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Power transformed when resistance is zero and infinite

  1. Oct 18, 2015 #1
    1. The problem statement, all variables and given/known data
    image.jpg
    image.jpg
    2. Relevant equations
    P=IV,
    =I2R
    =V2/R

    3. The attempt at a solution
    For zero resistance, I used P=V2/R formula, and sub. R=0 , power would be infinite. But if I sub. Into P=I2R, power will be zero. The correct answer should be zero. But why do we need to use the second relevant equation I typed above?
    For infinite resistance, I used P=V2/R and so will result in power tending to zero.
    I am confused on when to we use which equations to calculate power
     
  2. jcsd
  3. Oct 18, 2015 #2

    mfb

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    Both R and V go to zero, so you have "0/0" which is undefined. You cannot use this formula.
     
  4. Oct 18, 2015 #3
    Oh I see.. Thanks
     
  5. Oct 18, 2015 #4
    But if resistance is infinite, then shouldn't voltage be infinite too?
     
  6. Oct 18, 2015 #5

    mfb

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    No. Why do you expect this?
     
  7. Oct 18, 2015 #6
    Because V=IR, V α R, though I feel that it doesn't make sense to have an infinite voltage..
     
  8. Oct 19, 2015 #7

    CWatters

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    Remember we are talking about the voltage drop across Q.
     
  9. Oct 19, 2015 #8

    mfb

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    V α R is only true at fixed current.
    What is the current you expect for infinite resistance?
    And, as cross-check, can the voltage at the resistor exceed the source voltage?
     
  10. Oct 22, 2015 #9
    Zero current :biggrin:
    Oh yeah.. Never thought of that too, thanks!
     
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