# Photoelectric Effect Help

## Homework Statement

In an experiment to determine the value of Planck's constant a negative potential was applied to the anode of a photoelectric cell and the minimum potential required to reduce the photocurrent to zero was mesure for incident light of various frequencies. For the frequencies of 8.5 x 10^14 Hz and 2.35 x 10^15 Hz the minimum potentials required were found to be 0.4V and 6.4V, respectively. Calculate Planck"s constant h.

## Homework Equations

E=hf=hc/wavelength
E=mc^2

3. Attempt

I do not know how to start this. IN NEED OF DIRE HELP.
Thank you to anyone who helps.

Last edited:

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rock.freak667
Homework Helper
Even though I shouldn't reply since you didn't attempt anything, I will.

but you know that $E=\phi +E_{k(max)}$ which is the same as $hf=hf_0+\frac{1}{2}mv_{max}^2$

When the photocurrent reaches zero, it means that the photoelectrons with max ke. just fail to reach the anode. So that the work done in stopping the electrons with max ke from reaching the anode=Loss in ke of the photoelectrons with max ke.

Can you express the last line in terms of an equation?