Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

20 CHAPTER 2
energy of the electron with respect to the anode is
W = qV =−1.6 × 10 −19 C × (−1 V)
= 1.6 × 10 −19 J
This amount of energy, 1.6 × 10 −19 J, is called an electron volt and is symbolized by eV.
Since the magnitude of the electron volt is convenient in dealing with the energetics
of atomic and nuclear mechanics, this quantity of energy is taken as a unit and is
frequently used in health physics. Multiples of the electron volt are the keV (10 3 eV),
the MeV (10 6 eV), and the GeV (10 9 eV).
W EXAMPLE 2.7
How many electron volts of energy correspond to the mass of a resting electron?
Solution
E = mc 2
= 9.11 × 10 −31 kg ×
= 81.99 × 10 −15 J
3 × 10
Since there are 1.6 × 10 −19 J
eV ,
8 m2
E = 81.99 × 10−15 J
1.6 × 10−19 = 0.51 × 10
J
eV
6 eV.
s
It should be emphasized that, although the numerical value for the electron volt
was calculated by computing the potential energy of an electron at a potential of 1
V, the electron volt is not a unit of electrons or volts; it is a unit of energy and may
be interchanged (after numerical correction) with any other unit of energy.
W EXAMPLE 2.8
How many electron volts of heat must be added to change 1 L of water whose
temperature is 50 ◦ C to completely dry steam?