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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

    rl.bhat

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    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

    rl.bhat

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    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.
     
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