Introductory Physics Volume Two

4 Electric Field 1.1
Fact: Two Types of Charge
From observations, such as the demonstration with the rubber rods
and fur, we have the following rules:
• There are two types of electric charge: + and -.
• Objects with the same sign charge repel.
• Object with oppositely signed charges attract
⊙ Do This Now 1.1
Your friend Albert claims that he has isolated a third type of electric charge
in nature. He calls the three types of charge A, B and C. Explain how he
would prove this to you. Hint: you might think about the rock-paper-scissors
game, and make a table showing which pairs are attractive and which are
repulsive.
Does the fur rubbing against the rubber (plexiglass) create the
negative (positive) electric charge? It turns out that the answer is
no. Careful experiments show that whenever a negative electric charge
shows up an equal amount of positive charge will be found somewhere
else, and vice versa. The net charge in the universe remains zero. To
see this for the rod-fur case, consider the following. Charge a rubber
rod with fur, hang the rod and then bring the fur near the rod. The
negatively charged rod will be attracted to the fur, showing that the
fur has gained a positive charge. We can understand what is going
on by considering the fundamental building blocks of normal matter:
protons, neutrons and electrons. Protons have a positive charge +e,
neutrons have no charge, and electrons have a negative charge −e.
Uncharged objects contains an equal number of electrons and protons.
A net electric charge is usually established on an object by adding
or removing electrons. In the case of the fur and rod, electrons are
removed form the fur and collected by the rod. This leaves the fur
with a net positive charge (more protons than electrons) and the rod
with a net negative charge (more electrons than protons).
⊙ Do This Now 1.2
Explain why the cat’s fur stands on end when it is pet.
Fact: Electric Charge
Electric Charge is neither created nor destroyed.
Demo
Charging Without Rubbing: Hang two small metal balls as indicated
below: