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Homework Help: Help, with finding electron radius for a given condition

  1. Oct 12, 2009 #1
    1. The problem statement, all variables and given/known data

    A classical view of the electron pictures it as a purely electric entity, whose Einstein rest mass energy,E = mc^2 is the energy stored in its electric field.

    If the electron were a sphere with charge distributed uniformly over its surface, what radius would it have in order to satisfy this condition? Note: Your answer for the electron's "size" isn't consistent with modern quantum mechanics or with experiments that suggest the electron is a true point particle.

    Equations :

    E = mc^2
    F = qE
    F = ma
    E_sphere = Q/ (2*pi*r*epsilon_0);
    U = 1/2CV^2
    U = qV

    I am not sure what to equate with E = mc^2 ?
  2. jcsd
  3. Oct 12, 2009 #2


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    Do you know calculus? If so you could calculate the work needed to move charge dq in from infinity to the sphere that already has charge q against the electric force repelling dq from q. Then you could integrate that to find the total work done in bringing charge q together. I think it will be W = Q^2/(4*pi*epsilon*r). This is what you set equal to m*c^2.
  4. Oct 12, 2009 #3
    If I do that then :

    r = ke^2 / (m * c^2 );

    r = 2.8 fm

    But its not correct.
  5. Oct 12, 2009 #4


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  6. Oct 12, 2009 #5
    So the answer seems to be right according to wiki, but masteringPhysics, my online h.w
    is not accepting it.

    The answer should be in form ____ fm, but its not correct? Any ideas.
  7. Oct 13, 2009 #6
    Anyone got a clue ?
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