Capacitor - Fact of the humor

On Fri, 4 Apr 2008 22:45:07 -0700 (PDT) Aditya wrote: | This is a well known joke :- | | " A capacitor blocks DC components but allows to pass Ac. Why ? | Answer : The AC jumps ( as in the sine wave peak ) over it ! " | | Funny one. | But what is the exact detailed explanation of this. Can anyone please | explain the mechanism ???

AC current flows by charging up the capacitor (as opposed to actually flowing between the two capacitor connections), and subequently doing a discharge after the zero crossing.

DC will flow, too. It will just do so briefly and stop once the capacitor is charged to the same voltage.

Exact explanation of what? AC jumping?

A capacitor stores charge. As it charges, it develops a voltage across it that tries to discharge the capacitor. If you pass a current of 1 A through a 1 F (yes, one farad) capacitor, the voltage rises at one volt per second. To keep the current at 1 A, the supply voltage needs to keep rising at 1 V/sec indefinitely. In general, it can't, and so the current flow reduces as the voltage across the capacitor rises. When the capacitor voltage equals the supply voltage, no more current flows.

Now, when you apply a DC voltage to a circuit containing a capacitor, the capacitor charges, once, to a constant voltage, and then no more current flows. If it's in series, we say it blocks DC.

If you apply AC voltage across a capacitor, it alternately charges and discharges, and no average voltage builds up. So the AC "passes through" the capacitor. It might be attenuated somewhat, depending on the AC impedance of the capacitor compared to the source and load impedances surrounding it.

Dave

In steady state circuit theory, impedance of a capacitor is proportional to 1/frequency. At dc, impedance goes towards infinity.

The dielectric in a capacitor is an electrical insulator. So it's natural that a capacitor blocks DC. So why does it seem to pass AC?

Consider a water analogy. Imagine a closed system that includes two sections of water pipe (conductors). Connect a water pump (battery) between the two pieces of pipe. Between the other two ends of the pipe sections, connect a special device. Let the device contain a rubber bladder that prevents water from flowing through the device. That is, water pressure can stretch the rubber bladder but the water can't pass through it. Let the entire system be filled with water.

So you now have a complete circuit: When the pump starts, water flows momentarily through a section of pipe and stretches the rubber bladder; the water on the other side of the bladder is displaced and returns to the pump. The stretched bladder is now an energy storage device. This is exactly what happens when a battery is connected through wires to a capacitor: A current moves from the battery to the capacitor, placing a charge on the capacitor (storing energy); an equal current on the other side of the capacitor returns to the battery. But once the capacitor is charged, all current flow stops. In the water system, when the pump is applying as much pressure as it can (and the bladder doesn't break) the water flow stops. This is a DC system. What happens in an AC system?

For AC, imagine the same closed water system. But now let the pump first pump water in one direction then in the other, just like an AC generator pumps current. In this case, the rubber bladder stretches in one direction, then in the other, and back again. And it continues to do this as long as the pump is operating. So the alternating water--like AC current--continues to flow back and forth--even though the rubber bladder never lets water on one side pass through to the other side. This is how a capacitor "passes" AC current but blocks DC current.

Dear Bob, jay, dave nd phil..... Thx for all the replies... :) Got the answer !!!