[alocheim]

I think I'm headed in the right direction on this problem but am struggling to find how do I find velocity of an electron expelled from a Cu plate if I know the max KE of the light that was used?  The question stated that the longest wavelength of light that causes electrons to be ejected from the surface of a Cu plate is 243 nm.  Then it asked me to find the max velocity of the electrons ejected when light of wavelength 200 nm shines on a Cu plate.

So far I've determined that since the longest wavelength of light that causes electrons to be ejected is 243 nm since it would correspond to the threshold frequency.  I then solved for the work function using this and came up with approx. 8.18x10^-19.  To find the max KE i took the energy of the 200nm wave - work function.  Does it look good up to this point? I was thinking that I could somehow use KE = 1/2mu^2, but then, i'm confused about the mass?  How would I find the velocity from the max KE?

[Nicolas88]

ok this should be easy :

as you know hv= 1/2mu²+w
where w =hv₀
v₀=threshold frequency

now you can easily calculate v and v₀ , h is planck's constant , m is the mass of the electron , only thing left is to calculate the value of u which is the velocity .

[Borek]

Electron mass is not something difficult to find.

[alocheim]

Thanks much gentlemen.  I guess I was confused about whether or not I needed to use the mass of one electron or try and find the number of electrons that this energy of photons would eject...etc...so thanks again.

[Nicolas88]

Thanks much gentlemen.  I guess I was confused about whether or not I needed to use the mass of one electron or try and find the number of electrons that this energy of photons would eject...etc...so thanks again.

number of electrons ejected are calculated from the intensity of light not the energy of the photons