Ohm's law for alternating-current circuits uses the quantity called "impedance," (represented by the letter Z), in place of resistance. The voltage and current quantities are "effective" values instead of the actual cyclically varying instantaneous values. In this form, the equation for Ohm's law becomes the following for single phase circuits:
While this looks just like the DC equation from before, it is complicated by the fact that the impedance is made up of the resistance and the reactance of the circuit. And, as we indicated earlier, this reactance depends upon the inductive and capacitive elements in the circuit. While this reactance can be measured or calculated, it is not as easy as in pure resistance circuits where heat is dissipated or work is performed when current flows. Reactance absorbs energy when the current is increasing and returns energy to the circuit when current is decreasing. The average power is canceled out but there is always a power flow over the connecting wires.
This is called the "reactive power" (or "magnetizing power") and is measured in Volt-Amperes-Reactive (VARs). Therefore, there are two types of power in AC circuits. These can be calculated in a simple way:
The "apparent power" is a combination of these two powers and is a measure of the current carrying capacity investment required at the designated voltage. It is the number used to determine the electrical size (or rating) of a device such as a transformer or generator:
These types of AC power are related by the power factor (PF) and the reactive factor (RF).
Likewise, the three types of impedance are related:
The apparent power for a single-phase impedance is the product of the voltage with the current.
These are the same quantities that are used with Ohm's law.
Frequently, it is more convenient to use larger units for describing these "powers." The prefix, "kilo," means "one thousand" and "mega" means "one million." Thus, one kVA is equal to 1,000 volt-amperes and one megaVAR is one million volt amperes, reactive. Similarly, prefixes "milli" and "micro" mean "one-thousandth" and "one-millionth," respectively.
Example of Single-Phase Power
