physics

A point charge Q= 4.60 uC is held fixed at the origin. A second point charge q=1.20 uC with mass of 2.80 * 10^-4 is placed on the x-axis 0.250 m away from the origin. (b) The second point charge is released from rest. What is its speed withn its distance from the origin is (i) .500 m (ii) 5.00 m (iii) 50.0 m


I am not sure how to get the speed from the different distances

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asked by J
  1. A quick way to do this is to use conservation of energy. The potential energy of the moving charge in the E-field of the fixed charge is:
    P.E. = k Q q/R
    where k = 8.99*10^9 N m^2/C^2
    The kinetic energy of the particle is
    (1/2) m V^2

    The initial energy, which is all potential, is the total energy, which is constant. Therefore
    kQq/(0.25) = kQq/r + (1/2) m V^2

    V = sqrt [(2/m)kQq (4 - 1/r)]

    Solve for V at each or the r values in your question 1/2, 5 and 50 m.

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    posted by drwls

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