# Homework Help: Frequency of EMR

1. Oct 7, 2005

### ussrasu

Can some please review this question and tell me if the working looks correct? Please advise of any mistakes i may have made. Thanks

Q: UV radiation (wavelength = 250 nm) falls on a metal target, and electrons are liberated. If the maximum kinetic energy of these electrons is 1.00*10^-19 J, what is the lowest frequency EMR that will initiate a photocurrent on this target?

A: E = Ø + K = h*(c/λ)

Therfore Ø = ((h*c)/250*10^-9) - 1*10^-19
Which = 6.96*10^-19 J
= h*f

Therefore f = 1.05*10^15 Hz

Is this what i am supposed to do? Thanks for any help!

2. Oct 7, 2005

### lightgrav

I find it peculiar that you group " Ø + K " together
- it's NOT the total Energy " E " of any configuration!
Rather, it's the NEGATIVE Energy change of the electron
(- E_initial + E_final) ... so it does equal E_photon,initial.

Most of us learn conservation laws as
"final = initial + change" or "change = final - initial" ,
once in a while as "in - out = change".
Is there a word for this "negative change"?

3. Oct 9, 2005

### ussrasu

This is what my demonstrator told me to use?

4. Oct 9, 2005

### lightgrav

Do you see how it comes from E conservation?
Ø is just the negative of the electron's initial PE, etc.