1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Capacitor Equation

  1. Jan 20, 2013 #1
    1. The problem statement, all variables and given/known data
    Derive a relationship for Cs for a two capacitor series circuit with a resistor. Start by showing that the same charge separation q is present across the capacitor and each of the capacitors in series and that the voltage across Cs is equal to the sum of the potential differences across each capacitor.


    2. Relevant equations

    1/Cs=(1/C1)+(1/C2)

    3. The attempt at a solution

    not sure how to approach this
     
  2. jcsd
  3. Jan 21, 2013 #2

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    If they are all in series what does that say about the current?

    What is the definition of current?
     
  4. Jan 21, 2013 #3
    I tried to work it out and i was wondering if this was the correct way to show wht the question is asking for
    q1=C1V and q2=C2V
    q=q1+q2=(C1+C2)V
    CsV=q=(C1+C2)V
    Cs=C1+C2
     
  5. Jan 21, 2013 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    This would apply to parallel capacitors, but not to capacitors in series.

    The approach CWatters suggested gives you the correct equations for the charge.
     
  6. Jan 21, 2013 #5

    gneill

    User Avatar

    Staff: Mentor

    It is a series circuit, and as the problem states, "...the same charge separation q is present across the capacitor and each of the capacitors". But writing q = q1 + q2 not only assumes that the charges are different, but that the current can be different in different components of a series circuit!

    First you should argue why the charge separation should be the same on both capacitors. Then you can use that fact and the q = C*V relationship to work out the rest.

    {Note: If you happen to have a knowledge of calculus you could write the KVL equation for the circuit (Integral version) and answer all the questions of the problem from its characteristics}
     
  7. Jan 21, 2013 #6
    To show that they are equal charges q1=q2
    And
    q=Cs^-1V
    q=(C1^-1+C2^-1)V
    (C1^-1+C2^-1)V=Cs^-1V
    Cs^-1=C1^-1+C2^-1
     
  8. Jan 21, 2013 #7

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    You did not show it, you just assumed it.

    Why ^(-1)?

    How did you get that?

    Your solution will need V1, V2 in some way...
     
  9. Jan 21, 2013 #8
    Ok

    V=q/C
    V=V1+V2
    q/C=q/C1+q/C2
    C^-1=C1^-1+C2^-1
     
  10. Jan 21, 2013 #9

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    That is correct. You can argue that q1=q2 to satisfy charge conservation, or something similar.
     
  11. Jan 21, 2013 #10
    Ok thank you
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Capacitor Equation
  1. Capacitor equations (Replies: 4)

Loading...