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Measurement of unknown resistance using potentiometer

  1. Jul 20, 2015 #1
    [Mentor's Note: Thread moved to homework forums from another forum]

    Figure shows a potentiometer circuit for comparison of two resistances. The balance point with a standard resistor R = 10.0 Ω is found to be 58.3 cm, while that with the unknown resistance X is 68.5 cm. Determine the value of X.
    upload_2015-7-20_14-56-35.png
    In the solution of this problem every book using this method:-
    upload_2015-7-20_15-3-49.png
    my question is how can we take equal i for each resistance?
     

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    Last edited by a moderator: Jul 20, 2015
  2. jcsd
  3. Jul 20, 2015 #2

    NascentOxygen

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    Staff: Mentor

    We've seen this arrangement presented before, and for a time it had us purplexed. I believe the conclusion was that the lower-drawn source ε is actually a voltage source having significant resistance. So mark in some fixed internal resistance there, and re-examine the question.

    Does your textbook give an explanation of the circuit and its operation, and mentioning that source having internal resistance? Without the recognition of that internal resistance, the circuit makes no sense.
     
  4. Jul 20, 2015 #3
    i copied the whole question from text book. nothing was told about internal resistance. in general we have to take an arbitrary internal resistance here.
     
  5. Jul 20, 2015 #4

    NascentOxygen

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    Yes, arbitrary, but not zero.
     
  6. Jul 23, 2015 #5
    ok if there is a non-zero internal resistance, then what about the current through resistor R and X? are they equal? why? please answer it. I need it very badly.
     
  7. Jul 23, 2015 #6

    NascentOxygen

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    R and X are different valued resistances, so when separately connected to source ε will have different currents.
     
  8. Jul 23, 2015 #7

    NascentOxygen

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    In your first post you provided an "explanation" of how the arrangement works. I don't understand that description.
     
  9. Jul 23, 2015 #8
    So you agree that the solution given above is wrong right?
     
  10. Jul 23, 2015 #9

    NascentOxygen

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    Staff: Mentor

  11. Jul 24, 2015 #10
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