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When the two ends of a dipole can be separated, this method of charging by induction    may be used to create charged objects without transferring charge. In [link] , we see two neutral metal spheres in contact with one another but insulated from the rest of the world. A positively charged rod is brought near one of them, attracting negative charge to that side, leaving the other sphere positively charged.

In part a, a pair of neutral metal spheres are in contact. In part b, a rod with positive charge is close to the surface of one of the sphere. Negative signs are shown on this surface near the rod and positive signs are shown on the farthest part of the surface of the other sphere. The charged rod causes separation of charge. In part c, the positively charged rod is near the spheres, and the spheres are not in contact. As in figure b, the outer surface of the sphere nearest the rod has negative signs and the far surface of the other sphere has positive signs. In part d, the glass rod is not shown. The charges are now on the inner surfaces of the metallic spheres. One sphere has negative signs and the other has positive signs facing each other.
Charging by induction. (a) Two uncharged or neutral metal spheres are in contact with each other but insulated from the rest of the world. (b) A positively charged glass rod is brought near the sphere on the left, attracting negative charge and leaving the other sphere positively charged. (c) The spheres are separated before the rod is removed, thus separating negative and positive charges. (d) The spheres retain net charges after the inducing rod is removed—without ever having been touched by a charged object.

Another method of charging by induction is shown in [link] . The neutral metal sphere is polarized when a charged rod is brought near it. The sphere is then grounded, meaning that a conducting wire is run from the sphere to the ground. Since Earth is large and most of the ground is a good conductor, it can supply or accept excess charge easily. In this case, electrons are attracted to the sphere through a wire called the ground wire, because it supplies a conducting path to the ground. The ground connection is broken before the charged rod is removed, leaving the sphere with an excess charge opposite to that of the rod. Again, an opposite charge is achieved when charging by induction, and the charged rod loses none of its excess charge.

In part a, a rod with positive sign is brought near a neutral metal sphere. The surface of the sphere near the rod has negative signs and the surface far from it has positive signs. In part b, the sphere is connected to ground by a wire attached to the surface farthest from the rod. Negative charge is shown moving from the ground up to the sphere. The negative charges on the sphere near the rod are unaffected but there are fewer positive charges where the sphere is grounded. In part c, the sphere is disconnected from ground. The rod with positive sign is close to one surface of the sphere where negative charges are shown, and the other surface has no charges shown. In part d, the positive rod is absent, and the sphere has negative signs on it uniformly distributed on its surface.
Charging by induction using a ground connection. (a) A positively charged rod is brought near a neutral metal sphere, polarizing it. (b) The sphere is grounded, allowing electrons to be attracted from Earth’s ample supply. (c) The ground connection is broken. (d) The positive rod is removed, leaving the sphere with an induced negative charge.

Summary

  • A conductor is a substance that allows charge to flow freely through its atomic structure.
  • An insulator holds charge fixed in place.
  • Polarization is the separation of positive and negative charges in a neutral object. Polarized objects have their positive and negative charges concentrated in different areas, giving them a charge distribution.

Conceptual questions

An eccentric inventor attempts to levitate a cork ball by wrapping it with foil and placing a large negative charge on the ball and then putting a large positive charge on the ceiling of his workshop. Instead, while attempting to place a large negative charge on the ball, the foil flies off. Explain.

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When a glass rod is rubbed with silk, it becomes positive and the silk becomes negative—yet both attract dust. Does the dust have a third type of charge that is attracted to both positive and negative? Explain.

No, the dust is attracted to both because the dust particle molecules become polarized in the direction of the silk.

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Why does a car always attract dust right after it is polished? (Note that car wax and car tires are insulators.)

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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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