| 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.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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.
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
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