P25_043.PDF

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Chapter 25 - 25.43
43. We use the conservation of energy principle. The initial potential energy is U i = q 2 / 4 πε 0 r 1 , the initial
kinetic energy is K i = 0, the final potential energy is U f = q 2 / 4 πε 0 r 2 , and the final kinetic energy is
K f = 2 mv 2 ,where v is the final speed of the particle. Conservation of energy yields
q 2
4 πε 0 r 1 =
4 πε 0 r 2 + 1
q 2
2 mv 2
.
The solution for v is
v =
2 q 2
4 πε 0 m
1
r 1
1
r 2
(8 . 99
=
×
10 9 N
·
m 2 / C 2 )(2)(3 . 1
×
10 6 C) 2
1
0 . 90
10 3 m
1
20
×
10 6 kg
×
2 . 5
×
10 3 m
=2 . 5
×
10 3 m / s .
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