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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 28th Jun 2015, 11:02 am   #1
ms660
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Default Does current flow through a capacitor?

Does it?

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Old 28th Jun 2015, 11:29 am   #2
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Default Re: Does current flow through a capacitor?

It comes down to your definitions of 'current' and 'flow'

Electrons go in one end
Electrons come out the other end
But it's not the same electrons which flow through it.

There has to be a saturation limit when all electrons have been removed from one electrode.

Distortion of electron orbits in a dielectric material acts to reduce the electric field strength in the gap, which acts to decrease the voltage across the capacitor after a given amount of electron-moving has been done, so this makes the capacitance look larger.

The bigger fun question in this area relates to 'displacement current'.

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Old 28th Jun 2015, 11:50 am   #3
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Default Re: Does current flow through a capacitor?

All electrons everywhere are identical and indistinguishable, so it is not meaningful to say whether the ones which come out of one capacitor lead are the same ones or different ones to those which went in the other lead.

Any 'saturation limit' is so far beyond what happens in electronics that it can be ignored.

In any reasonable definition of 'current' then yes current flows through a capacitor (if by 'capacitor' you mean a two-terminal electrical component). Things get a bit more tricky when you start to look at what happens inside a capacitor.
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Old 28th Jun 2015, 12:04 pm   #4
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Default Re: Does current flow through a capacitor?

The only DC current that flows through a capacitor is when it leaks!
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Old 28th Jun 2015, 12:26 pm   #5
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Default Re: Does current flow through a capacitor?

When explaining to a beginner how a circuit works I say that:-

1. AC flows through a capacitor.

2. DC is blocked by a capacitor causing a charge to build up on its plates until they are discharged.

Any more complicated explanation can wait until later.
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Old 28th Jun 2015, 12:34 pm   #6
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Default Re: Does current flow through a capacitor?

Think of something like a condom mounted centrally inside a long tube. Water flowing in one end will initially expand the condom and displace any water that was ahead of it from the other end -- up to a certain maximum amount that the apparatus will hold, anyway. The elasticity of the rubber will try to push the stored water back where it came from, when the external source of pressure is removed.
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Old 28th Jun 2015, 2:48 pm   #7
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Default Re: Does current flow through a capacitor?

And unwanted consequences can follow from dielectric/condom rupture, too
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Old 28th Jun 2015, 3:32 pm   #8
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Default Re: Does current flow through a capacitor?

I'd say Grahams answer (Station X), will cover 90% of situations where a capacitors function in a circuit has to be visualised. But with a strict interpretation of the question current should not flow through a capacitor, except for leakage current in electrolytic types.
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Old 28th Jun 2015, 4:03 pm   #9
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Default Re: Does current flow through a capacitor?

Yeah, I'd put it higher than 90%

The question was so generally phrased that I suspected a certain amount of impish mischief-making behind it and so I concocted an equally general reply.

Besides, electrons are artificially-created models designed to fit observed phenomena...

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Old 28th Jun 2015, 4:09 pm   #10
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Default Re: Does current flow through a capacitor?

It was a simple enough question

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Old 28th Jun 2015, 4:52 pm   #11
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Default Re: Does current flow through a capacitor?

The problem with simple questions is that people often expect simple answers. To make things worse, those who can understand the answers probably don't need to ask the questions.
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Old 28th Jun 2015, 4:57 pm   #12
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Default Re: Does current flow through a capacitor?

Quote:
Originally Posted by Radio Wrangler View Post
Besides, electrons are artificially-created models designed to fit observed phenomena.
Well said, sir!
Too often, we forget that physics and maths are just a way of describing the real universe. As such, we can define our models in whatever way we choose (although there are, of course, conventions!).
When asked a question like the OP's, my response is usually 'what exactly do you want to know?'.
Sometimes there is no conclusive answer: nature isn't that convenient.
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Old 28th Jun 2015, 4:59 pm   #13
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Default Re: Does current flow through a capacitor?

Does AC even flow down a wire?

What I am getting at is that the drift velocity of electrons in a wire is sufficiently low that for a reasonable length of wire and a reasonable frequency, when you pass AC down a wire, an electron that enters one end will most likely never get to the other end, it will just come out of the first end when the polarity reverses.

My view, FWIW, is that as electrons are indistinguisable (you can't label a particular electron) then the fact that different electrons come out of the end of the wire, or from the other plate of a capacitor is irrelevant. All the effects associated with current flow are there, and it makes more sense to say that the current does flow down the wire or through the capacitor than to have some pedantic arguement as to why it doesn't
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Old 28th Jun 2015, 4:59 pm   #14
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Default Re: Does current flow through a capacitor?

I have also used the condom between two funnels to demonstrate a capacitor with a piece of card between the funnels to demonstrate a very low value capacitor , I have also made the analogy between a turbine connected to a flywheel to explain an inductor !
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Old 28th Jun 2015, 5:01 pm   #15
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Default Re: Does current flow through a capacitor?

Just because the question was simple, does not mean the answer will necessarily be simple!

Current is simply the rate at which electrons move from one place to another. The plates of a capacitor provide both a stash of mobile electrons, and room to store some more. The bigger the plates, the more electrons they can give up / accept.

A current can flow into a capacitor, as electrons flow away from one of the plates -- or out of one, as electrons flow towards one of the plates -- until there are no more spare electrons / no room for any more. And they enter the negative plate at the same rate as they leave the positive plate.

But whether that actually counts, given that the two sets of electrons are coming from different places, as a current flowing through a capacitor, is a philosophical question that has parallels with Trigger's Broom.
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Old 28th Jun 2015, 5:01 pm   #16
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Default Re: Does current flow through a capacitor?

Did you manage to get any oscillations in a parallel 'circuit' pf turbine + flywheel against funnels + rubber sheet?
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Old 28th Jun 2015, 5:04 pm   #17
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Default Re: Does current flow through a capacitor?

I've searched on the web for the answer, some say that current flows through a capacitor some say it doesn't so I'm not much further forward, Tyso Bl gave his answer in a previous post, that's the only one so far.

"When asked a question like the OP's, my response is usually 'what exactly do you want to know?'."

The answer...

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Old 28th Jun 2015, 5:06 pm   #18
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Default Re: Does current flow through a capacitor?

The answer is no.

You didn't ask for any further explanation, or specify AC or DC.
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Old 28th Jun 2015, 5:31 pm   #19
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Default Re: Does current flow through a capacitor?

Ok girls, put the claws away...

Look at it this way, if current did flow through a capacitor, it wouldn't be a capacitor, it would be a resistor, or a piece of wire, clearer now?
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Old 28th Jun 2015, 5:35 pm   #20
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Default Re: Does current flow through a capacitor?

Thanks for the answers so far, they seem to match what I found here in section 3-10:

http://www.navymars.org/national/tra.../14174_ch3.pdf

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