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I have a simple circuit as in the picture below. My question is why the negative charge -Q at the bottom plate of C1 moves and neutralizes with the positive charge +Q at the top plate of C2?

enter image description here

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    \$\begingroup\$ The charges in the green circle are the same and should not be written as +/-Q. \$\endgroup\$ – Andy aka Dec 19 '16 at 10:41
  • \$\begingroup\$ When one plate is charged to +Q, should the other plate also be charged to -Q? \$\endgroup\$ – anhnha Dec 19 '16 at 10:43
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    \$\begingroup\$ The two inner plates in the green circle are at the same potential so there is nothing to move charges. \$\endgroup\$ – JIm Dearden Dec 19 '16 at 11:49
  • \$\begingroup\$ Agree, but new problem now. Why there is no potential difference when there is a charge separation? \$\endgroup\$ – anhnha Dec 19 '16 at 11:50
  • \$\begingroup\$ electric field. \$\endgroup\$ – Jasen Dec 19 '16 at 18:22
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The area inside the green circle is electrically isolated from the rest of the circuit so there is always the same amount of charge inside the circle.

What charge flows into the top of C1, the same amount flows out of the bottom of C1 and into the top of C2, the same amount flows out of the bottom of C2, this is how capacitors work (or at-least a very good approximation)

Why does it move? Electric potential pushes it..

See also: Kirchoff's loop law

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  • \$\begingroup\$ I may not express my question well enough. What I am confused is what keep the +Q and -Q in the green circle separated from neutralization? \$\endgroup\$ – anhnha Dec 19 '16 at 10:46
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    \$\begingroup\$ They are separated because of attraction to the -Q and +Q outside the circle. \$\endgroup\$ – Jasen Dec 19 '16 at 10:48
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If you removed the top plate of the top capacitor and the bottom plate of the bottom capacitor, that's exactly what would happen. However, the presence of those charged plates creates an electric field that keeps the charges separated.

For the negative charge at the top capacitor to flow down, it would have to get away from the positive charge at the top plate of the top capacitor, which attracts it very strongly. Ditto for the bottom capacitor.

In fact, the attraction between the opposite charges at the two plates of a capacitor are exactly what makes it work - otherwise, you wouldn't be able to keep a lot of charge together.

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The charges in the green circle will neutralize. It is impossible for them not to neutralize. The forces pulling them apart and the forces pulling them together are the same. The forces balance each other out and they will neutralize.

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