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Does internal resistance affect balance point?

  1. Feb 6, 2015 #1
    1. The problem statement, all variables and given/known data
    if a potentiometer has driver cell and wire AB ,then an unknown emf (emf<driver cell) is balanced at some length L. does the balance point change if internal resistance of driver cell is changed?

    2. Relevant equations

    V= kl K= potential gradient

    3. The attempt at a solution
    if we increase the internal resistance ,the potential drop across wire AB should decrease and so balance point must shift.. But the solution says it will not change.

    Untitled.jpg
     
  2. jcsd
  3. Feb 6, 2015 #2

    ehild

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    You are right, the balance point will shift if you change the internal resistance of the driver cell. It will not depend on the internal resistance of the unknown cell
    .
     
  4. Feb 7, 2015 #3
    But in solution( i have checked in 4 books) says it will not change since we have set constant potential gradient across the wire.. and i dont even understand it
     
  5. Feb 7, 2015 #4

    ehild

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    It is wrong. The potential gradient across the wire is E/(ri+Rwire), where ri is the internal resistance of the driving source. Book solutions are wrong quite often. They certainly thought of the internal resistance of the measured cell.
     
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