Main Page | See live article | Alphabetical index

# Power (physics)

This article discusses power in physics. For alternative uses, see Exponentiation, Power (international), Electric power, 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

,

where
• P is power
• E is energy or work
• t is time.

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.

Non-SI units of power include horsepower (HP), Pferdstarke (PS) and the cheval vapeur (CV). One unit of horsepower 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.

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 consumed by a two-terminal electrical device is the product of the voltage across the terminals and the current passing through the device. That is,

where I is the instantaneous or average direct current (DC) and U is the instantaneous or average voltage. If I is in amperes and U is in volts then P is in watts.

## For sinusoidal alternating current (AC) and voltage

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

where I is the root mean square value of the sinusoidal alternating current (AC) and U is the root mean square value of the sinusoidal alternating voltage. φ is the phase angle between the voltage and the current sine functions. If I is in amperes and U is in volts then P is in watts.

This can also be called the effective power, as compared to the larger apparent power which is expressed in volt-amperes reactive (VAR) and does not include the term due to the current and voltage being out of phase.

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