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Thread: momentum of the photon

  1. #1 momentum of the photon 
    Forum Junior whizkid's Avatar
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    If a photon has energy 511 keV its momentum is hv/c :
    1.2356 *10^20 hv/3*10^10 = 4.1215 * 10^9 what, gr*cm/s or Kg*m/s?


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    Quote Originally Posted by whizkid View Post
    If a photon has energy 511 keV its momentum is hv/c :
    1.2356 *10^20 hv/3*10^10 = 4.1215 * 10^9 what, gr*cm/s or Kg*m/s?
    Uhhh... What units were the references you used in?


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  4. #3  
    Bullshit Intolerant PhDemon's Avatar
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    Convert everything to SI units, it makes life much simpler...
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by whizkid View Post
    If a photon has energy 511 keV its momentum is hv/c :
    1.2356 *10^20 hv/3*10^10 = 4.1215 * 10^9 what, gr*cm/s or Kg*m/s?
    What is 1.2356 *10^20? And what units does it have?
    What value have you used for h?
    What value have you used for v? (frequency?)
    You appear to have used cm/s for c.
    ei incumbit probatio qui dicit, non qui negat
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  6. #5  
    Bullshit Intolerant PhDemon's Avatar
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    I get this a lot at work with students (often research students who should know better). They look up an equation for, say, calculating the composition of an aerosol particle from it's refractive index, and plug the numbers in without thinking and are surprised when they get gibberish. Then you point out that they have used concentration in moles/L instead of kg/m3, their density is in kg/m3 but they've used a mass of sample in grams and a volume in nm3 etc.
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    Quote Originally Posted by Neverfly View Post
    Quote Originally Posted by whizkid View Post
    If a photon has energy 511 keV its momentum is hv/c :
    1.2356 *10^20 hv/3*10^10 = 4.1215 * 10^9 what, gr*cm/s or Kg*m/s?
    Uhhh... What units were the references you used in?
    I used the formula for energy of the photon E= hv (nu) then divided by 2.99* 10^10, is that OK?
    I suppose momentum is = gr*cm/s
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  8. #7  
    Bullshit Intolerant PhDemon's Avatar
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    Equations only give the right answer if all of the variables are in consistent units. Convert the energy into J, and use c in m/s, you have used an Energy in electron volts and c in cm/s these are not consistent units.

    For reference:

    1 eV =1.6 x 10-19 J
    1 cm = 0.01 m
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  9. #8  
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    Thanks,
    is it possible to use the cgs? what is 511keV in cgs?
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  10. #9  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by whizkid View Post
    is it possible to use the cgs?
    As long as you are consistent.

    what is 511keV in cgs?
    1 eV is 1.6021772*10-12erg
    Physical Constants and Astronomical Data
    ei incumbit probatio qui dicit, non qui negat
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  11. #10  
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    so momentum is E/c = 5.34374 * 10^-23 gr*cm/s?
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  12. #11  
    Brassica oleracea Strange's Avatar
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    I'm not sure where you get that from. I thought you were talking about a 511 keV photon?

    Putting "(511000 ev / c) in (cm g) /s" into Google returns: 2.73093002 10-17 (cm g) / s
    ei incumbit probatio qui dicit, non qui negat
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  13. #12  
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    Quote Originally Posted by Strange View Post
    1 eV is 1.6021772*10-12erg
    Physical Constants and Astronomical Data
    I multiplied yhat by 511000
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  14. #13  
    Bullshit Intolerant PhDemon's Avatar
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    Then send your calcualtor back for a refund (or press the correct buttons).
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  15. #14  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by Strange View Post
    1 eV is 1.6021772*10-12erg
    Physical Constants and Astronomical Data
    I multiplied yhat by 511000
    It is hard to see what you are doing wrong; Google (511000 * 1.6021772e-12) erg / c in cm g /s returns 2.73093111 10-17 (cm g) / s
    ei incumbit probatio qui dicit, non qui negat
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    Quote Originally Posted by Strange View Post
    It is hard to see what you are doing wrong;
    WhizKid, walk through what you're doing step by step.
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  17. #16  
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    Thanks, I forgot to multiply
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