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Determining a charge at higher potential & potential energy

  1. Jul 6, 2014 #1
    **sorry, the title should probably read determining a point at higher potential and potential energy

    So I feel like this is a really simple example, but I have a test tomorrow and this potential and potential energy stuff is really starting to not make sense...

    I know that a charge going in the direction of the electric field goes from a higher potential to lower potential and from lower to higher if it's going in the opposite direction.
    The problem is that I don't know how potential energy is different from potential.

    So a diagram I have has two points A and B and an electric field going from right to left. There is also a positive charge just sitting between A and B. And it says PE for B is bigger than PE for A. Is this just because B is at a higher potential?

    I also have a diagram that is the same, but only with a negative charge between A and B instead of a positive charge. This one I'm not really sure about.. I assumed it would be the opposite for a negative charge, but it says Vb is bigger than Va, but Ua is bigger than Ub.

    Can somebody help me with this please? The more I think about it the more confusing it gets...
  2. jcsd
  3. Jul 6, 2014 #2


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    Science Advisor
    Gold Member

    If the potential is V, then the potential energy is qV.

    That is, it needs to be multiplied by the charge. You can remember this easily if you are familiar with electron volts: eV. One eV is the energy acquired by an electron moving through a potential difference of 1 volt.
  4. Jul 6, 2014 #3
    Hm, that's something we never learned in class..
    Could you also explain the difference between what happens when you have a positive charge or a negative charge in the field?
  5. Jul 7, 2014 #4


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    Staff: Mentor

    Now you know that U = qV. If q is negative, what happens to U?

    When you have a quantity that can be either positive or negative, it's less confusing to say "higher" instead of "bigger", and "lower" instead of "smaller". If Vb = (say) 5 volts, and Va = 3 volts, then Vb is higher than Va (Vb > Va).

    However, if If Ub = -5 joules, and Ua = -3 joules, then Ub is lower than Ua (Ub < Ua).
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