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: Question on emission of electrons

  1. Jan 15, 2010 #1
    1. The problem statement, all variables and given/known data
    Monochromatic light of wavelength 4.5 x 10-7 m will eject electrons from the surface of a
    metal with a maximum KE of 3.2 x 10-19 Joules. Will light having a wavelength of 6.8 x 10-7
    m also cause emission of electrons from this metal

    2. Relevant equations
    E = h.c/lambda
    KEmax = E - work function

    3. The attempt at a solution
    In order for the emission to occur, the E > wo
    E = [(6.67x10^-34)(3.0x10^8)]/wavelength
    so we can find E for wavelength 4.5x10-7 and for wavelength 6.8x10-7

    How do we find wo? (work function)
  2. jcsd
  3. Jan 15, 2010 #2


    User Avatar
    Homework Helper

    KEmax = E - work function
    In the problem, KEmax and E is given. Find work function using the above equation. Compare this work function with the energy of the new wave length.
  4. Jan 15, 2010 #3
    E = [(6.67x10^-34)(3.0x10^8)]/4.5x10^-7 = 4.446x10-33
    since KE = 3.2x10^-19, wo = -3.2x10^-19

    Enew = [(6.67x10^-34)(3.0x10^8)]/6.8x10-7 = 2.94 x 10^ -33
    since wo is negative, though mag of E is smaller than Wo, E > wo and so there will be emission.

    I got these answers but the answer specified in the book is E = 2.9x10^-19 and wo = 1.22x10-19 which I didn't get... any ideas what I am doing wrong?
  5. Jan 15, 2010 #4


    User Avatar
    Homework Helper

    E = [(6.67x10^-34)(3.0x10^8)]/4.5x10^-7 = 4.446x10-33
    In this the calculation power is wrong. It should be (-34 + 8 + 7) = ...?
  6. Jan 15, 2010 #5
    Thanks. Since the wo is the same for the metal the one found in 1st case can be used in the 2nd case. Thanks much.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook