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Potentiometer circuit

  1. Oct 2, 2015 #1
    1. The problem statement, all variables and given/known data
    image.jpg

    2. Relevant equations


    3. The attempt at a solution
    The answer is B. But I thought E1 needs to be be known first in order to find E2... And why not the resistance of the wire XY?

    E1/ Rxy = E2/ Rxt

    Then, in order to find Rxt, we need E1, Rxy and E2?
     
  2. jcsd
  3. Oct 2, 2015 #2

    gneill

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    Staff: Mentor

    Suppose that you know E2. What can you say about the potential at T with respect to X?

    Note that you're looking for the potential difference per unit length, not the resistance or resistance per unit length.
     
  4. Oct 2, 2015 #3
    E1-E2?
     
  5. Oct 2, 2015 #4

    gneill

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    No. E2 is not connected to point T.
     
  6. Oct 2, 2015 #5
    I'm confused .. Why isn't E2 connected to T?
     
  7. Oct 2, 2015 #6

    gneill

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    Because there's the lengths XT and TY of resistive wire between E2 and T. There is some (unknown) current running through those lengths, causing potential differences.
     
  8. Oct 2, 2015 #7
    Okay.. So the p.d. across XT is XT/XY x E1?
     
  9. Oct 2, 2015 #8

    gneill

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    No. Forget about E1 for now; you don't have a value for it. What you are to assume is that you know E2, and that the galvanometer reading is zero.
     
  10. Oct 3, 2015 #9
    Potential at T is equal to E2
     
  11. Oct 3, 2015 #10

    gneill

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    Right! Note that it doesn't matter what E1 is. As soon as the galvanometer shows zero current, the potential at point T must match E2.

    Can you now work out the potential difference per unit length of the wire XY?
     
  12. Oct 3, 2015 #11
    Yeah! It's E2 divide by the length XT. Thanks !
     
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