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NCERT Class XII Chapter
Electric Charges and Fields
Questions With Solutions

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Question : 21
Total: 34
A conducting sphere of radius 10 cm has an unknown charge. If the electric field 20 cm from the centre of the sphere is 1.5 × 103 N C–1 and points radially inward. What is the net charge on the sphere?
Solution:  
r = 20 cm = 0.20 m, E = 1.5 × 103 N C–1], R = 10 cm = 0.1 m
E =
1
4πε0
q
r2
 or 1.5 × 103 = 9 × 109 ×
q
(0.20)2
or q =
1.5×13×0.04
9×109
 = 6.67 × 10−9
Since electric field points radially inwards, so charge is negative i.e.
q = –6.67 nC.
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