# Photoelectric Effect: Working out kinetic energy of photoelectron

1. The problem statement, all variables and given/known data
With the work function of copper being 4.65eV calculate the kinetic energy of a photoelectron knocked out with a photon with wavelength of 200nm . Then calculate the velocity of the electron.

2. Relevant equations

KE=hf-ø

3. The attempt at a solution
FIrst time I have done a question on this and what is confusing me is the KE comes out to be almost exactly equal to the work function but with a negative value, because hf is so small. This is what I have done

$$KE=hf-ø \\ KE=(6.621 \times 10^{-34})(\frac{3 \times 10^8}{2 \times 10^{-7}})-4.65 \\ KE=-4.649999999999999999006085 \\$$

I had to use wolfram alpha to get that result as every calculator I have used just rounded to -4.65, I dont know whether this is the whole point or not? If it is correct then I can just calculator the velocity with 1/2mv^2 and using the mass of the electron right?

Any help is appreciated,

thanks 🙂

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