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Bare potential energy equation

  1. Oct 24, 2004 #1
    I am having a problem understanding this problem which references this exercise .

    I tried it a couple different ways. I used
    \Delta U(Potential Energy) = Uf - Ui = Uf - U(r = infinity) = Uf - 0 \\
    dU = Uf = -W = \int F * ds = \int E * Q * ds \\
    Uf = -\int E * Q * dr = -\int \frac{kqQ}{r^2} = \frac{kqQ}{r}
    I also tried using the bare potential energy equation and using two arbitrary surface areas (dA) on opposite sides of the sphere with a distance between charges of 2r , then integrating each over just half of the sphere, which provided the same result.
    The problem I have is that this result gives me a pretty decent result for the mass of an electron if I assume that q = Q = charge of an electron. But this assumption seems to say that the electron is pushing against itself by its full charge.
    That is a force [itex]\frac{QQ}{r^2}[/itex] is present instead of a [itex]\frac {(Q)(q}{r^2}[/itex] where [itex]q = \frac {Q}{n}[/itex] or some other fractional or modified charge.
    This almost makes sense to me in that if a sphere were to have a charge spread evenly over its surface and its radius is infinity, it still acts as if it has all its charge at its center and therefore would resist a decreasing radius of like charge.

    If anyone can find a way to explain why this problem makes sense I would really appreciate it. I just dont understand how the Potential energy can contain the electron's charge twice, when there is only one charge to begin with.
    Last edited: Oct 24, 2004
  2. jcsd
  3. Oct 24, 2004 #2
    Oops, I posted this in the wrong place, i'll repost in the homework section.
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