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In the picture below, there are two capacitors right next to each other in an open circuit (ie the wire with V2 written above it does not go to ground). Assume that each capacitor had been previously 'charged' up so that they have different amounts of charge on them (each was charged in a different circuit). Then, in the scenario below, would the capacitors be "in series"? I'm asking because I made the assumption that the charge on both capacitors would be the same (I thought they were in series) and I got the problem wrong (the solution did not make the assumption that the charges would be the same).

So, essentially, what I'm asking is when can we assume two capacitors are in series and why are they not in series in the image below? Also bonus question if anyone can answer: the solution said that no charge would leave either of the capacitors. Why is this so? If one capacitor has an excess of electrons (like the left plate of C2) why don't they migrate to a capacitor with a deficiency of electrons (like the right plate of C1).

Circuit described above

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  • \$\begingroup\$ they are connected in series. Why would they not be in series? Your assumption being wrong has nothing to do with them being in series after having been disconnected from the circuit they were charged in. \$\endgroup\$ Nov 2, 2019 at 0:21

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So, essentially, what I'm asking is when can we assume two capacitors are in series and why are they not in series in the image below?

They are in series.

Only your conclusion that they have the same charge now that they are in series is wrong. Why would they? They've got charged somewhere else, and since the charge on the rightmost plate of the capacitor can't go anywhere, things stay like they were.

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  • \$\begingroup\$ Just mulled over this problem for like 2 hours and finally got it (really funny how such a simple thing can take so long for your mind to figure out.) Thanks for the answer and the insight about the charge on the rightmost plate not being able to go anywhere \$\endgroup\$ Nov 2, 2019 at 0:46
  • \$\begingroup\$ If it helps, when capacitors are in series the amount that their charge changes must be the same. So if they start out with zero charge (which is a very common assumption) then the charge is the same. But if their starting charges are different, then (as you now know!) their ending charges will be different. \$\endgroup\$
    – TimWescott
    Nov 2, 2019 at 1:04

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