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# Power in a Circuit With Resistance Only

Author: E.E. Kimberly

The instantaneous power in a circuit is always the product of the instantaneous current and instantaneous voltage. Fig. 5-21 shows the power wave as a locus of successive products of v and i when the voltage and current are in phase. The power passes through two cycles while the emf passes through one. All the power values are positive, and all of the energy represented by the area between the power wave and the axis is energy flowing in one direction in the circuit. If V is the voltage applied to a resistor and I is the resulting current, then the energy is that transformed into heat in the resistor in every cycle multiplied by the number of cycles. The power in the resistor is

 [5-18]

in which

P = average power;
F = rms volts;
I = rms amperes.
 Fig. 5-21. Power in a Circuit With Resistance Only

Last Update: 2010-10-05