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Homework Help: Determining EMF and Internal Resistance

  1. Nov 8, 2009 #1
    Basically I have been given this question:

    A battery of unknown EMF and internal resistance is connected in series with an ammeter and a resistance box. The current was 2.0A when R= 4.0 Ohms and 1.5A when R= 6.0 Ohms. Calculate the EMF and internal resistance.

    I feel you may have to arrange this somehow although I am probably wrong:

    Internal resistance = v1-v2/I2-I1

    I have had no attempt at a solution as I am utterly confused by this question.

    Thank you for reading.
  2. jcsd
  3. Nov 8, 2009 #2


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    Homework Helper

    Hi joseppi, welcome to PF.
    You have to write two equations for two resistances.
    Use the equation
    I = E/(R + r). Here r is the internal resistance.
    Then solve for r.
  4. Nov 8, 2009 #3
    Hi, sorry for me not having understood your instruction but how can I use the below equation for the answer if I have neither the: r or the E value?

    I = E/(R + r)

  5. Nov 8, 2009 #4
    E in this case is the emf, which you can work out with Ohm's Law.
  6. Nov 8, 2009 #5


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    You can write two equations for two resistances. With two equations you can find two unknowns.
    I1 = E/(R1 + r)-------(1)
    I2 = E/(R2 + r)-------(2)
    Substitute the values given in the problem. Try to eliminate one unknown to find the other unknown.
  7. Nov 8, 2009 #6
    2A = ?E/ (4.0 Ohms + ?r)

    1.5A = ?E/ (6.0 Ohms + ?r)

    I'll be honest I am quite bad with Maths even though I take AS level physics. I have no clue how to eliminate the r or E unknown or even really how to move it around ;/ You couldn't explain how and why you do this? It would be a great help, thank you.
  8. Nov 8, 2009 #7


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    From the first equation
    E = 2A(4.0 Ohm + r)----(1)
    Similarly from the second equation write E = ........---(2)
    Equate eq 1 and 2, and solve for r.
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