# Problems using photon's energy to calculate electron's velocity

1. Feb 8, 2006

### mgeoghe2

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. Feb 8, 2006

### Tide

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

$$\frac {v^2}{c^2} = \frac {2 \times 1.3 eV}{m c^2}$$