ABC CLASSES
T.E. School of Physical Sciences
1
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Two point charges are located on the
positive x-axis of
a coordinate
system (as shown in figure). Charge
q1 = 1.0 nC is 2.0 cm from the origin,
and charge q2 = −3.0 nC is 4.0 cm
from the origin. What is the total
force exerted by these two charges
on a charge q3 = 5.0 nC located at the
origin ? Gravitational
forces are
negligible.
Example 1.
Solution
3
31
32
F
F
F
=
+
18
3
2
2
4
5 1 5 3
10
|
|
2
4
10
F
k
−
−
×
×
=
−
×
9
14 5 15
4 10 10
4 16
−
= ×
×
−
5
2.5 10 N
−
=
×
2
In figure, two equal positive point charges q1 = q2 = 2.0 µC interact with a third
point charge Q = 4.0 µC. Find the magnitude and direction of the net force on
Q.
Example 2.
3
Solution
Fnet = 2 |F31| cos α
= 2 .
12
2
1
2 4 10
4
.
.
0.46
4
(.5)
5
N
−
× ×
=
π ∈
4
A certain charge ‘Q’ is to be divided into two parts q, Q – q. What is the
relationship of ‘Q’ to ‘q’ if two parts, placed at a given distance ‘r’ apart are to
have maximum coulomb repulsion ?
Example 3.
Solution
2
0
1
(
)
4
q Q q
F
r
−
=
πε
2
1
[
] 0
dF
d
Qq q
=
−
=
2
0
4
dq
r dq
πε
⇒
2
2
,
2
Q d F
q
dq
=
is negative.
Hence force will be maximum if q = Q / 2.
5
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IIT JEE/PMT in Delhi/NCR . Call 011-47041845 for
more Information or visit www.abcclasses.in
Two small identical conducting balls of charges of + 10 µC and +30 µC are
kept at a separation of 50 cm. Find the forces of interaction between the balls
after these balls have been connected by a wire for a short time.
Example 4.
Solution
Charge on each ball =
10 30
20
2
C
+
=
µ
F = 9 × 109 ×
12
20 20 10−
×
×
2
(1/ 2)
= 14.4 N
6
ABC CLASSES : The No 1 Coaching Institute for
IIT JEE/PMT in Delhi/NCR . Call 011-47041845 for
more Information or visit www.abcclasses.in
Two particles A and B having charges 8×10−6C and −2 × 10−6C respectively
hold fixed with separation of 2