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Circuit problem homework

  1. Mar 30, 2013 #1
    1. The problem statement, all variables and given/known data


    http://i.imgur.com/ij4eHqM.png?1?5000

    2. Relevant equations

    KVL, KCL, Ohm's Law, I guess


    3. The attempt at a solution

    Tried node analysis but I get 4 equations in 5 variables, so no specific solution. I don't think the circuit can be simplified either. A friend suggested mesh analysis but I don't think that works here because we have loops but they aren't meshes. I'd like just a hint in the right direction please, perhaps what approach I should try, I'd still like to do most of it myself. Thanks :)
     
  2. jcsd
  3. Mar 30, 2013 #2

    gneill

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    Staff: Mentor

    Consider that no current can flow across the open circuit where Vo is measured. Thus you have a current divider situation for the two branches, and you should be able to determine the currents and thus the individual potentials for Va and Vb:
    attachment.php?attachmentid=57295&stc=1&d=1364705207.gif
     

    Attached Files:

  4. Mar 31, 2013 #3
    Thank you, I managed to get the right answer with that help. Physically, what happens in terms of the resistors at the open terminals + and - of v0 once the current is turned on?

    I imagine that initially (for a brief amount of time) we have current flowing one-way into both open terminals, where charge builds up until the terminals reach the same potential as at Va and Vb, respectively. At that point current stops flowing, so there's no drop across the terminal resistors (2k and 4k) and the terminal potentials are simply equal to Va and Vb? Is this right?
     
  5. Mar 31, 2013 #4

    gneill

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    Staff: Mentor

    Yes, that's it for 'real' components. Ideal components have no physical size, so the transient time would be infinitesimally short.
     
  6. Mar 31, 2013 #5
    You're a champion, thanks for your help :)
     
  7. Mar 31, 2013 #6
    Actually one more question; what would be the final potential inside each resistor in a realistic and ideal circuit? Are they just at the same potential as the connecting wire?
     
  8. Mar 31, 2013 #7

    gneill

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    Staff: Mentor

    Yup. Same potential. No current.
     
  9. Mar 31, 2013 #8
    Thanks :)
     
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