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Homework Help: Internal Resistance and Circuits

  1. May 11, 2005 #1
    Hey, I'm revising for my summer exams, and gotten stuck on this particluar question.

    _______ 6V ______2.0 ohms___
    I I
    A I I
    I I
    I_____4.0 ohms______________ I
    I C I
    I I
    B I I
    I I
    I________1.5V_______________I

    SORRY I HOPE THIS CIRCUIT IS READABLE

    Question

    The 6V cell has a 2ohm internal resistance...The 1.5V cell has a 1ohm internal resistance. Work out the currents A, B and C!

    I have split the circuit into two series circuits to make it easier for me

    okay....as V=IR

    6V = (C x 4ohms) + (A x 2ohms) + (internal resistance - A x 2ohms)
    6V = (C x 4) + (A x 4)

    ......and for the other series circuit

    1.5V = (C x 4ohms) + (internal resistance - B x 1ohm)
    1.5V = (c x 4) + B


    errr....now i'm stuck :cry: ....have i made a mess of trying to work this out....or do i need to use simultanous equations to work it out. I haven't a clue!

    Please Help!

    Thanx :smile:

    James
     
  2. jcsd
  3. May 11, 2005 #2
    okay it didn't come out that great....

    current A is across the 2 ohm resistor, and 6V cell
    current B is across the 1.5V cell
    current C is the junction of A and C across the 4 ohm resistor.

    I kno i may have to use Kirchhoff's laws! but not sure how

    hope this helps if your as confused as i am! heehee :rofl:
     
  4. May 11, 2005 #3

    Doc Al

    User Avatar

    Staff: Mentor

    Looks good to me. But you need one more equation relating the three currents: A + B = C.

    Now you have three equations and three unknowns. Solve!
     
  5. May 11, 2005 #4
    okay......

    6V = 4C + 4A
    6V = 4C + B

    combining the equations together i get:

    B = 4 x A

    i can't seem to be able to get any values for either A or B, to be able to work out the current of C using Kirchoff's first law!

    Is this problem actually solveable?
     
  6. May 11, 2005 #5

    Doc Al

    User Avatar

    Staff: Mentor

    First equation is OK. Second should be:
    1.5 = 4C + B

    But don't forget the third equation:
    A + B = C.

    Solve it systematically. Start by using the last equation to eliminate C from the first two.
     
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