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. TEST YOUR UNDERSTANDING: This lesson on AC theory enables you to graphically view and compare the result in terms of voltage and current for individual resistor, capacitor and inductor components. AC voltages and currents by their nature are continually changing and apart from using RMS values are measured as instantaneous values throughout a sinewave cycle of 360° Make yourself familiar with the topic theory and then follow the calculations to see how AC signals can be measured on the representation of an oscilloscope screen. Current Phasors will show how two AC currents at different phase angles can be calculated when applied across an AC circuit. Don't worry too much about the detail, but appreciate that AC voltages and currents behave in a much more complex way than those for DC. What additional factors do you think cause this?
