57. Let the distance in question be
r
. The initial kinetic energy of the electron is
Km
v
ie
i
=
1
2
2
, where
v
i
= 3.2
×
10
5
m/s. As the speed doubles,
K
becomes 4
K
i
. Thus
∆∆
U
e
r
KK
K
K
m
v
ii
i
e
i
=
−
=−
−
2
2
4
43
3
2
π
0
ε
()
,
or
( )
2
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 Fall '10
 RW
 Physics, Energy, Kinetic Energy, Mass, Potential Energy, Joule, initial kinetic energy

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