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Acceleration of a proton in an electric field

  1. Aug 18, 2012 #1
    1. The problem statement, all variables and given/known data
    A proton (+e) accelerates from rest in a uniform electric field of 640 N/C. At some later time, the proton's speed is 1.20 x 10^6 m/s. Find the acceleration of the proton. The mass of a proton is 1.67x10^-27 kg. Charge on a proton is +1.6x10^-19C.


    2. Relevant equations
    F=ma
    F=qE


    3. The attempt at a solution
    F=qE
    F=ma
    ma=qE
    (1.6x10^-19)(640)=(1.67x10^-27)(a)
    a=6.13x10^10m/s^2

    I don't think this is correct. I ignored the (+e) because I didn't know what to do with it. Wouldn't an electron and a proton mean a net neutral charge? Also, the acceleration is just way too high. The particle would have to travel faster than the speed of light for a short time.
     
  2. jcsd
  3. Aug 18, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Looks fine to me.
    +e is just the proton's charge, which you used just fine. e is the elementary charge; the charge on an electron is -e, on a proton it's +e.

    There's no electron in this problem.

    The acceleration is high. Once the speed gets too high, Newton's laws must be modified using relativity. Note that the speed they quote is only a small fraction of light speed.
     
  4. Aug 18, 2012 #3
    Okay. I thought that by +e the problem meant that the electron and the proton were a pair (as in a hydrogen atom). They just meant that the charge on a proton is the same as the charge on an electron, but positive. Thanks for clearing that up.
     
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