Ohm's Law

Ohm's law is a mathematical formula that expresses the relationship between the electromotive force, electric current, and resistance in a circuit. This relationship was discovered by the German physicist Georg Ohm, and was published in 1827. When applied to a direct-current circuit, Ohm's law states that the electromotive force (E), measured in volts, equals the current (I) in amperes multiplied by the resistance (R) in ohms:

E = IR

When the law is used for an alternating-current circuit, resistance is replaced by impedance (Z), also in ohms. The flow of alternating current produces a counter-electromotive force, which resists the current. The strength of such resistance depends on how rapidly the current alternates. Impedance consists of this resistance, called reactance, combined with the circuit's regular resistance.

Electricians use Ohm's law to determine the efficiency of circuits. For example, they can calculate how the flow of current will be affected by various arrangements of such circuit components as connecting wires, capacitors, and resistors.

Contributor: Gregory Benford, Ph.D., Prof. of Physics, Univ. of California, Irvine.

Ohm's Law

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