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Finding speed of two electrons initially at rest

  1. May 4, 2015 #1
    1. The problem statement, all variables and given/known data
    Two electrons start at rest with a separation of 5.0 x 10^-12 m. Once released, the electrons accelerate away from each other. Calculate the speed of each electron when they are a very large distance apart (Ee = 0).

    2. Relevant equations
    EE = kQq/r
    Ek = (1/2)mv2

    3. The attempt at a solution
    EE2 - EE1 = - (Ek2 - Ek1)
    - EE1 = - Ek2

    (kq2) / r = (1/2)mv2
    v = sqrt[ 2(8.99 x 109)(-1.6 x 10-19)2 / (9.11 x 10-31)(5.0 x 10-12) ]
    v ≅ 1.0 x 107 m/s

    The answer is apparently 7.1 x 106 m/s
  2. jcsd
  3. May 4, 2015 #2


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    Hello, and welcome to PF!

    Do both electrons end up with some kinetic energy? Did you take that into account?
  4. May 4, 2015 #3
    Thanks for replying!
    Oh I don't think I did. So it would be Ee1 = (1/2)mv^2 + (1/2)mv^2.
  5. May 4, 2015 #4


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    Yes. Are you assuming that the two electrons end up with the same speed? Can you justify that?
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