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Homework Help: I need the form of potential energy for a simple problem

  1. Jan 14, 2010 #1
    1. The problem statement, all variables and given/known data

    A particle of mass "m" and electric charge "q" moves without friction along an ellipse in the horizontal plane, in the presence of a constant electric field of intensity E directed along the large semiaxis of the ellipse.

    Write the Lagrange function

    2. Relevant equations

    We now, obviously the ellipse equation so we choose as general coordinates:

    X1 = a*cos(phi) X2 = b*sin(phi)

    3. The attempt at a solution

    L = T - U;

    T = m/2*(X1'squared + X2' squared)

    But U?
  2. jcsd
  3. Jan 14, 2010 #2


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    Gold Member

    We know that F=qE

    How can we get U from F? (notice that there are no dissipative forces here)
  4. Jan 15, 2010 #3
    Hehe. Yes. U = - integral (F - along x direction in our case - dx) because Fx = - dU/dx
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