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What units are used after eV, MeV, GeV, etc, are divided by c^2? 
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#1
Feb1611, 07:41 PM

P: 12

I know that the mass of subatomic particles is usually given in electronvolts, and that c[tex]^{2}[/tex] is set to 1 so that we can say, "This particle has a mass of 13 eV." However, if you divide 13 eV by c[tex]^{2}[/tex], you get an answer, x. What units is this in?
Oh, also, what units do we use c in in the first place? km/s, m/s, mi/s? 


#2
Feb1611, 08:00 PM

Mentor
P: 11,501

I believe it is Kilograms or grams since that is the unit of mass. Cant say for the c.



#3
Feb1611, 08:40 PM

Mentor
P: 11,602

The electronvolt (eV) is a unit of energy, equal to 1.602e19 joule. When someone says "the mass of an electron is 511 keV" he really means, "the energyequivalent of the mass of an electron is 511 keV" or "the restenergy of an electron is 511 keV" or "the mass of an electron is 511 keV/c^2." Mathematically, [tex]m_e c^2 = 511 \rm{ keV}[/tex] Dividing through by c^2 we get [tex]m_e = 511 \rm{ keV}/c^2[/tex] so the eV/c^2 is a unit of mass, equal to (1.602e19 J)/(2.998e8 m/s)^2 = 1.782e36 kg. To check this, 511 keV = 511 x 1000 x 1.782e36 kg = 9.108e31 kg which is indeed the mass of an electron in kg. 


#4
Feb1711, 10:48 AM

Sci Advisor
P: 1,256

What units are used after eV, MeV, GeV, etc, are divided by c^2?
This language is fortunately commonly used by physicists.
There are several systems of units in which c=1 and is dimensionless. (Distance in lightyears and time in years is one.) All of those systems are more useful than kg or joules in describing an electron. 


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