Q. My lessons say that the current through a capacitor is 90 degrees ahead of the voltage. Why is this?

A. A capacitor draws current whenever the voltage across it changes. The faster the voltage changes, the more current it will draw. Now think about the shape of the sine wave for the voltage across the capacitor. It is steepest at the point where it goes through zero and changes polarity.

In other words the voltage changes fastest in this part of the AC cycle. This occurs at 180° and 360° in the sine wave for the voltage. So the current will peak at these points. The voltage change slows down and stops momentarily when its sine wave reaches 90° and 270°. The current reaches zero at these points. So the sine wave for the current peaks at 180° and 360°. It goes through zero at 90° and 270°. This describes a sine wave that is 90 degrees ahead of the sine wave for the voltage.

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7/25/2016 8:44 PM
True, Voltage is always 90 degrees ahead of the current, in the ac.