# Homework Help: Finding planks constant from photoelectric effect

1. Nov 22, 2008

### hayyan1

1. The problem statement, all variables and given/known data

Hello, i was doing an experiment in which you shine different colours of light onto a cathode and apply negative voltage to the anode so that the photo current reduced to a constant value.
I obtained the average stopping potential for each wavelength of light (green yellow blue violet turquoise) and now i was going to plot average stopping potential versus frequency. Now the gradient of this graph multiplied by the charge of the electron gives me plancks constant (h=e(ΔV/Δf)).
However my problem is, does the line of best fit go through the origin or not?

2. Nov 22, 2008

### naresh

What is the equation that describes the photoelectric effect? Why does the slope give you Planck's constant?

3. Nov 22, 2008

### hayyan1

i have the equation
e(electron charge) x V(stopping potential)= h(plancks constant) x f(frequency)

4. Nov 22, 2008

### naresh

Well, if that is the equation, you should know what the intercept on the axis is!

How do you derive that equation?

5. Nov 23, 2008

### hayyan1

huh? all im saying is that i plotted average stopping potential on y-axis and frequency on x-axis, and all i want to know is that would my line of best fit go through (0,0) or not.

6. Nov 23, 2008

### hayyan1

Does the photocurrent drop to zero when a potential across it is equal to the kinetic energy of electrons?, because i found this not to be the case, the photocurrent reached a steady value that didnt decrease further, as i increased the potential across the anode and cathode..

7. Nov 24, 2008

### naresh

Yes, and I'm saying you should be able to work this out from the equation for the photoelectric effect. So please plot the theoretical stopping potential vs. frequency curve, and see if it goes through (0,0).

Hint: Your equation is not quite correct.