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Homework Help: Change in ion's electrical potential energy

  1. Feb 16, 2010 #1
    1. The problem statement, all variables and given/known data

    A Na+ ion moves from inside a cell, where the electric potential is -70 mV, to outside the cell, where the potential is 0 V.

    Uelec = ? eV

    2. Relevant equations

    I thought you would convert -70 mV to V (therefore it would be -.07) and multiply it by 1.6*10e-19 C. But it is rejecting my answer so I don't know what formula to use.

    3. The attempt at a solution
    See above.
  2. jcsd
  3. Feb 16, 2010 #2
    First off, the change in voltage is positive. To see this, use final minus initial:
    0 - (-.07) = +.07. Second, if you multiply by just 1.6e-19, your answer is in Joules. From what you've said, it seems to want eV. 1 eV is the amount of Joules required to move one electron through one volt. [U = qV = (1.6e-19 C) (1V) = 1.6e-19 J]
  4. Feb 16, 2010 #3
    Ohhh. Thank you.
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