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Homework Help: Electric circuits, finding current with terminal voltage

  1. May 19, 2013 #1
    1. What current flows through the 11.0 Ω resistor?

    a) 0.21 A
    b) 0.27 A
    c) 0.93 A
    d) 1.2 A

    To summarize:

    V1 = 8.0 V
    V2 = 5.0 V
    R1 = 2.0 Ω
    R2 = 11.0 Ω
    R2 = 1.0 Ω

    I know we shouldn't attach pictures, but the diagram does make the question a lot easier to understand.

    2. Relevant equations

    VT = ε-Ir

    3. The attempt at a solution

    RT = 11.0 + 2.0 + 1.0 = 14.0 Ω
    VT = 8.0 + 5.0 = 13.0 V
    IT = V/R = 13.0/14.0 = 0.93 A.

    And since current should be the same throughout a series circuit, the current through the 11.0 Ω resistor should be 0.93 too. My answer is wrong though.

    Correct answer: a) 0.21 A, but I don't understand why. Please help me?

    Attached Files:

    Last edited: May 19, 2013
  2. jcsd
  3. May 19, 2013 #2
    You are close but notice that the battery's oppose each other
  4. May 19, 2013 #3
    Yes, but what affect does that have? Does it mean the current is different in different parts of the circuit?

    Am I doing the VT and the RT correctly?
  5. May 19, 2013 #4
    If you write the kirchoff voltage law around the loop, you will find that the battery voltages will subtract rather than add as you have done.
  6. May 19, 2013 #5
    So are you saying VT= 8.0-5.0 = 3.0 V?

    But why would that happen? Kirchoff's Loops Rule just states that potential differences around a complete loop is equal to zero. Wouldn't both batteries create a positive potential change though?
  7. May 19, 2013 #6
    Check the schematic diagram, the batteries do not add, they will subtract.
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