Electric Force & Charge

Introduction Certain particles in nature exert forces on each other. If you rub two balloons on your hair and then hang them from strings and bring them close to each other, they will clearly repel each other. The same balloons will attract other things, such as little bits of paper. This simple experiment tells us that, whatever property of nature causes these forces, there are at least two types. In the 1700s, when experiments of this type were first being done, the name charge was used to label the mysterious property of matter responsible for these forces.

Electric charges are of two types: positive and negative.

Electrons are negative; protons are positive, and neutrons are neutral (have no charge). It is a strange and interesting fact that electrons are protons have exactly the opposite charge. In other words, an electron and proton together are exactly neutral. This, despite the fact that the proton has 1836 times the mass of the electron!

Most things around us have just as many negative electrons as positive protons, making the object neutral overall.

Units for Measuring Charge

The basic unit for measuring charge is the magnitude of the electron charge, e. This is positive, so we say that the charge on a single electron is -e, and the charge on a single proton is +e.

One small problem: the elementary charge e is very small. Even in small, low-power electrical circuits there are billions of billions of electrons flowing through the wire every second. We'd like to have a unit of charge that is much larger for everyday use with typical electrical circuits. This unit is called the Coulomb.

1 coulomb = 1 coul = 6.24x10¹⁸ electrons

1. There are about 2x10²⁸ electrons in your body. How many coulombs is that?

2. If there are so many electrons in your body, how come you don't think of yourself as 'charged'? In other words, how come you're not shooting sparks out all the time?

3. What is the charge of a single electron, in coulombs? (HINT: Use the FL method.)