Introductory Physics Volume Two

1.1 Electric Charge 5
Then charge a rubber rod with fur, place it in contact with the metal
balls, and then remove it. The two balls will be observed to repel each
other.
Here is how we can explain the demonstration. When the rubber rod
comes in contact with the metal, some of the charge (which is negative
since it is from a rubber rod rubbed with fur) moves over to the metal.
This is because metals easily accept or give up electric charge; electric
charge moves easily through metals. If we indicate the charges using
negative signs, then we can represent what happens with the following
picture:
- - -
-
-
-
- -
- - - -
-
-
-
-
-
- -
-
-
-
-
- -
-
-
-
The charges move from the rubber rod onto the metal balls because
of the repulsive forces between them. Once isolated, the two metal
spheres repel each other because they both have a net charge of the
same sign.
Example
Two equal masses, each with mass m = 4g, are electrically charged
and hung using string as shown. If the masses hang in equilibrium at
an angle θ = 10 ◦ , what is the magnitude of the electric force that each
mass exerts on the other?