# Thread: momentum of the photon

1. 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?  2.

3. Originally Posted by whizkid 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?  4. Convert everything to SI units, it makes life much simpler...  5. Originally Posted by whizkid 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.  6. 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.  7. Originally Posted by Neverfly  Originally Posted by whizkid 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  8. 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   9. Thanks,
is it possible to use the cgs? what is 511keV in cgs?  10. Originally Posted by whizkid 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  11. so momentum is E/c = 5.34374 * 10^-23 gr*cm/s?  12. 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  13. Originally Posted by Strange 1 eV is 1.6021772*10-12erg
Physical Constants and Astronomical Data
I multiplied yhat by 511000  14. Then send your calcualtor back for a refund (or press the correct buttons).  15. Originally Posted by whizkid  Originally Posted by Strange 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  16. Originally Posted by Strange It is hard to see what you are doing wrong;
WhizKid, walk through what you're doing step by step.  17. Thanks, I forgot to multiply  Bookmarks
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