To Charge a Body by Induction
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-328
2025-11-27
35
In the preceding demonstration we notice that the electroscope remains charged only so long as the inducing body is near it. It is possible, however, to charge the electroscope permanently, as follows:
Demonstration. - Bring the glass rod near the knob as before, and when the leaves have separated, touch the knob with the finger. The leaves instantly fall together. Now remove first the finger and then the rod, and the leaves again diverge, showing that the electroscope has received a permanent charge.
To explain this action, we must remember that like electricities repel, and unlike electricities attract each other. When the electrified rod is brought near the knob, the + of the rod separates the electricities in the knob, wire, and leaves of the electroscope, driving the like kind, +, to the leaves, and holding the unlike kind, -, near to itself in the knob as in Fig. 1. When the knob is touched it is put in contact with the earth, and the + electricity, repelled by the + of the rod, escapes, while the - is held bound. Figure 2 shows this condition. Figure 304 shows the condition when the contact with the earth is broken and then the glass rod is removed.
The - electricity is no longer bound, but free, and so passes partly from the knob into the wire and leaves, which diverge less than before, but with a charge of - electricity.
From this we see that a body can be charged by induction, if only some way is provided by which to carry off the electricity that is repelled by the inducing body.
The inductive action of a charged body is well shown in an experiment first made by Faraday, called the ice-pail experiment because he used ice pails in making it :
Demonstration. - Place a thin metallic vessel, like a tin can, upon an insulating stand and connect it by a wire with the knob of an electroscope. Suspend a positively charged metal ball by a silk thread and lower it within the can; the leaves at once diverge. Remove the ball, and they fall together. Lower the ball again, and they diverge. Touch the can, and they fall. Remove the finger and then the charged ball and the leaves separate, charged with negative electricity. Explain.
Discharge the electroscope and again, lower the ball. Observe the amount of divergence of the leaves. Let the ball touch the inside of the can. Notice that the divergence of the leaves is not changed. Remove the ball, and the leaves will still be separated, charged with positive electricity. Explain.
A thorough study and understanding of the above experiment will give the student a correct idea of the phenomena of induction.
The attraction of an unelectrified body by an electrified one is the direct result of induction. The positive electricity on a charged glass rod repels the positive electricity of a pith ball and attracts the negative. Since the distance between the positive charge on the pith ball and the glass rod is greater than the distance between the negative charge and the rod, the repulsion is less than the attraction, and the pith ball is attracted.
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