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Homework Help: EMF's in series how does this work ?

  1. Jul 4, 2008 #1
    1. The problem statement, all variables and given/known data

    A secondary cell having an e.m.f. of 2V and an internal resistance of 1ohm is connected in series with a primary cell having an e.m.f. of 1.5V and an internal resistance of 100ohm the negative terminals of each cell is connected to the positive terminal of the other cell. A voltmeter having a resistance of 50ohm is connected to measure the terminal volatage of the cells. Calculate the voltmeter reading and the current in each cell.

    Ans given by book:72.8mA, 34.3mA, 1.93V

    2. Relevant equations
    Kirchoff voltage law.

    3. The attempt at a solution

    I can not understand how 2 cells in series can have a different current ? What concept am I missing ?
  2. jcsd
  3. Jul 4, 2008 #2
    The current will change according to which cell voltage you are measuring.

    The voltmeter has a resistance of 50R... (that's some "voltmeter"...)
  4. Jul 4, 2008 #3


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    Homework Helper

    Is there a picture for the question? I'm not sure how the voltmeter is connected to measure the terminal voltage.
  5. Jul 4, 2008 #4
    Sorry no picture because the textbook has no picture only the words
  6. Jul 4, 2008 #5
    The book answer gives one voltage and 2 currents.
  7. Jul 4, 2008 #6


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Instead of thinking about 2 batteries in series, try picturing the situation as 3 objects all in parallel (the 2 cells and the 50-ohm resistor).

    Don't forget to include the internal resistances as part of the cells.
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