INDUCTORS AND AC

Suppose that you change the voltage source connected across a coil from dc to ac. Imagine that you can vary the frequency of the ac from a few hertz to hundreds of hertz, then kilohertz, and then megahertz.
At first, the ac will be high, just as is the case with dc. However, the coil has a certain amount of inductance, and it takes a little time for current to establish itself in the coil. Depending on how many turns there are and on whether the core is air or a ferromagnetic material, you’ll reach a point, as the ac frequency increases, when the coil starts to get sluggish. That is, the current won’t have time to get established in the coil before the polarity reverses. At high ac frequencies, the current through the coil has difficulty following the voltage placed across the coil. Just as the coil starts to “think” that it’s making a good short circuit, the ac voltage wave passes its peak, goes back to zero, and then tries to pull the electrons the other way. This sluggishness in a coil for ac is, in effect, similar to dc resistance. As the frequency is raised, the effect gets more pronounced. Eventually, if you keep on increasing the frequency of the ac source, the coil does not even come near establishing a current with each cycle. It then acts like a large resistance. Hardly any ac current flows through it.
The opposition that the coil offers to ac is called inductive reactance. It, like resistance, is measured in ohms (Ω). It can vary just as resistance does, from near zero (a short piece of wire) to a few ohms (a small coil) to kilohms or megohms (bigger and bigger coils or coils with ferromagnetic cores at high frequencies). Inductive reactance can be depicted on a ray, just like resistance, as shown in Fig. 1.

Fig. 1. Inductive reactance can be represented on half-line or ray. There is no limit to how large it can get, but it can never be negative.