Positive and Negative Charge

It was Ben Franklin who introduced the concept of two kinds of electric charge. Franklin noticed that you get opposite electrical effects when you rub a glass rod with silk, or rub a rubber rod with cat fur. He decided to call the charge left on the glass rod positive charge, and the charge left on the rubber rod negative charge. What we will see in this section is how this choice of positive and negative charge leads to the electron having a negative charge – e, and a proton a positive charge + e.

charge on an electron = – e          (1a)
charge on a proton = + e               (1b)

Despite the fact that the electron’s charge turns out to be negative, e is still called the charge on an electron.
The basis for saying we have two kinds of charge is the fact that with the electric interaction we have both attractive and repulsive forces. With the choice that electrons are negative and protons are positive, then the rules shown in Figure 1 can be summarized as follows: like charges (2 protons or 2 electrons) repel, opposite charges (proton and electron) attract. We can explain the lack of any force on the neutron by saying that the neutron has no charge— that it is neutral.
Choosing one charge as positive and one as negative automatically gives us a reversal in the direction of the force when we switch from like to opposite charges

Figure 1: The electric interaction between protons and electrons at rest. There is no electric force on a neutron.