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SPECIAL THEORY OF RELATIVITY |
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all rights reserved 2002
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Units
of Mass, Energy, and Momentum
Instead
of using kilograms to measure mass, physicists use a unit of energy -- the
electron volt. It is the energy gained by one electron when it moves through a
potential difference of one volt. By definition, one electron volt (eV
Lets
look at an example of how this energy unit works. The rest mass of an electron
is 9.11 x 10-31 kg. Using E = mc2 and a
calculator we get: E
= 9.11
x 10-31 kg x (3 x 108 m/s)2 = 8.199 x 10-14
joules This
gives us the energy equivalent of one electron. So, whether we say we have 9.11
x 10-31 kg or 8.199 x 10-14 joules, we really talking
about the same thing -- an electron. Physicists go one stage further and convert
the joules to electron volts. This gives the mass of an electron as 0.511 MeV
(about half a million eV). So
if you ask a high energy physicist what the mass of an electron is, you'll be
told the answer in units of energy. You can blame Einstein for that! Eagle-eyed
readers will notice that if you solve E=mc2 for m,
you get m=E/c2, so the unit of energy should be eV/c2.
What happened to the c2? It's very simple, particle physicists
choose units of length so that the speed of light = 1! How can we do that?
Quite easily, as long as everyone understands the system. All we have to do is
use a conversion factor to get back the "real" (i.e. everyday) units,
if we want them. Not
only are mass and energy measured in eV, so is momentum. It makes life so much
easier than dividing by c2 or c all the time.
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is equivalent to 1.6 x 10-19 joules.