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Homework Help: E.M.F and internal resistance

  1. Mar 2, 2014 #1
    1. The problem statement, all variables and given/known data

    A cell of unknown e.m.f. , ε , and internal resistance, r, is connected in series with a
    variable resistor. A voltmeter is then connected across the terminals of the resistor.
    When the resistor has a value of 16.0 Ω the voltmeter reads 1.20 V. When the
    resistance is reduced to 8.0 Ω the voltmeter reading falls to 1.00 V.

    Calculate the e.m.f. and internal resistance of the cell

    2. Relevant equations

    emf = v + vr
    emf = v + ir
    (r = internal resistance)

    3. The attempt at a solution

    I (perhaps wrongly) worked out each current for the given voltages and resistances as

    1.2/16= 0.075A
    1/8 = 0.125A

    I then drew a graph of voltage against current, (I think I may be wrong here also) to find that the e.m.f is 1.5v and the internal resistance(gradient with the two points of (current , voltage)) as -0.25.

    But I cannot seem to get calculatiosn with these to add up for instance if the emf is 1.5 then v at either point 1.2 or 1 added to current x internal resistance should equate to 1.5.

    Anyhelp would be much appreciated.
  2. jcsd
  3. Mar 2, 2014 #2


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    Homework Helper

    Currents you have calculated can be written as I = ε /( r + R ). Write two equations for two currents and solve for r.
  4. Mar 2, 2014 #3

    Hi, Thanks alot.

    I saw the other thread in which you helped https://www.physicsforums.com/showthread.php?t=352782

    I was a little unsure if the currents were correct due to the location of the voltmeter but I have discovered and you have confirmed the location doesn't matter in this case.

    Thanks again!!!
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