# capacitor working

Discussion in 'General Electronics Chat' started by sharanbr, Apr 13, 2009.

1. ### sharanbr Thread Starter Active Member

Apr 13, 2009
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Hi,

I am going through capacitor basics. I can understand that capacitor passes ac signal through it (due to charge and discharge). But it is also mentioned that dc signal does not pass through it. Actually when a dc input is applied to capacitor, the other side of the node actually charges to opposite polarity. So this way the dc is passed though with reverse polarity. Am I wrong?

Regards

Apr 5, 2008
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3. ### italo New Member

Nov 20, 2005
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read and get confused? NO capacitors do not change polarity. they transfer the charge immidiately and decay at the rc time constant.

Jan 6, 2009
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5. ### RAH1379 Well-Known Member

Dec 13, 2005
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actually caps do change polarity from when they charge and discharge because the current flows in opposite directions, it dont actually go through the capacitor but electrons build up their numbers on the negative plate,compared to the positive plate, then once fully charged if that voltage is higher than the voltage that charged it,it will discharge the excessive electrons from the negative plate out through the circuit, towards the positive plate.

6. ### sharanbr Thread Starter Active Member

Apr 13, 2009
76
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Dear Bertus,

I will take a look at the links you have shared.

I have another question related to capacitors. Does charge actually flow through capacitors?

7. ### ErnieM AAC Fanatic!

Apr 24, 2011
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If you mean do electrons flow thru a capacitor no they do not, but current does. Long ago a Mr Maxwell defined this current as a displacement current, and a good thing he did as this is the same phenomena behind all radio transmissions.

A DC voltage applied to a capacitor will indeed charge it, but a steady state DC will produced a dwindling current so the steady state current is indeed zero, which is why it is said a cap will block DC.

One use of this fact: a cap at the input of an amplifier will prevent any long term DC voltage at the input from being amplified and overloading the output.

8. ### nsaspook AAC Fanatic!

Aug 27, 2009
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IMO 'displacement current' is one of the most confusing electrical terms still used in modern field theory. This fictitious pseudo-current of uncharged particles called photons is a holdover from aether theory. It's required by relativity but Maxwell had no idea of that and that makes his solution to the problem even more brilliant for the time.

http://scienceworld.wolfram.com/physics/DisplacementCurrent.html

A simple visualization is that the electromagnetic field that normally travels at ~c on the outside of the wire during changes in potentials to the capacitor plates 'that are very close (electrically)' bridges that gap so quickly IRT the change in potential the effect is of a continuous conductor to the varying field.

As the speed of the changes increase and/or the plate distance is increased the gap then becomes significant to the movement of the field across space causing it (the gap and the surface of the plates as an electrical system) to be a complex impedance that stores energy into a standing wave that can radiate into space (a antenna).

Last edited: Jun 13, 2015

Apr 13, 2009
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thanks ...