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Homework Help: 4 problems I need help with

  1. Dec 6, 2007 #1
    Last test coming up and I need help with these last 4 problems please.....

    (6.6e-34*2.57e15)-(3.80*1.6e10-19) to get 10.88 J what am i doing wrong here?

    Number of Photons = Energy of pulse / Energy of one photon...so Ephoton is 4.01e19...how do I get energy of pulse?

    I have no idea...I'm lost on this

    need anser to #3 to solve this...
  2. jcsd
  3. Dec 6, 2007 #2


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    Read the question again. Carefully. Might it be given?

    You'll have to solve the given equation for the momentum, p. You are given the temperature T and the value of k. So only the mass is still unknown. What other information is given in this question that would allow you to determine (in the broadest sense: looking it up is also determining :wink:) that mass?
  4. Dec 6, 2007 #3
    Well, I though it was given also but when I 4.01319/5960 J...the computer still says it is the wrong answer
  5. Dec 6, 2007 #4


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    Prob 1. Please check the units and orders of magntiude. 6.6e-34*2.57e15 = 1.6962e-18 J and 3.80*1.6e-19 = 6.08e-19.

    Prob 2. "how do I get energy of pulse?" - that is given in the problem statement "intense pulse of light having an energy of 5960 J". The pulse width is superfluous with respect to energy, but would be a factor in a calculation of power.

    Prob 3. See - http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html

    Think of how to relate momentum of a particle, p = mv, to kinetic energy KE or T = 1/2 mv2.

    Prob 4. It appears one knows the answer.

    This appears to be a homework problem. If so, I will move it to Introductory Physics HW forum.
  6. Dec 7, 2007 #5
    1.) stupid mistake
    2.) stupid mistake
    3.) using p2=(3/2)(1.38e-23)(293)([2][8])=4.32e-20 with p=2.0797e-10 kg m/s can someone verify that it is correct...computer still says wrong but I don't know what else I am missing
  7. Dec 7, 2007 #6


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    Argh! I edited what was here instead of adding a new reply... see below
    Last edited: Dec 7, 2007
  8. Dec 7, 2007 #7
    i thought so too...that is sqr(4.85e-22) which is 2.20e-11 and still incorrect
    Last edited: Dec 7, 2007
  9. Dec 7, 2007 #8


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    You started from

    KE = (3/2)kT ,

    but KE is also

    KE = (p^2)/2m .

    so you should have

    (p^2) = 3mkT = 3 · (4 · 1.67·10^-27 kg) · (1.38·10^-23 J/K) · (293 K) (kg·m/sec)^2
  10. Dec 8, 2007 #9
    ah...got it thanks alot!!!!!!

    WL=h/v -->6.6e-34/8.98e-34=7.35e-11 nm is that correct?
    Last edited: Dec 8, 2007
  11. Dec 8, 2007 #10


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    I am getting 9.00·10^-24 N·sec for the linear momentum of the helium atom.

    The deBroglie wavelength is given by h/p , for which I am getting

    (6.63·10^-34 J·sec) / (9.00·10^-24 N·sec) = 7.37·10^-11 m or 0.074 nm.

    So I basically agree with your result, but not with the way you've presented it.

    Be careful about what units the computer accepts your entries in.
  12. Dec 9, 2007 #11
    it is was a computer error...i emailed instructor and he fixed it...thanks
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