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See also, Electric power
In physics, power (symbol: P) is the amount of work P done per unit of time t. This can be modeled as an
energy flow, equivalent to the rate of change of the energy in a system, or the time
rate of doing work, as defined by
-
where
The units of power are therefore energy divided by time (e.g. foot-pounds per minute, 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), Pferdestärke (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 or more while playing a strenuous sport. Professional cyclists have been measured at 2000 W output for
short periods of time.
Electrical power
Instantaneous electrical power
One can find the instantaneous electrical power in a circuit with the following equations:
- P = IV,
- P = I2R,
or
-
where V is the potential difference (or voltage
drop) across a resistance R with current I flowing through it.
Average electrical power for sinusoidal voltages
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.
The amplitudes of sinusoidal voltages and currents, such as those used almost universally in mains electrical supplies, are
normally specified in terms of root mean square values. This makes the above calculation a simple matter of multiplying the two
stated numbers together.
This figure 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 cosφ term due to the current and voltage being out of phase. For simple domestic appliances, the "cos
φ" term (called the power factor) can often be assumed to be unity,
and can therefore be omitted from the equation. In this case, the effective and apparent power are assumed to be equal.
Electrical power transfer
The efficient transfer of electrical power is governed by the maximum power theorem.
Units of power
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