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82. (a) We are given
5
ˆˆ
i(
61
0T
)
i
x
BB
−
==
×
G
, so that
G
G
vB vB
yx
×=
−
±
k where
v
y
= 4
×
10
4
m/s.
We note that the magnetic force on the electron is
−−
ev
B
b
g
e
j
±
k and therefore points in
the
+
±
k direction, at the instant the electron enters the fieldfilled region. In these terms,
Eq. 2816 becomes
r
mv
eB
ey
x
0 0038
.m
.
(b) One revolution takes
T
= 2
π
r
/
v
y
= 0.60
µ
s, and during that time the “drift” of the
electron in the
x
direction (which is the
pitch
of the helix) is
∆
x = v
x
T
= 0.019 m where
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 Spring '08
 Any
 Physics, Force

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