THEORY: Ohm's law plays a very important role in the theory of DC circuits. Does it work in AC circuits? Yes, it does, if the resistance R is replaced by the impedance, which is determined by both the resistance and the reactance of the inductors and the capacitors. What are the impedance and the reactance? We will study these concepts in the following two experiments.
The reason that Ohm's law takes more complicated form in AC circuits is that the current passing through a capacitor or inductor is not in phase with the AC voltage applied across them. Physically, it reflects the fact that capacitors and inductors are devices that store energy, not consuming it. Such storage function makes the current advance (in the case of capacitors) or lag (in the case of inductors) the voltage by one quarter of a period. To see this, let us consider the inductor circuit of Figure 1.