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# capacitors; explain how the plates acquire equal and opposite charge Watch

1. Describe it in terms of the movement of electrons.

I wrote "Electrons move from the battery to the Y plate, making it negatively charged" I got one mark for that. But then I wrote "The X plate is repelled by this, making it positively charged." I got no marks for that.

The question is out of two marks. Could someone explain how I would get the other mark?
2. The electrons are flowing off the +ve plate at the same time that electrons are flowing onto the -ve plate.

If you were to connect ammeters to your capacitor leads while you were charging it to measure the flow of electrons, you'd see the same magnitude and direction of current on both sides at the same time.

When it's fully charged there is no current of course.
3. (Original post by Voltozonic)
I'm not sure what you mean by the X-Plate between repelled, sounds quite misleading.

When the battery is connected, the electrons being negatively charged are repelled by the negative terminal and build up on the Y plate causing it to be negatively charged.

The electrons on the X plate are attracted to the positive terminal of the battery and so there essentially comes a point where the X plate has more protons on it than electrons, hence it is positively charged.
So my model answer should be "Electrons are repelled from the negative terminal onto the Y plate, making it negatively charged. The electrons on the X plate are attracted to the positive terminal, making it positively charged."

Also when I said the X plate is being repelled, well since the Y plate has the repelled electrons from the negative terminal, then that plate should also repel to electrons on the Y plate.
4. (Original post by Joinedup)
The electrons are flowing off the +ve plate at the same time that electrons are flowing onto the -ve plate.

If you were to connect ammeters to your capacitor leads while you were charging it to measure the flow of electrons, you'd see the same magnitude and direction of current on both sides at the same time.

When it's fully charged there is no current of course.
Still doesn't explain how one plate becomes positively charged whilst the other negatively.
5. (Original post by Voltozonic)
Yes.

And to answer your second post, it does explain why it become positively charged, because the X plate loses it electrons leaving the overall charge density on the plate positive.

Think of rubbing a glass rod with silk. You rub the electrons off the rod and so the rod wants these electrons back to aquire a neutral charge. If the rod wants to gain electrons (which are negative) itself as become positively charged.

It's the same with the X-Plate of the capacitor. If electrons leave the plate, it aquires a net positive charge because it's nature now is to want electrons to balance the charge distribution.
Ok, I understand the concept now, thanks!

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Updated: April 18, 2013
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