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NCERT Class XII Chapter
Electric Charges and Fields
Questions With Solutions

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Question : 8
Total: 34
Two point charges qA = 3 μC and qB = –3 μC are located 20 cm apart in vacuum.
(a) What is the electric field at the midpoint O of the line AB joining the two charges?
(b) If a negative test charge of magnitude 1.5 × 10–9 C is placed at this point, what is the force experienced by the test charge?
Solution:  
(a) Electric fields at O due to the charges at A and B are
EOA = EOB =
1
4πε0
q
r2
 = 9 × 109 ×
3×10−6C
(20∕2×10−2m)2
or EOA = EOB = 27 × 105 NC−1
As, they are directed in same direction, so net electric field at midpoint O is
E = EOA + EOB = 2EOB = 2 × 27 × 105
or E = 5.4 × 106NC−1 directed along OB.
(b) When test charge q0 = - 1.5 × 10−9 C is placed at point O, it experiences a force
F = q0E = 1.5 × 10−9 × 5.4 × 106 or F = 8.1 × 10−3 N
In direction opposite to that of E i.e. along OA.
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