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Photons and the photoelectric effect

  1. Apr 19, 2008 #1
    1. The problem statement, all variables and given/known data
    the maximum wavelegnth for which an electromagnetic wave can eject electrons from a platinum surface is 196 nm. when radiation with a wavelength 141 nm shines on the surface, what is the max speed of the ejected electrons?


    2. Relevant equations
    f=c/λ
    E=hf
    E=Φ+K
    Φ=hf0


    3. The attempt at a solution
    λ=196nm
    f=(3E8)/(196E-9)
    f=1.5306E15
    E=Φ+K
    (6.63E-34)(f??)=(Φ??)+K
    K=??

    λ=141nm
    f=(3E8)/(141E-9)
    f=2.127E15
    E=Φ+K
    (6.63E-34)(f??)=(Φ??)+K
    K=??

    K1-K2=_______
    K=1/2mv^2 to find v


    but i dont know what numbers to plug in as f?? and Φ??
    i tried putting in the f values i found using the given wavelengths but i kept getting K=0

    please help

    thank you for your time
     
  2. jcsd
  3. Apr 19, 2008 #2
    phi is the workfunction of the metal, which is related to the minimum wavelength required to eject an electron. If the energy input is the energy of the workfunction and nothing else, the kinetic energy will be zero of the electron
     
  4. May 17, 2008 #3
    im having the same problem where im supposed to find its max. kinetic energy of the photoelectron in electron volts(eV) and joules(J), but im completely stuck.

    if i use hf = Emax + work, i have planks constant (h) and frequency, but there are two variables left to find...plz help me out
     
  5. May 17, 2008 #4

    alphysicist

    User Avatar
    Homework Helper

    Hi sodr2,

    If it's the same type of problem as in the original problem, use the equation twice. Once for the case of the maximum wavelength (and think about what being the maximum wavelength means for the other terms in the equation).

    Then use it separately for the next given (smaller) wavelength to find the kinetic energy.

    With two equations, you can have two unknowns. What do you get?
     
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