# Homework Help: Calculations from photoelectric experiments.

1. Apr 10, 2010

### lyrebird

1. The problem statement, all variables and given/known data
In a photoelectric experiment using a sodium surface, you find a stopping potential of 1.85 V for a wavelength of 300 nm and a stopping potential of 0.820 V for a wavelength of 400 nm. From these data find the following:
a) a value for the Planck constant
b) the work function for sodium
c) the cutoff wavelength λ0 (the wavelength corresponding to the cutoff frequency) for sodium

2. Relevant equations

3. The attempt at a solution
a) For this, I calculated the frequencies (using c = fλ) to be 10^15 Hz and 7.5*10^14 Hz.

I then used my graphics calculator to graph the stopping potentials vs. the frequencies. This gave me the line
y = 2.427*10^14*x + 5.5097*10^14​

The slope should be equal to Planck's constant divided by the charge on an electron. Thus,
Planck's constant = 92.427*10^14)(1.602*10^-19) = 3.89*10^-5​
which clearly isn't right.

Also, I am supposed to give the answer in eV·s. But isn't Planck's constant measured in m^2·kg·s?

b) The y-intercept of the line on the graph is the work function divided by the charge on an electron.

Work function = (5.5097*10^14)(1.602*10^-19) = 8.826*10^-5​

Again, this doesn't seem right?

c) I presume that I'll be able to solve this once I know the answers to the previous parts, but I have absolutely no idea how to do it.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution