We say that in capacitive circuit the voltage and current are out of phase. Current is 90 (degrees) ahead of voltage. What is the physical explanation for this effect? How can current flow through a capacitive circuit, when voltage is zero i.e when voltage has a phase angle of 0 and current has a phase angle of 90?
Answer
If you want to gain an intuitive understanding of how this can be true, let's consider first an inductor, because this makes a better physical analogy. In an AC circuit with an inductive load, voltage leads current by 90 degrees. It's the opposite of a capacitive load.
Why? Well, an inductor is like a flywheel that gives inertia to current. The proper name for voltage is electromotive force. That is, it's a force that causes electricity to move. When electricity moves, we call it a current.
Imagine a flywheel. The angular inertia (size and weight) of the flywheel is the value of the inductor. The voltage is a force you apply to the flywheel. The current is the speed the flywheel is spinning. Now, say you apply a force to this flywheel. It does not begin spinning instantly. Rather, the force you apply accelerates it. Now, you apply force in the other direction. It does not immediately reverse direction. First it must slow, and eventually it will turn the other way. But by the time it has done this, you have moved on and have changed your direction of force yet again.
If the force you apply is sinusoidal, and there is no friction (resistance) in the spinning of the flywheel, then the speed of the flywheel will be 90 degrees out of phase with the force that's being applied to it.
Now, go develop a good mental model of a capacitor, and consider the same sort of thing. It should make more sense, just with current and voltage reversed, or the phase shift in the other direction.
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