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What is ohm's law?

Ohm's law states that the current through a metallic conductor is directly proportional to the potential difference across it, provided temperature stays constant.

Ohm's law explained

A component that obeys the law is ohmic and gives a straight-line current-voltage graph through the origin. Filament lamps and diodes are non-ohmic and curve.
In circuit questions, combine the law with series and parallel rules: current is shared in parallel and the same throughout a series loop.

Key formula

V=IRV = IR

Worked example

A 12 V12\,\text{V} supply across a 4 Ω4\,\Omega resistor drives a current of 3 A3\,\text{A}.

Examiner tip

Ohm's law is not a definition of resistance — resistance is always V ÷ I, but only ohmic components keep that ratio constant.

Ohm's law: common questions

Why does a filament lamp not obey Ohm's law?
As current rises the filament heats up, ions vibrate more, and resistance increases — so the I-V graph curves.
Does Ohm's law work for AC circuits?
It applies to the resistive part, but capacitors and inductors need impedance, which is beyond most A Level specifications.

Related terms

Resistance

Resistance measures how strongly a component opposes the flow of electric current, and is defined as potential difference divided by current.

Electric current

Electric current is the rate of flow of electric charge, measured in amperes — one ampere is one coulomb per second.

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