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Terminal voltage for 2 batteries

  1. Oct 28, 2009 #1
    1. The problem statement, all variables and given/known data

    two batteries A and B are connected in parallel, and an 80 ohm resistor is connected across the battery terminals. the e.m.f and the internal resistance of battery A are 100V and 5ohms respectively, and the corresponding values for battery B are 95V and 3ohms respectively.


    2. Relevant equations

    find (a) the value and direction of the current in each battery and (b) the terminal voltage

    3. The attempt at a solution

    I've done part (a) but i'm stuck on part (b). i know the answer is 94.66V from looking at the back of the text book but how do i get this answer? please help!!!
     
  2. jcsd
  3. Oct 28, 2009 #2

    Delphi51

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    From (a) you know the current through the 80 ohm resister, so you can use V = IR to find the voltage on the whole parallel system including the two batteries.
     
  4. Oct 28, 2009 #3
    oh so is it the current through battery A, B and the resistor, multiplied by the resistor?
     
  5. Oct 28, 2009 #4

    Delphi51

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    The current through the resistor is the sum of the two currents from the batteries. That's the "sum of the currents into a node is zero" rule.
     
  6. Oct 28, 2009 #5
    oh i see. so the terminal voltage is the sum of the 2 currents from the batteries multiplied by the resistor?
     
  7. Oct 28, 2009 #6

    Delphi51

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    Right. You are actually calculating the voltage on the resistor, but the batteries have the same voltage because they are connected to the resistor by conductors with essentially zero resistance. No change in voltage along a wire with no resistance.
     
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