bei48482_FM

a massless particle—the photon—indeed exists and its behavior is as expected, as we

shall find in Chap. 2.

Electronvolts

In atomic physics the usual unit of energy is the electronvolt(eV), where 1 eV is the

energy gained by an electron accelerated through a potential difference of 1 volt. Since

WQV,

1 eV(1.602 10 ^19 C)(1.000 V)1.602 10 ^19 J

Two quantities normally expressed in electronvolts are the ionization energy of an atom

(the work needed to remove one of its electrons) and the binding energy of a mole-

cule (the energy needed to break it apart into separate atoms). Thus the ionization

energy of nitrogen is 14.5 eV and the binding energy of the hydrogen molecule H 2 is

4.5 eV. Higher energies in the atomic realm are expressed in kiloelectronvolts(keV),

where 1 keV  103 eV.

In nuclear and elementary-particle physics even the keV is too small a unit in most

cases, and the megaelectronvolt(MeV) and gigaelectronvolt(GeV) are more appro-

priate, where

1 MeV  106 eV 1 GeV  109 eV

An example of a quantity expressed in MeV is the energy liberated when the nucleus

of a certain type of uranium atom splits into two parts. Each such fission event releases

about 200 MeV; this is the process that powers nuclear reactors and weapons.

The rest energies of elementary particles are often expressed in MeV and GeV and

the corresponding rest masses in MeV/c^2 and GeV/c^2. The advantage of the latter units

is that the rest energy equivalent to a rest mass of, say, 0.938 GeV/c^2 (the rest mass of

the proton) is just E 0 mc^2 0.938 GeV. If the proton’s kinetic energy is 5.000 GeV,

finding its total energy is simple:

EE 0 KE (0.938 5.000) GeV 5.938 GeV

In a similar way the MeV/cand GeV/care sometimes convenient units of linear mo-

mentum. Suppose we want to know the momentum of a proton whose speed is 0.800c.

From Eq. (1.17) we have

p 

1.25 GeVc

Example 1.8

An electron (m0.511 MeV/c^2 ) and a photon (m0) both have momenta of 2.000 MeV/c.

Find the total energy of each.

0.750 GeVc



0.600

(0.938 GeVc^2 )(0.800c)



 1 (0.800c)^2 c^2

m



 1 ^2 c^2

32 Chapter One

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