Photoelectric Effect: Question on Quantum Physics

[ghostbuster25]

Ok guys got a question on quantum physics - photoelectric effect i want to 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.

I don't really know where to start as I am not completely sure on what the question is asking for.
I know how to work out the maximum kinetic energy of the ejected electrons by way of hf-work funtion, but i KNOW this isn't the way to approach it.
Any ideas or suggestions?

 

[Stonebridge]

No you are correct. Calculate the max ke of the electrons as you suggest.
Then it's a case of what pd would stop electrons with this ke.
A hint: it's exactly the opposite case of what ke electrons would gain if accelerated from rest through that pd. Same formula. Same principle. Just deceleration rather than acceleration.

 

[ghostbuster25]

No you are correct. Calculate the max ke of the electrons as you suggest.
Then it's a case of what pd would stop electrons with this ke.
A hint: it's exactly the opposite case of what ke electrons would gain if accelerated from rest through that pd. Same formula. Same principle. Just deceleration rather than acceleration.

hi thanks for the reply,
when you say pd are do you mean the potential difference?

 

[Stonebridge]

hi thanks for the reply,
when you say pd are do you mean the potential difference?

Yes, sure. Pardon my abbreviations when in a hurry!
The potential difference you calculate in this case (deceleration rather than acceleration) is the "stopping potential" you need.

 

[jrosen13]

This problem seems like a trick to get you to calculate the electron energy in eV!