Why are the charges on capacitors A and B the same, and C and D equivalent?

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Homework Help Overview

The discussion revolves around understanding the behavior of charges on capacitors A and B, and the equivalence of charges on capacitors C and D, particularly in the context of series connections. The subject area pertains to circuit theory and capacitor behavior.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the reasoning behind the equal charges on capacitors in series, discussing the implications of current flow and charge distribution. Questions arise regarding the initial conditions of the capacitors and the role of the battery in charge movement.

Discussion Status

Multiple perspectives on the charge behavior of the capacitors have been presented, with participants acknowledging that different explanations can coexist. There is an exploration of how charge is transferred and redistributed in the circuit, but no consensus has been reached on which explanation is preferable.

Contextual Notes

Some participants mention the possibility of unusual setups affecting charge distribution, and there is a focus on the assumption that the wire connecting the capacitors was initially neutral.

maiad
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http://capa.physics.mcmaster.ca/figures/sb/Graph26/sb-pic2654.png
This is more of a concept understanding if anything but I'm interested to know why the charge on capacitors A and B are the same and similarly, the charge on capacitor C and D are equivalent.
 
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maiad said:
http://capa.physics.mcmaster.ca/figures/sb/Graph26/sb-pic2654.png
This is more of a concept understanding if anything but I'm interested to know why the charge on capacitors A and B are the same and similarly, the charge on capacitor C and D are equivalent.

Capacitors A and B are connected in series, as are capacitors C and D. Components in series always carry the same current -- so any charge that moves into or out of capacitor A must also move into or out of capacitor B. Similarly with capacitors C and D.
 
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Yes they are, unless it is a weird setup where one had a charge before they were connected. You can see the reason easily. The wire between Ca and Cb was originally neutral. When a current flows into that branch, it cannot flow through a capacitor because of the gap between the plates. So it just separates the charge on the wire between so a +q appears on one capacitor and a -q on the other, keeping the whole wire neutral as it must be.
 
Yours seem more reasonable so does that mean gneill's response is incorrect? You explanation would also explain why The charge in A and B is simpliy Q=C(eq)ΔV
 
Both explanations are correct. They are just different ways conceptually of looking at the situation.
 
Another good way to look at it is from the point of view of the battery. The + of the battery is connected to the plate on the left (of Cc) and the - of the battery is connected to the plate on the right (of Cd). These are the only plates connected to the battery.
The battery transfers -charge from the left plate to the right plate. This is the only charge that flows round the external circuit, through the battery. The other + and - charges you see on the capacitors are just charges that have been re-distributed, they have not come from 'outside'.
It does not matter how many capacitors are in series. The charge on each one is the same and only one amount of charge has passed from the battery.
 

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