# Power

This article discusses power in physics. For alternative uses, see Power (mathematics), Power (international), Statistical power and Power (sociology).

In physics, power is the amount of work done per unit of time. This is equivalent to the rate of change of the energy in a system, or the time rate of doing work, as defined by

$P=\frac{dE}{dt}$,

where

The units of power are therefore work divided by time (e.g. foot-pounds per minute, ergs per second or joules per second). The SI unit of power is the watt, which is equal to one joule per second.

The commonest non-SI unit of power is the horsepower, which is equivalent to 33,000 foot-pounds per minute, or the power required to lift 550 pounds one foot in one second, and is equivalent to about 746 watts. Other European countries have similar units, such as the Pferdstarke[] (PS) and the cheval vapeur (CV).

The power consumption of a human is on average roughly 100 watts, ranging from 85 W during sleep to 800 W while playing a strenuous sport.

## For direct current (DC) and voltage

In electrical engineering, the instantaneous power load by a two-terminal electrical device is the product of the voltage across the terminals and the current passing through the device. That is,

$P(W) = I(A) U(V)$

where I is the instantaneous or average direct current (DC) in ampere (A) and U is the instantaneous or average voltage in volt (V).

## For sine-shaped alternating current (AC) and voltage

The average power load by a two-terminal electrical device is a function of the root mean square values of the sine-shaped voltage across the terminals and the sine-shaped current passing through the device. That is,

$P(W)=U(V) I(A) cos(phi)$.

where I is the root mean square value of the sine-shaped alternating current (AC) in ampere (A) and U is the root mean square value of the sine-shaped alternating voltage in volt (V). Phi is the phase angle between the voltage and the current sine functions.

The efficient transfer of electrical power is governed by the power theorem">maximum power theorem.

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