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Homework Help: Finding the final velocity of a proton

  1. Feb 3, 2013 #1
    1. The problem statement, all variables and given/known data
    A charged particle (either an electron or a proton) is moving rightward between two parallel charged plates separated by distance d = 1.90 mm. The particle is slowing from an initial speed of 93.5 km/s at the left plate. The left plate has V=-70V and the right plate has V=-50V

    q=1.602e-19 C
    m=9.11e-31 kg

    2. Relevant equations
    Conservation of Energy:

    3. The attempt at a solution
    Alright, so I just used the conservation of energy, assuming that the energy at the right plate will equal the energy at the left plate. Rearranging that equation I solved for v_2. However, I always get a nonreal answer. I tried this problem again using kinematics and solving for the force then the acceleration using E=-V/d=F/q and F=ma and I got the same negative number within the square root. Someone please solve this and tell me if it's possible. Then let me know where I'm going wrong! Thanks!
  2. jcsd
  3. Feb 3, 2013 #2


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    What is the problem asking for?

    Show your work; no one can tell what you may have done wrong if they can't see what you did.
  4. Feb 3, 2013 #3
    Nevermind, I figured it out. I was using the mass for an electron and not for a proton.
  5. Feb 3, 2013 #4


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    What's a factor of 1800 among friends?
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