Introductory Physics Volume Two

1.1 Electric Charge 3
Demo
1 Electric Field
§ 1.1 Electric Charge
Sometimes when you pet a cat, the cat’s fur begins to stand up
on end and if you then touch the cats nose, you and the cat are both
shocked. Through petting, the cat has been charged with static electricity.
The shock occurs when the cat discharges the built up static
charge to your finger. She has not done this on purpose, it is simply
the inevitable force of nature taking its course, so don’t get mad at the
cat. Of course if you had not touched the cat’s nose she would have lost
her charge more gradually through the air, so she might have reason to
be mad at you for sucking all the charge from her nose.
Let’s investigate static electricity further.
Static Electricity: Rub a rubber rod with fur and hang it by a string.
Rub a second rubber rod with fur and bring it near the first:
Rubber Rods rubbed with fur
Observe that the charge rods repel one another. Thus, for two rods
of the same material rubbed in the same way we find there will be
a repulsive force between them. Remove the second rubber rod, and
replace it with a plexiglass rod that has been rubbed with the fur.
Observe that the hanging rubber rod is attracted to the plexiglass rod.
So it is possible by rubbing different objects with fur to cause either
attractive or repulsive forces between the objects.
The observations above can be explained by the existence of two
types of “electric charge.” Objects with the same charge repel each
other, while objects with different charges attract one another. The
two types of charge, following the convention of Benjamin Franklin,
are called positive and negative. Rubbing rubber with fur leaves the
rubber with a negative charge, while rubbing plexiglass with fur leaves
the plexiglass with a positive charge.