# Find current generated from photoelectric effect.

1. Oct 29, 2009

### lagwagon555

1. The problem statement, all variables and given/known data
A beam of light of 300nm is shone at a metal surface of work function 2.25eV. The power of the light striking each square centimeter is 1 microW. A total of 3cm^2 is illuminated by the beam. If each photon caused an electron to be ejected, what current is produced?

2. Relevant equations

E = hf - (work function)]
P/V = I
E = hf

3. The attempt at a solution

I calculated each electron emitted to have 1.89eV, and for each square centimeter, 1.51x10^12 photons strike per second, So, I try to use P/V = I, using P = 3microwatts and V = 1.89eV, which gives me an answer of 1.59microamps. However, the answer given in the book is 0.726microamps. Can anyone see where I'm going wrong, or any concepts I'm not understanding correctly? Thanks for any help!

2. Oct 30, 2009

### Delphi51

The I = P/V doesn't make sense to me.
You get one electron for each photon, so you have 3*1.51x10^12 electrons flowing per second. Use I = Q/t to figure out the resulting current.

It is incorrect to use the 3 microwatts on the electron side because much of the power is lost in knocking those electrons loose.

3. Oct 30, 2009

### ehild

That 1.89 eV you calculated, is the kinetic energy of the emitted electron- it is not voltage. (voltage is work/charge).You need the current- the charge flowing through a cross section in one second. You got 1.51x10^12 photons/cm2, they will kick out the same number of electrons in one second and three times that from 3 cm2. The out-flowing electrons carry e=1.6x10^19 charge each what is the total charge supplied by the photocell in one second?

ehild