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!

Equivalent capacitance A hard one?

  1. Oct 29, 2011 #1
    Equivalent capacitance... A hard one!?

    1. The problem statement, all variables and given/known data
    Calculate Equivalent capacitance between A and B:
    [PLAIN]http://scsupport.org/moodle/file.php/13/Chapter26/imgch26q14.gif [Broken]
    EDIT: If you can't see the picture, see 3rd post for file attachment, sorry!


    2. Relevant equations
    C1 = 3.55 µF
    C2 = 1.90 µF


    3. The attempt at a solution
    I know how to solve for equivalent capacitance with capacitors in series and parallel, but not both at the same time! Unless I'm seeing the problem wrong this is a complicated problem that I have never seen before! My mind is blown...

    Any help at how to approach this would be greatly appreciated!
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 29, 2011 #2

    Borek

    User Avatar

    Staff: Mentor

    Re: Equivalent capacitance... A hard one!?

    Hard to help without seeing a picture - but usual approach is to simplify the task by finding those that can be treated as just parallel or just in series, calculating their equivalent capacitance - and then treat them as one capacitor.
     
  4. Oct 29, 2011 #3
    Re: Equivalent capacitance... A hard one!?

    Shoot, is the picture not showing up for you guys? It must've been cached in my browser and not visible to you guys since it requires a login. Diagram attached to this reply.

    C1 = 3.55 µF
    C2 = 1.90 µF
     

    Attached Files:

    Last edited: Oct 29, 2011
  5. Oct 29, 2011 #4

    cmb

    User Avatar

    Re: Equivalent capacitance... A hard one!?

    If you know how to do it as resistors, then treat each as a resistor with value 1/(k.C) if you prefer.

    Once you have a value for that 'resistance', 'X', reconvert it to C ( = 1/k.X)
     
  6. Oct 29, 2011 #5
    Re: Equivalent capacitance... A hard one!?

    I suggest redrawing the circuit more symmetrically for yourself to reviel the role played by the 8 uF capacitor that is neither in series nor parallel with any other capacitor.
     
  7. Oct 29, 2011 #6

    gneill

    User Avatar

    Staff: Mentor

    Re: Equivalent capacitance... A hard one!?

    You could use a Delta-Y transformation on one of the Δ configurations and then proceed as usual to identify parallel and serial opportunities.

    Or you could slap a voltage source V between a and b, creating a third loop, and solve the mesh equations for the current being driven by that source. The impedance of the circuit is then V/I. I'd start by assigning symbols Z1, Z2, Z3 to the capacitor impedances and finding a symbolic result... you may find that the result has an obvious form that you can exploit.
     
  8. Oct 29, 2011 #7
    Re: Equivalent capacitance... A hard one!?

    Applying KVL to the circuit, could something be said about Vc in relation to V1 and V2 (Vc is the voltage across the 8µF capacitor)? Looking at several different routes, I think I found a result that would massively simplify this problem.
     
  9. Oct 29, 2011 #8

    gneill

    User Avatar

    Staff: Mentor

    Re: Equivalent capacitance... A hard one!?

    Could be! :wink:
     
  10. Oct 30, 2011 #9
    Re: Equivalent capacitance... A hard one!?

    I'm not exactly sure how to apply KVL to the circuit because there's no voltage source.
     
  11. Oct 30, 2011 #10

    gneill

    User Avatar

    Staff: Mentor

    Re: Equivalent capacitance... A hard one!?

    For purposes of analysis it's perfectly acceptable to add one as long as it doesn't change the results. You can assume a voltage Vab is applied across terminals a and b. It won't change the impedance.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Equivalent capacitance A hard one?
  1. Equivalent capacitance (Replies: 7)

  2. Equivalent capacitance (Replies: 4)

  3. Equivalent Capacitance (Replies: 6)

  4. Equivalent Capacitance (Replies: 7)

Loading...