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Why does a capacitor block DC but AC? - Quora

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Why does a capacitor block DC but AC? Answer

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23 Answers Gautham Ashok, Electrical Engineer 36.3k Views

More people should wonder why it does AC, rather than wondering why it blocks DC! After all, a simple capacitor, represented in diagrams, has two metallic plates in parallel separated by a small distance by air, a dielectric. By default, current can't cross the dielectric, so that's simply why DC can't . So what about AC: Actually when you say that a capacitor allows AC, it is true, but even in AC the current doesn't actually "through" the capacitor(a dielectric is still present). Before I can explain that, let me clarify a common misconception: A capacitor does not store charge. It stores energy. Of course, one plate of the capacitor, say the plate connected to the negative terminal of a DC supply, will accumulate electrons, ing for negative charge. BUT here's what actually happens next: First, that the space between the plates is separated by the dielectric. Therefore, these negative charge cannot through the dielectric itself, but the plate connected to the positive terminal(which is metallic) will deplete an equal amount of electrons, and that places a positive charge on that PLATE. So actually there is no crossing over. Just a depletion in one plate as a reaction to the accumulation in the other. Since there is not gain of charge, the NET CHARGE IS ZERO. Now after equal but opposite charge is depleted from the opposite plate the voltage appearing across the capacitor is the DC source voltage. It's in an equilibrium and stays like that, so it's an open circuit since there is a dielectric in between (unless the voltage is so high that the dielectric breaks down). Energy is stored in the electrostatic field. The only current that appears before the Capacitor charges to the DC voltage level is the current that appears because of the opposite plate depleting electrons. You'd have seen graphs with an exponential increase in voltage till the DC level and an exponential drop in current to zero. Upvote 8

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So that clears blocking formula Q=CV means.

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3/3/2016 9:36 PM