Until this point, we have been discussing the motion of charges in the absence of any other materials, i.e., in a vacuum.
Charges can also flow freely through some materials.
We refer to such materials as conductors . If a potential difference exists in two different parts of a conductor, charge will flow. For example, on a 12 volt battery, a potential difference of 12 volts exists between the terminals. If we attach a light bulb in the middle, we get the circuit shown below. We can think of the charges flowing in analogy with the flow of water.