Static Charges and Forces
In this activity, you noticed that a force was present between the pieces of tape. It's an invisible force. You couldn't see the force, but you saw evidence that it was there because it moved the tape. It's tough to believe in invisible things, but since this force had (and consistently does have) visible and measurable effects, it would be tougher not to believe in it.

You also observed that the force could be attractive or repulsive. One explanation might be that there are two types of charges and that the interaction of the charges creates a force. If the two pieces of tape have the same charges, the force is repulsive. If the two pieces of tape have the opposite charges, the force is attractive. An alternative explanation is that there is one type of charge. If the two pieces of tape have an excess amount of charge, there is repulsion. If the two pieces of tape have a deficiency of charge (they've each lost some charge), there is repulsion. If one piece of tape has a deficiency and the other piece of tape has an excess of the charge, there is attraction.

You then found evidence for forces that you could attribute to excess charges. The rubber rod removed excess charges from the wool. These excess (negative) charges were shared with the Styrofoam ball. The rod and the ball now had the same charge and you observed a force of repulsion that pushed the ball and rod apart from each other. When the glass rod, which had a deficiency of (negative) charges, was placed near the (negatively) charged ball, there was a force of attraction.

In the 1700s Benjamin Franklin performed experiments similar to those you did in this activity. He defined which charge would be

called positive and which would be called negative. A rubber rod rubbed in wool gains a negative charge. A glass rod rubbed in silk becomes positively charged. The glass rod did not gain positive charges but became positive by losing negative charges. Before Benjamin Franklin, investigators supposed that, since there was both attraction and repulsion, there were two kinds of electricity.

Conservation of Charge
A rubber rod gains electrons, while a piece of wool loses electrons. A glass rod loses electrons, while a piece of silk gains electrons. These situations are good examples of one of the major organizing principles of physics—the conservation of charge. In any system, the total amount of charge must remain constant. If negative charge is removed from the wool, that negative charge must go somewhere. For example, if 15 bits of negative charge are removed from the wool, then 15 bits of negative charge are transferred to the rubber rod. There are only a few quantities discovered that nature conserves — charge is one of them.