Physics 21, Solutions for HW 2:
due online 1 AM, Friday, January 21, 2011
Chapter 22, Problem 4
Two particles are fixed to an
x
axis: particle 1 of charge 6.76 x 10
7
C is at the origin and particle 2
of charge +6.76 x 10
7
C is at
x
= 21.8 cm. Midway between the particles, what is the magnitude of
the net electric field?
Solutions:. With
x
1
= 6.00 cm and
x
2
= 21.00 cm, the point midway between the two charges is located at
x
=
13.5 cm. The values of the charge are
q
1
=
–
q
2
=
–
2.00
10
–
7
C,
and the magnitudes and directions of the individual fields are given by:
9
2
2
7
5
1
1
2
2
0
1
9
2
2
7
5
2
2
2
2
0
2


(8.99 10 N m
C )
2.00 10
C
ˆ
ˆ
ˆ
i
i
(3.196 10 N C)i
4
(
)
0.135 m
0.060 m
(8.99 10 N m
C )(2.00 10
C)
ˆ
ˆ
ˆ
i
i
(3.196 10 N C)i
4
(
)
0.135 m
0.210 m
q
E
x
x
q
E
x
x
Thus, the net electric field is
5
net
1
2
ˆ
(6.39 10 N C)i
E
E
E
Chapter 22, Problem 10
Part (a) of Fig. 2233 shows two charged particles fixed in place on an
x
axis with separation
L
. The
ratio
q
1
/
q
2
of their charge magnitudes is 4.05. Part (b) of the figure shows the
x
component
E
net,x
of
their net electric field along the
x
axis just to the right of particle 2. The
x
axis scale is set by
x
s
=
30.3 cm.
(a)
At what value of
x
> 0 is
E
net,x
maximum?
(b)
If particle 2 has charge 5
e
, what is the
value of that maximum? Assume
e
= 1.602 × 10
19
C.
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 Spring '08
 Hickman
 Charge, Electric charge, 6 m, 3 L, Enet, 3.97 cm

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