Electronics Principles V11

Clive W. Humphris

AC THEORY: AC Ohm's Law.  

Ohm's Law can also be applied to AC circuits. However, alternating currents and voltages are continually changing. At the beginning of the cycle the voltage and current are zero, building to peak positive values at 90°, before declining back to zero, and then repeated in a negative direction.

It is therefore only possible to calculate instantaneous values of V or I throughout the cycle. Peak or RMS values are normally used.

The AC resistance of capacitors and inductors is called 'reactance' (measured in Ohms). As the frequency is increased, capacitive reactance decreases, whereas inductive reactance increases.

Once the reactance is calculated for C or L at the applied frequency, the value can be inserted in the formula as for resistance. Where there is a combination of resistance and reactance the calculation refers to 'impedance', symbol Z.

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