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: Photoelectric effect/photon stuff

  1. Sep 15, 2010 #1
    got a couple of questions....

    firstly,

    A blue supergiant star has surface temperature 29kK and has total power output 100000 times that of the Sun.

    where is the wein peak of this stars balckbody curve?

    I got lambda=9.99*10^-8m, which i know is right.

    now, radius of sun is 6.96*108m, find radius of supergiant..

    so ratio here should give the supergiant a surface area of A=100000A of sun.. solving for that gives r = 2.11*10^11m... but it tells me its wrong???





    secondly,

    A photoelectric experiment uses 265 nm light and a silver target.
    Find max kinetic energy of photoelectrons.

    Kmax=hf-W0
    h=6.626*10-34
    f=c/lambda=1.13*10^15
    W0 for silver = 7.43*10^-19

    subbing in gives Kmax = 5.738*10^-21J.... and it also tells me that is wrong???




    thirdly,

    Suppose you want to observe neutron diffraction in a crystal with 0.28 nm spacing between lattice planes.

    Find the momentum of neutrons having wavelength equal to this spacing.
    found p=2.4*10^-24 kg*m/s

    Find the kinetic energy of neutrons having wavelength equal to this spacing.
    found K=1.7*10^.21 J

    If these neutrons constituted an ideal gas, what would be the gas temperature?
    How the heck do i do that :S??



    thanksss so much for any help!
     
  2. jcsd
  3. Sep 15, 2010 #2
    super star Q is easy
    Psun=[tex]\sigma[/tex]4[tex]\pi[/tex]r2*T4
    from the text book temp of sun is about 5800K
    solve for Power (also in the text - could just look up)
    then 100000*3.91*10^26=5.65*10-8A*(29*103)4 or whatever your temp value was.
    A=9.73976*1020=4[tex]\pi[/tex]r2
    solve for r

    the second question I got wrong twice, you have to use a more accurate value for c then 3*10^8 or it wont except- mine was out by 0.04 and it told me I was wrong

    as for the last question I don't have a clue yet.
     
  4. Sep 15, 2010 #3
    accept!
     
  5. Sep 15, 2010 #4
    Mastering physics is a bit stupid like that.
    You can be wrong if your too accurate, and wrong if your not accurate enough.
     
  6. Sep 15, 2010 #5
    for Kmax i got 6.59427*10^-21 J and its still telling me its wrong. and i tried 0.04116 eV and its telling me its wrong too.
     
  7. Sep 15, 2010 #6

    I had a wavelength of 264*10^-9 m and w_o of 7.43*10^-19 (silver)
    my first answer was 1.0*10^-20J and it finally accepted 1.04*10^-20J

    not sure why it wont accept your answer, procedure is the same as mine.

    btw MP always seems to like SI units (J).
     
    Last edited: Sep 15, 2010
  8. Sep 16, 2010 #7
    howcome you used 264 nm? when it says 265?

    the way i did it was K= hf - W and f=c/265nm = 299792458/265*10-9 = 1.131292294*1015 Hz
    i used all exact values for constants so...

    K = (6.62606821*10-34*1.131292294*1015) - 7.43*10-19

    = 6.60*10-21 J what did i do different?
     
  9. Sep 16, 2010 #8
    my value was 264!!!!!! not sure why it won't accept that answer - it was screwed up for me too - maybe go see him and ask whats going on.
     
  10. Sep 16, 2010 #9
    dont worry about it, i used your answer and it still told me it was wrong, which was my last attempt...

    so the answer it gave me was 7.57*10^-21 J so go figure. i sent the uni an email so we'll see. thanks anyway.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook