Coils and Alternating Current

An inductor connected across a source of ac.
Suppose you change the voltage source, connected across the coil, from dc to ac (above figure). Imagine that you can vary the frequency of the ac, from a few hertz to hundreds of hertz, then kilohertz, then megahertz.

At first, the current will be high, just as it is with dc. But the coil has a certain amount of inductance, and it takes some 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 of the ac voltage reverses. At high ac frequencies, the current through the coil will have difficulty following the voltage placed across the coil. 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 increasing the frequency of the ac source, the coil will not even come near establishing a current with each cycle. Then the coil will act like a high resistance.

The opposition that the coil offers to ac is called inductive reactance. It, like resistance, is measured in ohms. 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). Like resistance, inductive reactance affects the current in an ac circuit. But, unlike simple resistance, reactance changes with frequency. This effect is not merely a decrease in the current, although in practice this does happen. Inductive reactance produces a change in the way the current flows with respect to the voltage.