1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Frequency of EMR

  1. Oct 7, 2005 #1
    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! :smile:
  2. jcsd
  3. Oct 7, 2005 #2


    User Avatar
    Homework Helper

    Your final answer is correct.

    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"?
  4. Oct 9, 2005 #3
    This is what my demonstrator told me to use?
  5. Oct 9, 2005 #4


    User Avatar
    Homework Helper

    Do you see how it comes from E conservation?
    Ø is just the negative of the electron's initial PE, etc.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook