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Charges and Materials

There are some materials, most of them metals, in which electrons are able to move freely from atom to atom. Such materials are called conductors. A material which does not conduct electrons is called either a nonconductor, or an insulator, or a dielectric, depending on the mood of the speaker.

If we place a bunch of electrons (a negative charge) on an isolated conducting sphere (like a metal ball), the electrons repel and move away from one another. So all the charge ends up evenly distributed on the surface. (See Figure 14-2.)

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Figure 14-2


If we examine the interior of a piece of the sphere, the total charge on that piece will be zero. (This has to be. If there were any excess electrons, they would repel each other and move out of the piece, until they cannot go any further.) All the excess charge is on the surface.


The same holds for positive charge. If a positive charge is placed on a conducting sphere, then the excess positive charge will spread out on the surface, with no positive excess charge in the interior. In other words, any interior piece has a total charge of zero (Figure 14-3). (Although the positive charge is caused by a deficit of electrons, you will never go wrong by thinking in terms of positive charges repelling each other.)

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Figure 14-3


This is a general rule: The total charge of a piece of the interior of a conductor is zero. Any excess charge lies on the surface of the conductor.
If a charged object is brought near a neutral conductor, electrons will move away or toward the charged object, depending on the object's charge. This is called induced charge, since the charged object induces a charge on the neutral conductor (Figure 14-4).

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Figure 14-4


We can induce a charge on a nonconductor, too. Even though charges do not move freely in it, each molecule in a nonconductor can have its electron density move to one side (Figure 14-5a).
Generally the induced charge on nonconductors is smaller than that on conductors. Instead of drawing all the polarized molecules, we generally summarize these with a picture like Figure 14-5b.

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Figure 14-5a

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Figure 14-5b


A ground is any hug reservoir and depository of electrons. If you touch a charged object to a wire connected to ground, the charge will be neutralized where you touch it (Figure 14-6).

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Figure 14-6


The following example illustrates these concepts. In the illustrations, a slight excess in the number of electrons over the number of protons is indicated by a – sign, and a deficit of electrons is indicated by a + sign.


In a physics lab text, the following procedure is given for charging by induction a metal sphere connected to an insulating rod:

  1. Rub a piece of amber with cotton cloth.
  2. Bring the amber near but not touching the metal sphere.
  3. Touch the other side of the sphere with your finger (which acts as a ground).
  4. Remove your finger.
  5. Remove the amber.

Sketch what happens to charges. (Hint: Amber tends to pick up electrons.)


1. We rub amber with a cloth.


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Figure 14-7a

2. We bring the amber near the sphere.


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Figure 14-7b

3. We touch the other side with a finger.


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Figure 14-7c

4. We remove the finger


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Figure 14-7d

5. We remove the amber.


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Figure 14-7e


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