The rate at which energy is transferred or work is done.Power, in physics, is the rate at which energy is transferred or work is done. It measures how quickly a system can convert one form of energy into another or move energy from one place to another. The standard unit of power is the watt (W), equal to one joule per second. Power is a scalar quantity that appears in virtually every branch of physics, from mechanics (e.g., an engine's output) to electromagnetism (e.g., electrical power in circuits) and thermodynamics (e.g., heat flow rates).
The term 'power' was formalized by James Watt in the 18th century, who introduced the unit 'horsepower' to compare steam engines with draft horses. The watt was later named in his honor.
Mathematically, power is defined as P = W/t for constant work over time, or more generally as the derivative P = dW/dt. In electrical systems, power is the product of voltage and current (P = VI). In mechanical systems, it is the product of force and velocity (P = Fv). The concept allows engineers and scientists to compare the capabilities of different devices, such as engines, generators, or lasers, regardless of how long they operate.
Power ratings are used everywhere, from specifying light bulbs and electric motors to designing power plants and renewable energy systems.
A human at rest dissipates about 100 watts of power as heat, roughly the same as a bright incandescent light bulb.
| P | variable | Power — power watt |
| N | variable | Number of atoms — particle number |
| P | variable | accretion power — power watt |
| η | variable | efficiency of mass-to-energy conversion |
| Ṁ | variable | mass accretion rate — mass flow rate kilogram per second |
| c | constant | Speed of light in vacuum — speed of light = 299792458 metre per second |
| P | variable | Transmitter power — power watt |
| d | variable | Distance from the transmitter — distance metre |
| ∝ | operator | Indicates that the left side scales with the square of the right side — proportionality |
| L_H | variable | Instantaneous non-thermal emission (luminosity) — luminosity watt |
| L_S | variable | Plasma heating rate (energy per time) — power watt |
| \frac{d}{dt} | operator | Computes the rate of change with respect to time — time derivative |
| G | variable | optomechanical gain |
| Q_m | variable | mechanical quality factor — quality factor |
| P | variable | optical power — power watt |
| G_n | variable | optical response function (complex) |
| M | variable | effective mass of mechanical mode — mass kilogram |
| ω_m | variable | mechanical angular frequency — angular frequency radian per second |
| Re | operator | extracts the real component of a complex number — real part |