Power (physics)

Template:Short description Template:Use dmy dates Template:Infobox physical quantity Template:Classical mechanics Power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. Power is a scalar quantity.

The output power of a motor is the product of the torque that the motor generates and the angular velocity of its output shaft. Likewise, the power dissipated in an electrical element of a circuit is the product of the current flowing through the element and of the voltage across the element.

Definition

Power is the rate with respect to time at which work is done or, more generally, the rate of change of total mechanical energy. It is given by: $$P=\frac{dE}{dt},$$ where Template:Mvar is power, Template:Mvar is the total mechanical energy (sum of kinetic and potential energy), and Template:Mvar is time.

For cases where only work is considered, power is also expressed as: $$P=\frac{dW}{dt},$$ where Template:Mvar is the work done on the system. However, in systems where potential energy changes without explicit work being done (e.g., changing fields or conservative forces), the total energy definition is more general.

We will now show that the mechanical power generated by a force F on a body moving at the velocity v can be expressed as the product: $$P=\frac{dW}{dt}=\mathbf{𝐅}\cdot \mathbf{𝐯}$$

If a constant force F is applied throughout a distance x, the work done is defined as $W=\mathbf{𝐅}\cdot \mathbf{𝐱}$. In this case, power can be written as: $$P=\frac{dW}{dt}=\frac{d}{dt}(\mathbf{𝐅}\cdot \mathbf{𝐱})=\mathbf{𝐅}\cdot \frac{d\mathbf{𝐱}}{dt}=\mathbf{𝐅}\cdot \mathbf{𝐯}.$$

If instead the force is variable over a three-dimensional curve C, then the work is expressed in terms of the line integral: $$W=\underset{C}{\int }\mathbf{𝐅}\cdot d\mathbf{𝐫}=\underset{\mathrm{\Delta }t}{\int }\mathbf{𝐅}\cdot \frac{d\mathbf{𝐫}}{dt}dt=\underset{\mathrm{\Delta }t}{\int }\mathbf{𝐅}\cdot \mathbf{𝐯}dt.$$

From the fundamental theorem of calculus, we know that $$P=\frac{dW}{dt}=\frac{d}{dt}\underset{\mathrm{\Delta }t}{\int }\mathbf{𝐅}\cdot \mathbf{𝐯}dt=\mathbf{𝐅}\cdot \mathbf{𝐯}.$$ Hence the formula is valid for any general situation.

In older works, power is sometimes called activity.

Units

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.

Average power and instantaneous power

As a simple example, burning one kilogram of coal releases more energy than detonating a kilogram of TNT, but because the TNT reaction releases energy more quickly, it delivers more power than the coal. If Template:Math is the amount of work performed during a period of time of duration Template:Math, the average power Template:Math over that period is given by the formula $$P_{\mathrm{a}\mathrm{v}\mathrm{g}}=\frac{\mathrm{\Delta }W}{\mathrm{\Delta }t}.$$ It is the average amount of work done or energy converted per unit of time. Average power is often called "power" when the context makes it clear.

Instantaneous power is the limiting value of the average power as the time interval Template:Math approaches zero. $$P=\underset{\mathrm{\Delta }t\to 0}{\lim }{P}_{\mathrm{a}\mathrm{v}\mathrm{g}}=\underset{\mathrm{\Delta }t\to 0}{\lim }\frac{\mathrm{\Delta }W}{\mathrm{\Delta }t}=\frac{dW}{dt}.$$

When power Template:Math is constant, the amount of work performed in time period Template:Mvar can be calculated as $$W=Pt.$$

In the context of energy conversion, it is more customary to use the symbol Template:Mvar rather than Template:Mvar.

Mechanical power

File:Horsepower plain.svg

One metric horsepower is needed to lift 75 kilograms by 1 metre 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 Template:Math on an object that travels along a curve Template:Mvar is given by the line integral: $$W_C=\underset{C}{\int }\mathbf{𝐅}\cdot \mathbf{𝐯}dt=\underset{C}{\int }\mathbf{𝐅}\cdot d\mathbf{𝐱},$$ where Template:Math defines the path Template:Mvar and Template:Math is the velocity along this path.

