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

Solution
REVIEW OF PHYSICAL PRINCIPLES 21
The specific heat of water is 1 cal
, and the heat of vaporization of water is 539
g
cal
. Therefore,
g
cal
cal
heat energy added = 1000 g 1 × (100 − 50) deg + 539
g · deg g
= 589,000 cal
Since there are 4.186 J
J
and 1.6 × 10−19 , we have
cal eV
heat energy added =
5.89 × 10 5 cal × 4.186 J
cal
1.6 × 10 −19 J
eV
= 1.54 × 10 25 eV
The answer to Example 2.8 is an astronomically large number (but not very much
energy on the scale of ordinary physical and chemical reactions) and shows why the
electron volt is a useful energy unit only for reactions in the atomic world.
W EXAMPLE 2.9
An alpha particle, whose charge is +(2 × 1.6 × 10 −19 ) C and whose mass is 6.645 ×
10 −27 kg, is accelerated across a potential difference of 100,000 V. What is its kinetic
energy, in joules and in electron volts, and how fast is it moving?
Solution
The potential energy of the alpha particle at the moment it starts to accelerate is,
from Eq. (2.32),
W = qV = 2 × 1.6 × 10 −19 C × 10 5 V
= 3.2 × 10 −14 J
In terms of electron volts,
W = 3.2 × 10−14 J
1.6 × 10−19 = 2 × 10
J
eV
5 eV