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Potential energy in a battery

  1. Nov 29, 2015 #1
    1. The problem statement, all variables and given/known data
    An electron moves from the positive to the negative terminal of a battery (9V). How much potential energy did it gain or lose?

    2. Relevant equations

    3. The attempt at a solution
    Solving for the amount is basic. That's just qV = 14.4 * 10^-19 J
    I'm not sure whether it will gain the energy or lose it.
    I believe that since the positive terminal is at a higher potential, a positive charge would lose energy, and therefore, the electron will gain energy. But the answer at the back says that it would lose energy. I'm not sure what's wrong.
  2. jcsd
  3. Nov 29, 2015 #2
    Wouldn't it take work to move an electron away from a positive charge?
  4. Nov 29, 2015 #3
    The more general relation is ##\Delta U=q \ \Delta V## where ##\Delta V## is the change in potential. If ##q## and ##\Delta V## have different signs, then ##\Delta U## is negative. If they have the same sign then ##\Delta U## is positive.
  5. Nov 29, 2015 #4


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    you are starting with an incorrect assumption
    try again
  6. Nov 29, 2015 #5


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    No he isn't. The problem statement ask what happens if an electron moves from positive to negative. That occurs when a battery is being charged.
  7. Nov 29, 2015 #6


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    we shouldn't nitpick :wink:
    he didn't say what if ... he stated it as a fact :wink:

    this isn't likely to happen as it goes against the current flow

    Noting that he hasn't stated if this is internal or external to the battery ... the norm being an external reference
    battery charging also wasn't mentioned

    To the OP
    you need to write a clearer idea of what you are referring to, to stop the ambiguity :smile:

  8. Nov 29, 2015 #7
    The OP didn't state this, he's just trying to understand the statement he was given and answer a question about it. It makes no difference how the electron gets there. The positive terminal is at a potential of +9 volts relative to the negative terminal. An electron in the latter location has how much potential energy relative to an electron in the former location? That's an equivalent question.
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