Discussion in 'General Electronics Chat' started by sharanbr, Jun 26, 2015.

1. ### sharanbr Thread Starter Active Member

Apr 13, 2009
76
1
Hello,

I would like to get some clarifications on a capacitor ...

I have uploaded a picture to explain my questions ...

1) when the capacitor is charging, I assume that electrons build on lower plate of capacitor.
What happens to the upper plate. Physically, is there an opposite of electrons?

2) what would happen if upper plate of capacitor is open or is not connected to positive terminal of supply?
Would the capacitor still charge?
Even this case, the lower plate of capacitor's voltage is lower than negative terminal of supply.
So, capacitor should charge.

3) In the figure, what would happen if instead of load we have another capacitor of same capacity?
Would the second capacitor charge half way through and then settle down?

Thanks a lot ...

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2. ### crutschow Expert

Mar 14, 2008
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1) For every electron that enters one plate, an electron has to leave the other plate (the total net charge on both capacitor plates is always zero).

2) If one capacitor terminal is open then it will not charge since there's no place for the electrons on the unconnected plate to go.

3) If you have two capacitors in parallel, then it acts like one capacitor with a capacitance equal to their sum (in this case twice the capacitance).

3. ### DickCappels Moderator

Aug 21, 2008
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This video should clear up any remaining questions.

Aug 27, 2009
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5. ### sharanbr Thread Starter Active Member

Apr 13, 2009
76
1
Dear crutschow,

Referring to the figure in the attachment, what would happen when lower terminal of capacitor is connected to ground.
Here ground cannot source positive charge on the lower plate (unlike a voltage source). How would the charge build up on the upper plate?

6. ### sharanbr Thread Starter Active Member

Apr 13, 2009
76
1
Thank you very much ...

7. ### crutschow Expert

Mar 14, 2008
16,186
4,324
It would make no difference if the lower capacitor terminal is connected to ground as long as there's still a connection to the battery or the lower battery terminal is also connected to ground.

Ground is just a reference point. It has no other special characteristics. It can carry electrons in either direction the same as a voltage source. Why do you think otherwise?

Note that positive charges do no move in conductors. Only electrons move. A positive charge on a plate is just from the absence of electrons. This leaves a net positive charge from the protons in the nucleus of the atoms.

Mar 31, 2012
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9. ### sharanbr Thread Starter Active Member

Apr 13, 2009
76
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Dear crutschow,

Thanks a lot. The answer really helps. I have a follow-up question on the last part of your answer.
Take the first figure I have attached. So, as electrons start to build up on the lower plate of the capacitor, does this result in expelling electrons from the opposite plate resulting in electrons movement back into the positive terminal of the voltage?

Aug 27, 2009
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Apr 13, 2009
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12. ### crutschow Expert

Mar 14, 2008
16,186
4,324
More or less, except the electrons don't "start to build up". Basically for every electron that goes into one plate of the capacitor, one leaves the other plate (the speed of this occurring limited only by the speed of light). The net total charge is always zero.
The same occurs in the battery.

13. ### sharanbr Thread Starter Active Member

Apr 13, 2009
76
1
Thanks a lot. I assume that both the plates should be able to add electrons (plate on which electrons are drawn from voltage source) and supply electrons also. I assume capacitor plates have ability to add electrons or deplete electrons from either of its plates ...

14. ### crutschow Expert

Mar 14, 2008
16,186
4,324
Yes. Any conductive material will do that.