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Homework Help: Ratio of velocities between proton and electron?

  1. Sep 25, 2013 #1
    1. The problem statement, all variables and given/known data
    An electron and a proton are separately placed at rest midway between two oppositely charged metal plates. A) Which way will the electron accelerate? B) Which way will the proton accelerate? C) Which particle, if either, will acquire more kinetic energy just before striking a plate? D) What is the ratio of their velocities just before they strike the plates?

    2. Relevant equations


    3. The attempt at a solution

    I've figured out parts a-c. Not sure about d thought.

    a) The electron will accelerate towards the postively charged plate.

    b) The proton will accelerate towards the negatively charged plate.

    c) Because the charge on either particle is the same, they will acquire the same kinetic energy.

    d) How would you solve for the ratio of velocities? Does it have to do with the difference in their masses?
  2. jcsd
  3. Sep 25, 2013 #2


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    Homework Helper

    Yes, the mass will control the velocity.
    I like to start questions like this with the definition of electric potential as the energy per charge.
    If V is the potential difference between plates, then each charge moves through potential difference V/2
    and so V/2 = energy/q = .5mv²/q
    Solve for v and you will see how the velocity depends on the mass.
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