Inductive Reasoning

Q. In my lessons on AC and resonant circuits, it gives the memory aid, ELI the ICEman meaning that the voltage leads the current in an inductor and the current leads the voltage in a capacitor. I understand this. But I do not understand why this is so - could you please explain?

A. A capacitor draws current when the voltage across it changes. In other words, it "reacts" to a change in voltage by drawing a current. This is why it is said to have "reactance". If the voltage across it is a sine wave, the voltage changes fastest where it goes through zero and changes from one polarity to the other. The current is also a sine wave, so it peaks at the points where the voltage sine wave goes through zero. This describes a sine wave that is 90 degrees ahead of the voltage sine wave.

An inductor does just the opposite. It produces a voltage when the current through it changes. This is another way of saying that it reacts to a change in current by producing a voltage. As with the capacitor, the voltage across and current through the inductor are sine waves. As the current sine wave goes through zero, the voltage sine wave peaks. This describes a sine wave that is 90 degrees ahead of the current sine wave.

Both the capacitor and inductor are energy storage devices. The capacitor stores energy in its electric field, where the inductor stores energy in its magnetic field. Electrically, they both react to changes, but the capacitor reacts to voltage changes, where the inductor reacts to current changes. So the two are sort of mirror images of each other.

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