The electrical relationship between voltage, current, and impedance is called Ohm's law. Ohm's law is a simple mathematical formula which says the voltage in a circuit can be computed by multiplying the current flowing in the circuit times the impedance of the circuit. The "impedance" of a circuit is measured in ohms and is represented by the letter Z. The term impedance is used to include both inductive and capacitive reactance and resistance because all three are forms of opposition to the flow of current.

Ohm's Law is written as V = A x Z where

A = current (amps)

Z = impedance (ohms)

The electrical impedance of a circuit is made up of both the electrical resistance and the electrical reactance of the elements in the circuit. Both the resistance and reactance impede the current flow through a circuit.

Resistance loads include incandescent light bulbs and electric heating elements. Reactive loads include electric motors and other devices where the magnetic field created around a coil of wire or in a capacitor is utilized as electrical energy.