If the force Template:Math 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: $$W_C=U(A)-U(B),$$ where Template:Mvar and Template:Mvar are the beginning and end of the path along which the work was done.

The power at any point along the curve Template:Mvar is the time derivative: $$P(t)=\frac{dW}{dt}=\mathbf{𝐅}\cdot \mathbf{𝐯}=-\frac{dU}{dt}.$$

In one dimension, this can be simplified to: $$P(t)=F\cdot v.$$

In rotational systems, power is the product of the torque Template:Math and angular velocity Template:Math, $$P(t)=\mathit{\tau }\cdot \mathit{\omega },$$ where Template:Math is angular frequency, measured in radians per second. The $\cdot$ represents scalar product.

In fluid power systems such as hydraulic actuators, power is given by $$P(t)=pQ,$$ where Template:Mvar is pressure in pascals or N/m², and Template:Mvar is volumetric flow rate in m³/s in SI units.

Mechanical advantage

If a mechanical system has no losses, then the input power must equal the output power. This provides a simple formula for the mechanical advantage of the system.

Let the input power to a device be a force Template:Math acting on a point that moves with velocity Template:Math and the output power be a force Template:Math acts on a point that moves with velocity Template:Math. If there are no losses in the system, then $$P=F_{\text{B}}{v}_{\text{B}}=F_{\text{A}}{v}_{\text{A}},$$ and the mechanical advantage of the system (output force per input force) is given by $$\mathrm{M}\mathrm{A}=\frac{F_{\text{B}}}{F_{\text{A}}}=\frac{v_{\text{A}}}{v_{\text{B}}}.$$

The similar relationship is obtained for rotating systems, where Template:Math and Template:Math are the torque and angular velocity of the input and Template:Math and Template:Math are the torque and angular velocity of the output. If there are no losses in the system, then $$P=T_{\text{A}}{\omega }_{\text{A}}=T_{\text{B}}{\omega }_{\text{B}},$$ which yields the mechanical advantage $$\mathrm{M}\mathrm{A}=\frac{T_{\text{B}}}{T_{\text{A}}}=\frac{{\omega }_{\text{A}}}{{\omega }_{\text{B}}}.$$

These relations are important because they define the maximum performance of a device in terms of velocity ratios determined by its physical dimensions. See for example gear ratios.

Electrical power

Ansel Adams photograph of electrical wires of the Boulder Dam Power 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 $$P(t)=I(t)\cdot V(t),$$ where

• $P(t)$ is the instantaneous power, measured in watts (joules per second),
• $V(t)$ is the potential difference (or voltage drop) across the component, measured in volts, and
• $I(t)$ is the current through it, measured in amperes.

If the component is a resistor with time-invariant voltage to current ratio, then: $$P=I\cdot V=I^2\cdot R=\frac{V^2}{R},$$ where $$R=\frac{V}{I}$$ is the electrical resistance, measured in ohms.

Peak power and duty cycle

File:Peak-power-average-power-tau-T.png

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 $s(t)$ of period $T$, like a train of identical pulses, the instantaneous power $p(t)=|s(t){|}^2$ is also a periodic function of period $T$. The peak power is simply defined by: $$P_0=\max [p(t)].$$

The peak power is not always readily measurable, however, and the measurement of the average power $P_{\mathrm{a}\mathrm{v}\mathrm{g}}$ is more commonly performed by an instrument. If one defines the energy per pulse as $${\epsilon }_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=\underset{0}{\overset{T}{\int }}p(t)dt$$ then the average power is $$P_{\mathrm{a}\mathrm{v}\mathrm{g}}=\frac{1}{T}\underset{0}{\overset{T}{\int }}p(t)dt=\frac{{\epsilon }_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}}{T}.$$

One may define the pulse length $\tau$ such that $P_0\tau ={\epsilon }_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}$ so that the ratios $$\frac{P_{\mathrm{a}\mathrm{v}\mathrm{g}}}{P_0}=\frac{\tau }{T}$$ are equal. These ratios are called the duty cycle of the pulse train.

Radiant power

Power is related to intensity at a radius $r$; the power emitted by a source can be written as: $$P(r)=I(4\pi r^2).$$

See also

• 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