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Homework Help: Change in electric potential energy

  1. Feb 13, 2010 #1
    1. The problem statement, all variables and given/known data

    A tire of radius 32.5 cm has a surface that is conducting. A charge of 4.89×10-8 C is then placed on the surface. The potential on the balloon's surface is 1.35E+03 V. Later, suppose that some of the air is let out of the tire, so that the radius shrinks to 10.5 cm, the new potential on the balloon's surface is 4.19E+03 V.

    What is the change in electrical potential energy during the shrinking process?

    2. Relevant equations

    Change in electric potential energy = q (change in electric potential)

    3. The attempt at a solution

    [tex]\Delta[/tex]U = q([tex]\Delta[/tex]V) = (4.89e-8C)(4190J/C-1350J/C) = 1.4e-4 J

    But that doesnt work.
  2. jcsd
  3. Feb 13, 2010 #2
    anyone have a clue, or a hint
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