1. Not finding help here? Sign up for a free 30min 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!

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



    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:

  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

    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.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?