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Homework Help: Electric potential word problem

  1. Mar 20, 2005 #1
    An electron starts from rest 72.5 cm from a fixed point charge with Q=-0.125µC. How fast will the electron be moving when it is very far away?

    The answer the book says is 2.33 x 10^7 m/s, but I don't know how they got that answer.

    The equation I think I need to use is V=k(Q/R)

    So I plugged in the numbers: V= (9.0 x 10^9)(-1.25 x 10^-7) /.725 m
    and got -1551.7 V (I got -1.25 x 10^-7 for Q b/c µ is10^-6 so I multiplied the two together)

    The trouble I'm having is I don't know how to convert V into m/s. What equation would I use?
  2. jcsd
  3. Mar 20, 2005 #2
    You'll want to approach this using energy. F = kq1q2/r^2, U (potential) = - kq1q2/R

    You know q1 and q2, and you know R. Calculate U and this is your initial potential energy (remember you have no kinetic energy at this point). Use the relationship KE = PE, where KE = (1/2)m*v^2. The mass of an electron is 9.1x10^-31
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