Summary

In electricity, as in all sciences, basic properties like charge are expressed in units. A unit is a standard quantity of something. For example, a pound is a unit of weight. In the case of charge, the standard unit is the coulomb. A coulomb is the charge carried by 6.24 x 1018 electrons. That's more than 6 million million million electrons.

Charge is normally most useful when it is flowing through a conductor, such as a wire. A conductor is a means of moving (conducting) electrons to a desired location. Metals, like copper-the most commonly used conductor-make good conductors, because they have loosely bound electrons that jump from atom to atom. These loosely bound electrons are called free electrons. The number of free electrons per volume of material determines how well a material conducts electricity. There are approximately 85,000 million, million, million free electrons in a cubic centimeter of copper. (±) •©· © •

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(±) Copper atoms minus free electrons • Free electrons jumping from atom to atom Materials that don't conduct electricity very well are called Insulators. They have a much lower number of free electrons per volume of material. Wire typically has an outer covering of insulation that keeps electrons from leaking into other conductors that may touch the wire. By using a source of electricity, such as a battery, to supply electrons to one end of a conductor and take them away at the other end, it is possible to get many trillions of free electrons to flow through the wire. This flow of electrons is called current. A material's tendency to resist the flow of current is called resistance.