In physics, power is the rate of doing work or of transferring heat, i.e. the amount of energy transferred or converted per unit time. Having no direction, it is a scalar quantity. In the International System of Units, the unit of power is the joule per second (J/s), known as the watt (W) in honour of James Watt, the eighteenth-century developer of the condenser steam engine. Another common and traditional measure is horsepower (comparing to the power of a horse); one mechanical horsepower equals about 745.7 watts. Being the rate of work, the equation for power can be written as:
|SI unit||watt (W)|
|In SI base units||kg⋅m2⋅s−3|
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As a physical concept, power requires both a change in the physical system and a specified time in which the change occurs. This is distinct from the concept of work, which is measured only in terms of a net change in the state of the physical system. The same amount of work is done when carrying a load up a flight of stairs whether the person carrying it walks or runs, but more power is needed for running because the work is done in a shorter amount of time.
The output power of an electric motor is the product of the torque that the motor generates and the angular velocity of its output shaft. The power involved in moving a ground vehicle is the product of the traction force on the wheels and the velocity of the vehicle. The power of a jet-propelled vehicle is the product of the engine thrust and the velocity of the vehicle. The rate at which a light bulb converts electrical energy into light and heat is measured in watts—the higher the wattage, the more power, or equivalently the more electrical energy is used per unit time.
The dimension of power is energy divided by time. The SI unit of power is the watt (W), which is equal to one joule per second. Other units of power include ergs per second (erg/s), horsepower (hp), metric horsepower (Pferdestärke (PS) or cheval vapeur (CV)), and foot-pounds per minute. Other units include dBm, a logarithmic measure relative to a reference of 1 milliwatt, calories per hour, BTU per hour (BTU/h), and tons of refrigeration.
Equations for power
Power, as a function of time, is the rate (i.e. derivative) at which work is done, so can be expressed by this equation:
for a constant force, power can be rewritten as:
In fact, this is valid for any force, as a consequence of applying the fundamental theorem of calculus.
As a simple example, burning one kilogram of coal releases much more energy than does detonating a kilogram of TNT, 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 Pavg over that period is given by the formula:
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.
In the case of constant power P, the amount of work performed during a period of duration t is given by:
In the context of energy conversion, it is more customary to use the symbol E rather than W.
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.
where x defines the path C and v is the velocity along this path.
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:
In one dimension, this can be simplified to:
In fluid power systems such as hydraulic actuators, power is given by
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 FA acting on a point that moves with velocity vA and the output power be a force FB acts on a point that moves with velocity vB. If there are no losses in the system, then
and the mechanical advantage of the system (output force per input force) is given by
The similar relationship is obtained for rotating systems, where TA and ωA are the torque and angular velocity of the input and TB and ωB are the torque and angular velocity of the output. If there are no losses in the system, then
which yields the mechanical advantage
The instantaneous electrical power P delivered to a component is given by
- is the instantaneous power, measured in watts (joules per second)
- is the potential difference (or voltage drop) across the component, measured in volts
- is the current through it, measured in amperes
Peak power and duty cycle
In the case of a periodic signal of period , like a train of identical pulses, the instantaneous power is also a periodic function of period . The peak power is simply defined by:
The peak power is not always readily measurable, however, and the measurement of the average power is more commonly performed by an instrument. If one defines the energy per pulse as:
then the average power is:
One may define the pulse length such that so that the ratios
are equal. These ratios are called the duty cycle of the pulse train.
Power is related to intensity at a radius ; the power emitted by a source can be written as:
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- Halliday and Resnick (1974). "6. Power". Fundamentals of Physics.CS1 maint: uses authors parameter (link)
- Chapter 13, § 3, pp 13-2,3 The Feynman Lectures on Physics Volume I, 1963
- Burning coal produces around 15-30 megajoules per kilogram, while detonating TNT produces about 4.7 megajoules per kilogram. For the coal value, see Fisher, Juliya (2003). "Energy Density of Coal". The Physics Factbook. Retrieved 30 May 2011. For the TNT value, see the article TNT equivalent. Neither value includes the weight of oxygen from the air used during combustion.