# Homework Help: HELP photoelectric effect

1. Apr 24, 2010

### ghostbuster25

HELP!!! photoelectric effect

Ok guys got a question on quantum physics - photoelectric effect i wanna run past you before i attempt a solution.

The question is as follows;

In a photoelectric-effect experiment, light of wavelength 420nm is incident on a sample of sodium. The work function of sodium is 3.67*10-19 J.
Calculate the stopping potential required to stop all electrons from reaching the anode.

Ok so im planig to use einsteins photoelectric equation of 1/2 mev2 = hf-psi(as in work function)
Im guessing i need to convert the wavelength to frequency so have come up with the solution 7.137915667e11

so this in the equation gives me 6.63*10-34 * 7.14285713*1014 - 3.67*10-19 J = n = -3.665267562

So this is the result of the maximum kinetic energy E of the electrons.
I am a bit worried as my result is a minus!!!!!!!! which i imagine is impossible

after that i believe i am to work out the potential differnence to stop the electrons.

That i am a bit unclear on how to do so any help would be much appreiciated :)

2. Apr 24, 2010

### ehild

Re: HELP!!! photoelectric effect

You have miscalculated the KE of the electrons. It is

6.63*10-34 * 7.14*1014 - 3.67*10-19 =1.07*10-19

ehild

3. Apr 24, 2010

### ghostbuster25

Re: HELP!!! photoelectric effect

ahhh ok thanks :)

for the potential differnce is it just KE=eV and therefore i need a force of the opposite charge to stop the electrons............this right? is this question that simple?

4. Apr 24, 2010

### ehild

Re: HELP!!! photoelectric effect

Yes. it is that simple :)

ehild

5. Apr 24, 2010

### ghostbuster25

Re: HELP!!! photoelectric effect

sweeeeet thanks :)

any good at schrodingers wave mechanics ha ha :)