Rub a balloon or styrofoam drinking cup against a wool sweater. It will then stick to a wall (at least on a dry day) or pick up small bits of paper. Why? The answer leads to the concept of electric charge.

Electromagnetic forces are one of the four fundamental forces in nature. The other three are gravitational, strong nuclear, and weak nuclear forces. The electromagnetic force unifies both electrical and magnetic forces. The magnetic forces occur whether the charges are moving or at rest. Electric charge is our way of measuring how much electric force an object can exert or feel.

Electric charge plays the same role in electric forces as mass plays in gravitational forces. The force between two electric charges is proportional to the product of the two charges divided by the distance between them squared, just as the force between two masses is...