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Ohm's Law and Power

Ohm's law links the voltage across a resistor, the current through it and its resistance. Knowing any two of the three quantities, you can always find the third. The power dissipated as heat in the resistor can be computed in three equivalent ways, depending on which quantities you know.

U = RI

Ohm's law: voltage is resistance times current

P = UI = RI^2 = \dfrac{U^2}{R}

power in a resistor, three equivalent forms

Symbols

$U$	voltage across the resistor	V
$R$	resistance	$\Omega$
$I$	current through the resistor	A
$P$	power (heat dissipated)	W

Example

$U = 12$ V across $R = 4$ Ω:

$I = U/R = 12/4 = 3$ A.

$P = UI = 12\cdot3 = 36$ W.

Given any two of U, R and I, Ohm's law always gives the third – sketch the circuit and mark what you know.

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Part of Electric Circuits: Direct current.