Electric Power
a physical quantity, a measure of the time rate of transmitting or transforming electric energy.
In DC electric circuits electric power is given as P = EI, where E is the voltage in volts and I is the current in amperes. In AC circuits the product of instantaneous values of voltage e and current i is equal to the instantaneous power: p = ei. The value of p is a variable quantity that represents the power at a given instant. The average value of the instantaneous electric power during the time period T is called the active power
In single-phase circuits with sinusoidal current P = El cos φ, where E and I are the effective values of voltage and current and φ is the phase shift angle between voltage and current. Active electric power characterizes the time rate of the irreversible conversion of electric energy into other kinds of energy (heat, light, and so on). Electric power that characterizes the time rate of energy transmission from the current source to the receiver and back is called reactive power Q = El sin φ. The apparent power is equal to the product of effective values for circuits with periodically changing currents. The relationship between apparent power and active and reactive power is expressed by the equation S2 = P2 + Q2. For circuits with nonsinusoidal currents, electric power is equal to the sum of the corresponding average powers of the individual harmonics
For three-phase circuits electric power is defined as the sum of the powers of individual phases. With symmetrical loading
Where El is the line current, Il, is the line voltage, and φφ is the phase dishift angle between voltage and current.
P. V. ERMURATSKII
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