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Capacitors and d.c. - HSN forum

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Capacitors and d.c.


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#1 HatingHighers

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Posted 21 May 2005 - 05:05 PM

If we were ever asked in an exam:

"Explain why capacitors allow a.c. signals but not d.c. signals"

what sort of thing would you write to ensure full marks?

#2 werlop

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Posted 21 May 2005 - 05:58 PM

A capacitor consists of 2 metal plates. When connected to a DC circuit the electrons flow from the negative terminal towards to positive terminal. However, the 2 plates act as an insulator. Electrons "pile up" on one plate - this plate therefore becomes negatively charged. As a result the other plate becomes positively charged. As the plates become more and more charged the current in the circuit decays - it becomes harder to push electrons onto the plate. Eventuall the current drops to zero.

In an AC circuit the direction of the current is constantly changing so the current never dacays to zero - current is directly proportional to frequency in a alternating current capacitive circuit.
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