# Photoelectric effect

1. Apr 8, 2010

### ghostbuster25

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.

I dont really know where to start as im not completly 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?

2. Apr 8, 2010

### 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.

3. Apr 8, 2010

### ghostbuster25

when you say pd are do you mean the potential difference?

4. Apr 8, 2010

### Stonebridge

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.

5. Apr 12, 2010

### jrosen13

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