Power (physics)

Power is the rate with respect to time at which work is done; it is the time derivative of work:

${\displaystyle P={\frac {dW}{dt}}}$

where P is power, W is work, and t is time. Because work is a force F applied over a distance x,

${\displaystyle W=\mathbf {F} \cdot \mathbf {x} }$

for a constant force, power can be rewritten as:

${\displaystyle P={\frac {dW}{dt}}={\frac {d}{dt}}\left(\mathbf {F} \cdot \mathbf {x} \right)=\mathbf {F} \cdot {\frac {d\mathbf {x} }{dt}}=\mathbf {F} \cdot \mathbf {v} }$

This is valid for any force, as a consequence of applying the fundamental theorem of calculus.

The dimension of power is energy divided by time. In the International System of Units (SI), the unit of power is the watt (W), which is equal to one joule per second. Other common and traditional measures are horsepower (hp), comparing to the power of a horse; one mechanical horsepower equals about 745.7 watts. Other units of power include ergs per second (erg/s), foot-pounds per minute, dBm, a logarithmic measure relative to a reference of 1 milliwatt, calories per hour, BTU per hour (BTU/h), and tons of refrigeration.

As a simple example, burning one kilogram of coal releases much more energy than does detonating a kilogram of TNT,[6] but because the TNT reaction releases energy much more quickly, it delivers far more power than the coal. If ΔW is the amount of work performed during a period of time of duration Δt, the average power P_avg over that period is given by the formula:

${\displaystyle P_{\mathrm {avg} }={\frac {\Delta W}{\Delta t}}}$

It is the average amount of work done or energy converted per unit of time. The average power is often simply called "power" when the context makes it clear.

The instantaneous power is then the limiting value of the average power as the time interval Δt approaches zero.

${\displaystyle P=\lim _{\Delta t\rightarrow 0}P_{\mathrm {avg} }=\lim _{\Delta t\rightarrow 0}{\frac {\Delta W}{\Delta t}}={\frac {\mathrm {d} W}{\mathrm {d} t}}}$

In the case of constant power P, the amount of work performed during a period of duration t is given by:

${\displaystyle W=Pt}$

In the context of energy conversion, it is more customary to use the symbol E rather than W.

One metric horsepower is needed to lift 75 kilograms by 1 meter in 1 second.

Power in mechanical systems is the combination of forces and movement. In particular, power is the product of a force on an object and the object's velocity, or the product of a torque on a shaft and the shaft's angular velocity.

Mechanical power is also described as the time derivative of work. In mechanics, the work done by a force F on an object that travels along a curve C is given by the line integral:

${\displaystyle W_{C}=\int _{C}\mathbf {F} \cdot \mathbf {v} \,\mathrm {d} t=\int _{C}\mathbf {F} \cdot \mathrm {d} \mathbf {x} }$

where x defines the path C and v is the velocity along this path.

If the force F is derivable from a potential (conservative), then applying the gradient theorem (and remembering that force is the negative of the gradient of the potential energy) yields:

${\displaystyle W_{C}=U(A)-U(B)}$

where A and B are the beginning and end of the path along which the work was done.

The power at any point along the curve C is the time derivative:

${\displaystyle P(t)={\frac {\mathrm {d} W}{\mathrm {d} t}}=\mathbf {F} \cdot \mathbf {v} =-{\frac {\mathrm {d} U}{\mathrm {d} t}}}$

In one dimension, this can be simplified to:

${\displaystyle P(t)=F\cdot v}$

In rotational systems, power is the product of the torque τ and angular velocity ω,

${\displaystyle P(t)={\boldsymbol {\tau }}\cdot {\boldsymbol {\omega }}}$

where ω measured in radians per second. The ${\displaystyle \cdot }$ represents scalar product.

In fluid power systems such as hydraulic actuators, power is given by

${\displaystyle P(t)=pQ}$

where p is pressure in pascals, or N/m² and Q is volumetric flow rate in m³/s in SI units.

Ansel Adams photograph of electrical wires of the Boulder Dam Power Units, 1941–1942

The instantaneous electrical power P delivered to a component is given by

${\displaystyle P(t)=I(t)\cdot V(t)}$

where

${\displaystyle P(t)}$ is the instantaneous power, measured in watts (joules per second)

${\displaystyle V(t)}$ is the potential difference (or voltage drop) across the component, measured in volts

${\displaystyle I(t)}$ is the current through it, measured in amperes

If the component is a resistor with time-invariant voltage to current ratio, then:

${\displaystyle P=I\cdot V=I^{2}\cdot R={\frac {V^{2}}{R}}}$

where

${\displaystyle R={\frac {V}{I}}}$

is the resistance, measured in ohms.

In a train of identical pulses, the instantaneous power is a periodic function of time. The ratio of the pulse duration to the period is equal to the ratio of the average power to the peak power. It is also called the duty cycle (see text for definitions).

In the case of a periodic signal ${\displaystyle s(t)}$ of period ${\displaystyle T}$, like a train of identical pulses, the instantaneous power ${\displaystyle p(t)=|s(t)|^{2}}$ is also a periodic function of period ${\displaystyle T}$. The peak power is simply defined by:

${\displaystyle P_{0}=\max[p(t)]}$

The peak power is not always readily measurable, however, and the measurement of the average power ${\displaystyle P_{\mathrm {avg} }}$ is more commonly performed by an instrument. If one defines the energy per pulse as:

${\displaystyle \epsilon _{\mathrm {pulse} }=\int _{0}^{T}p(t)\mathrm {d} t}$

then the average power is:

${\displaystyle P_{\mathrm {avg} }={\frac {1}{T}}\int _{0}^{T}p(t)\mathrm {d} t={\frac {\epsilon _{\mathrm {pulse} }}{T}}}$

One may define the pulse length ${\displaystyle \tau }$ such that ${\displaystyle P_{0}\tau =\epsilon _{\mathrm {pulse} }}$ so that the ratios

${\displaystyle {\frac {P_{\mathrm {avg} }}{P_{0}}}={\frac {\tau }{T}}}$

are equal. These ratios are called the duty cycle of the pulse train.

Power is related to intensity at a radius ${\displaystyle r}$; the power emitted by a source can be written as:

${\displaystyle P(r)=I(4\pi r^{2})}$

• Simple machines
• Orders of magnitude (power)
• Pulsed power
• Intensity — in the radiative sense, power per area
• Power gain — for linear, two-port networks
• Power density
• Signal strength
• Sound power