Introductory Physics Volume Two

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1.2 Coulomb’s Law 7
How can we understand this apparently new type of charge, a
charge that is attracted to both positive and negative charges? The
place to start is consider the aluminum of which the can is composed.
Each aluminum atom is composed of an equal number of protons and
electrons, so the net charge of the can is zero. But there is a difference
between the protons and electrons. The aluminum atoms are locked
together in a crystal lattice, with the protons locked together with the
neutrons in the nucleus of the atoms. The protons cannot move. On the
other hand some of the electrons are shared between all of the atoms,
and move around freely. This is what makes aluminum a conductor, it
has electrons that move freely through the bulk of the material.
So when a positively charged rod is brought near the can the electrons
are attracted toward the oppositely charged rod and the move
within the metal a little bit toward the rod. This leaves the can with a
net negative charge on the side toward the rod and a net positive charge
on the side away from the rod. The net positive charge is because the
protons did not move, so that when the electrons moved toward the
other side of the can, the protons were left alone, unpaired.
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can alone
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can next to
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showing
net charge
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One might think that the net force would still be zero once the electrons
have redistributed themselves, since we end up with a net positive
charge on one side that is repelled and a net negative charge on the
other side that is attracted. But it ends up that the force between two
charges decreases with distance, so that the repulsive force is less than
the attractive force.
⊙ Do This Now 1.3
Explain how a negatively charged rod also attracts the can.
§ 1.2 Coulomb’s Law
As we have seen there is a force between charged objects. After
careful observation it was determined that the electrical force is