AC Ohm’s Law
The AC analog to Ohm’s law is
where Z is the impedance of the circuit and V and I are the rms
or effective values of the voltage and current. Associated with
the impedance Z is a phase angle, so that even though Z is the
also the ratio of the voltage and current peaks, the peaks of
voltage and current do not occur at the same time. The phase
angle is necessary to characterize the circuit and allow the
calculation of the average power used by the circuit.
If an rms voltage of Vrms =
is applied to an impedance Z =
then the rms current will be Irms =
If the phase is φ = degrees,
then the power factor is cosφ =
and the average power is
Pavg = VrmsIrmscosφ = = watts.



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