Power in A.C Circuits

Power in an A.C. Circuit

The average power in an a.c. circuit is given by

P = IVcosɸ

where I, V, are the effective (r.m.s.) values of the current and voltage respectively and ɸ is the angle of lag or lead between them. The quantity cos ɸ is known as the power factor of the device. The power factor can have any value between zero and unity for ɸ varying from 90o (or cos ɸ = 0) average power P, is zero. A power of zero means that the device is a pure reactance, inductance or capacitance. Thus no power is dissipated in an inductance or capacitance. However if I is the r.m.s. value of the current in a circuit containing a resistance, R, then the power absorbed in the resistance is given by

P = I2R

For an a.c. circuit, the instantaneous power is given by P =IV (instantaneous values.)

Cos ɸ = R/Z = Resistance/Impedance

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