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Homework Help: Velocity of a proton question

  1. Nov 13, 2009 #1
    1. The problem statement, all variables and given/known data

    Calculate the veolcity of a proton of energy 0.12 keV

    proton mass = 1.7 x 10^-27

    2. Relevant equations

    3. The attempt at a solution

    I thought...

    0.12 keV = 120 eV

    120/1.60 x 10^-19 = 7.5 x 10^20 J

    E = 0.5mv^2

    sqrt(2E/m) = v

    but I'm getting something to x 10^23 so its obviously wrong.

    its A-Level physics so shouldnt be a really complex answer...

    Going wrong somewhere but cant work it out. Probably where I divided by the charge of an electron?

    Any help appreciated.

  2. jcsd
  3. Nov 13, 2009 #2
    What exactly is an electronVolt?
  4. Nov 13, 2009 #3
    You divided instead of multiplying. Try using dimentional analysis to convert from eV to Joules. It takes a lot of guess work out of the problem.
  5. Nov 14, 2009 #4
    By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt. (from Wiki)

    Oh, my tutor told me to divide...

    Is it because 1 Volt = 1 Joule/1 Coulomb

    There for Joules = V * C ?

    So eV multiplied but the charge of an electron give the Energy of the proton?

    Also, I've not heard of Dinentional Analysis before, what exactly is it?

    Thanks alot

  6. Nov 14, 2009 #5
    So would the correct answer be...

    120 * 1.6 x 10-19

    = 1.92 x 10 -17

    sqrt(2*1.92 x 10 -17/1.7 x 10-27) = v = 1.5 x 105 ms-1
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