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Homework Help: To find the velocity of the particle

  1. Mar 14, 2005 #1
    A metal sphere of radius 0.10m carries a positive charge of [tex] 1.0 x 10^{-4}C[/tex]. A particle of mass [tex] 2.0 x 10^{-5}kg[/tex] carrying a negative charge of [tex] 1.5 x 10^{-10}C[/tex] is released from rest at a distance of 1.0m from the centre of the sphere. Calculate the velocity of the particle when it srtikes the surface of the sphere. Neglect the gravitational effect.

    I cannot get the correct answer, which is 11m/s....anyway, here are my steps:

    KE gained = PE loss
    1/2 mv^2 = [tex] \frac{Qq}{4\pi\epsilon r} [/tex]
    v = 3.87 m/s
  2. jcsd
  3. Mar 14, 2005 #2


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    Your method is right, you're just using the wrong distance in the PE. Remember, the particle goes from r=1 m to r=0.1 m, not from r=infinity to r=1 m.
  4. Mar 14, 2005 #3
    Thank you SpaceTiger!
    I get the answer now, I thought that the PE on reaching the surface of the sphere is zero..... :tongue:
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