From the "Zap!" Lab you should know...

1. Various materials can be rubbed together, after which they will attract other materials such as bits of paper, hair, etc.
2. Rubbing these materials with a damp cloth will largely eliminate this attraction.
3. Two objects made of the same material, after being rubbed with another material, will repel one another.
4. When an object exhibits such behavior it is said to be charged.
5. We call the force causing these effects an electrostatic force.This electrostatic force differs from gravity in several ways.
   • No effort such as rubbing objects together is required to produce the gravitational force.
   • The electrostatic force is much stronger than gravity.
   • While a charged object can be discharged, thus losing its effect on other objects, gravity can't be eliminated.
   • Gravitation is never a repulsive force.
6. We can classify any charged body electrostatically into one of two categories.
   • Those that act the same - attract and repel the same things are said to have like charges.
   • If object A attracts something and object B repels it, then A & B are said to have unlike charges.

   We say that one object is positively charged and the other negatively charged. Since this naming is really arbitrary, some sort of standard must be set. That is, we must assign the positive type of charge to some standard object in a reproducible situation. It has been determined by comparison to recognized standards that anything that acts like our clear plastic strip after it (the clear strip) is rubbed with paper, is positively charged. If it acts the opposite, it is negatively charged. The white strip acted in this manner when rubbed with paper.

7. We sum up this behavior with the Law of Charges which says that likes repel and unlikes attract.
8. This law can be a bit hard to use in the case of attraction as we find that uncharged objects will always be somewhat attracted to charged objects.