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Homework Help: Terminal voltage and electromotive force

  1. Apr 26, 2007 #1
    Hi everybody!

    I'm a physics-interested guy from Norway, who have a question. We don't have any such exellent forums in Norway. :-) I hope you understand my english by the way. :-)

    1. The problem statement, all variables and given/known data
    We have a battery with internal resistance 2 Ohm. The voltmeter shows 9 volt when no current goes through the circuit (switch open). (Then, the electromotive force is 9 volt, right?)
    When this switch is closed, i.e. current goes through the circuit, the voltmeter shows 7,8 volt. In this circuit we therefore have a resistance with 13 ohm.

    If we now have two resistances (13 ohm and 30 ohm) switched parallell (do you understand my english. :-) ). (Then we have a resultant resistance 9,07 ohm, right?)

    Then the questions:

    1. What shows the voltmeter now? (we need to find the terminal voltage, right?)

    2. What's the current through the resistance with 13 ohm?

    2. Relevant equations

    3. The attempt at a solution

    1. According to my calculations, the terminal voltage now is 7,4 volt.

    2. The current through resistance 13 ohm: 0,567 Ampere.

    But I don't know whether this is right?

    Hope I'll get answer from some physics genius!! :-)
  2. jcsd
  3. Apr 26, 2007 #2


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    I think you've got it. I get SLIGHTLY different answers, but then you are rounding your numbers somewhat differently in different places.
  4. Apr 26, 2007 #3
    Ah!! Good to hear! :-)
    To be exact: I calculated the terminal voltage (question 1) to 7,374. Did you get the same answer?
  5. Apr 26, 2007 #4


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    No. I got 7.373949579831933. :wink:
  6. Apr 26, 2007 #5
    Heh. :-)
    Thank you very much by the way!
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