Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Circuits and capacitance

  1. Jan 30, 2012 #1
    http://capa.physics.mcmaster.ca/figures/sb/Graph26/sb-pic2654.png [Broken]
    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.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jan 30, 2012 #2


    User Avatar

    Staff: Mentor

    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.
    Last edited by a moderator: May 5, 2017
  4. Jan 30, 2012 #3


    User Avatar
    Homework Helper

    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.
  5. Jan 30, 2012 #4
    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
  6. Jan 31, 2012 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    Both explanations are correct. They are just different ways conceptually of looking at the situation.
  7. Jan 31, 2012 #6
    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook