Joule's law (also known as Joule effect) is a physical law expressing the relationship between the heat generated by the current flowing through a conductor. It is named for James Prescott Joule who studied the phenomenon in the 1840s. It is expressed as:

$Q\; =\; I^2\; \backslash cdot\; R\; \backslash cdot\; t$

Where Q is the heat generated by a constant current I flowing through a conductor of electrical resistance R, for a time t.

When current, resistance and time are expressed in amperes, ohms, and seconds respectively, are used the unit of Q is the joule.

Joule's law is sometimes called the Joule-Lenz law since it was later independently discovered by Heinrich Lenz. The heating effect of conductors carrying currents is known as Joule heating.

See also: Peltier-Seebeck effect

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