Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Problems using photon's energy to calculate electron's velocity

  1. Feb 8, 2006 #1
    ive got this problem in which i am given:
    the work function of tungsten = 4.5 eV
    light with photon energy 5.8 eV illuminates surface

    and i need to figure out what the velocity of the fastest electron ejected from the surface is.

    so i see work function and 'fastest' and i think, ok KEmax = hf - work function. and KE is just .5mv^2. the problems i run into are these:
    where do i get f? or hf?
    i see that E = hf , so i think, ok i will use 5.8 -4.5 = KEmax. but then i run into another problem.
    which mass do i use? i got really screwed up with the units... i can use 9.109e-31 kg but i dont know if and eV is kg m^2/s^2. i am also given the mass of an electron as .511 MeV/c^2 but i dont really understand what that means. is it .511e6 or .511e6/(3e8)^2??
    if you have any insightful comments, i would be very grateful.
  2. jcsd
  3. Feb 8, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper

    Since you know the rest energy of electron you can find the speed from

    [tex]\frac {v^2}{c^2} = \frac {2 \times 1.3 eV}{m c^2}[/tex]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook