tying over the can, in its place, a piece of thin muslin or cheesecloth. Rap the plate gently with a lead pencil; the filings will move at every blow, and will arrange themselves in lines that show clearly the paths of the lines of force.
Suspend a half-inch piece of magnetized sewing needle quite near the plate used in the above demonstration. Notice that it places itself in line with the filings.
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