Electric Power

The electrical power is the energy transferred per unit time by the charge carriers to the component.(the rate of energy transfer).

P	=	W  t
	=	W × Q Q      t
P	=	V I
watts	=	volts × amps

Power is the rate of energy transfer, measured in watts (W), where 1 W = 1 J s^-1. The derivation above uses two earlier ideas: the potential difference V is the work done per unit charge (V = W/Q), and the current I is the rate of flow of charge (I = Q/t). Multiplying them gives W/t, which is the power.P = IVThe energy transferred in a time t is thereforeW = IVtFor example, a 12 V lamp carrying a current of 2.0 A has a power of 12 × 2.0 = 24 W, so in 5.0 minutes (300 s) it transfers 24 × 300 = 7200 J.Once resistance R is introduced (V = IR), the power can also be written in two other useful forms:P = I²R     P = V²/RP = I²R is useful when the current is known, for example the heating effect in a cable. P = V²/R is useful when the p.d. is fixed, for example a heater connected to the mains supply.