Electrical wires have a certain amount of resistance per foot. A longer wire means more resistance and a greater voltage drop.

An electrical circuit is a path through which electricity flows. This path contains one or more components that create a load (something that is using electricity) and that load acts as resistance to the passage of electricity through the circuit. Electricity can only flow through a circuit when the path is closed and cannot flow through an open circuit.

Semiconductors have valence shells that are exacly half full and can conduct electricity under some conditions but not others. This property makes them useful for the control of electrical current.

The joule is a unit of measurement for energy.

Direct current flows in only one direction in a circuit, from the negative terminal of the voltage source to the positive. A common source of direct current (DC) is a battery.