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Finding the emf of a cell

  1. Nov 13, 2015 #1

    MBBphys

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    Gold Member

    1. The problem statement, all variables and given/known data
    Say we have a circuit where a cell with internal resistance r is connected to a voltmeter with extremely high resistance. This means practically all of the p.d would be across the voltmeter as the ratio of its resistance to the internal resistance is, say, infinitely high.
    So the voltmeter reading equals emf of cell?

    Is this reasoning correct?
    Thanks!

    2. Relevant equations
    (V1/V2=R1/R2)

    3. The attempt at a solution
    (n/a)
     
  2. jcsd
  3. Nov 13, 2015 #2
    I can't tell from what you've written. Which part is the question, which part is the answer, and which part is the reasoning.
     
  4. Nov 13, 2015 #3

    MBBphys

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    Well, I am saying that, for the circuit described, the voltmeter reading will equal approximately the emf of the cell. Is this right? Thank you!
     
  5. Nov 13, 2015 #4
    What is your reasoning for that answer?
     
  6. Nov 14, 2015 #5

    MBBphys

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    Well, I thought that if the voltmeter reading was infinitely high:

    We know:

    emf = terminal voltage - (current * internal resistance)
    Hence, if we increase the resistance of the load by putting a voltmeter with infinitely high resistance, current will be zero, so (current * internal resistance) will be zero, so the p.d. across the internal resistance would be zero, so the terminal voltage would equal the emf, and as the voltmeter reads the terminal voltage, the voltmeter reading will equal the emf.

    Is that right? Thanks
     
  7. Nov 14, 2015 #6

    CWatters

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    Correct.
     
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