AC2 Fundamentals Unit 4 – Power in AC Circuits
UNIT 4 – POWER IN AC CIRCUITS
UNIT OBJECTIVE
At the completion of this unit, you will be able to calculate and measure the apparent power, real
power, reactive power, and power factor in ac circuits.
UNIT FUNDAMENTALS
Power is the rate at which work is done. Power is energy expended over a period of time to
accomplish useful work. In an electrical circuit, energy moves electrons. This energy is
dissipated in the form of heat when electrons flow through a resistance. A watt is the unit of
electrical power. Real power (P) is dissipated only in a resistance.
Three basic formulas are used to compute real power in a resistance.
P = I x E P = I^2 x R P = E^2 /R
In these formulas, P is power in watts, I is the current through the resistance in amperes, E is the
voltage across the resistance in volts, and R is the resistance in ohms.
In this circuit, the sine wave of voltage (VGEN) produces a sine wave of current (I) through the
resistor (R).
Multiplying the instantaneous current value by the corresponding voltage produces the power
curve shown. The average power is indicated by the horizontal dashed line. When calculating the
average power, use the rms values of current and voltage.
P = I x E P = I^2 x R P = E^2 /R