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Circuit with 2 batteries is confusing me

  1. Nov 7, 2007 #1
    1. The problem statement, all variables and given/known data
    The circuit in the figure is composed of two batteries (E1 = 6 V and E2 = 4 V) and three resistors (R1 = 4 , R2 = 4 , R3 = 5 ) as shown.

    (a) What is the current I3 flowing through the batteries?

    I3 = .286A (no problem here)

    (b) What is the voltage Vb - Va between the points a and b in the circuit?

    Vb - Va = V

    So i combined the parallel Rs to get R1 + R2 = 2ohms
    So voltage in is 2V right?
    Then voltage on other side of Req should be .579V right? (because I=.286A)
    so then 2V - .579V = 1.43 = wrong answer
    please help!

    thanks


    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Nov 7, 2007 #2

    Doc Al

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    Staff: Mentor

    Good.
    How did you deduce this? You have the resistance and the current.
    Do you mean the voltage across R3? What's the current through R3?
     
  4. Nov 7, 2007 #3
    because picture says current is going cw, so 1st batt is -4V and 2nd batt is 6V = 2V
    so 2V enters Req.
    Req = 2ohms
    and total current in the circuit is .286A
    so V=IR = .286 X 2ohm = .579V
    2V - .579V = 1.421V
     
  5. Nov 8, 2007 #4

    Doc Al

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    Staff: Mentor

    Much better. (Unless the reference point is very clear, it's always a good idea to specify the voltage difference between two points or across something.)
     
  6. Nov 8, 2007 #5
    1.421V is wrong answer though, so can u please tell me what im doing wrong?
    thanks
     
  7. Nov 9, 2007 #6

    Doc Al

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    Staff: Mentor

    You did the work correctly, but you're getting confused about which voltage drop is the answer to the question.
    This is the equivalent resistance between points A and B.
    This is the total current going through both R3 and between points A and B.
    That's the voltage drop across A and B. That's the answer to the question.
    That's the voltage across R3.
     
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