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NCERT Class XII Chapter
Electric Charges and Fields
Questions With Solutions

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Question : 21 of 34
Marks: +1, -0
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 × 10310^3 N C−1C^{-1} and points radially inward. What is the net charge on the sphere?
Solution:  
r = 20 cm = 0.20 m, E = 1.5 × 10310^3 N C−1]C^{-1]}, R = 10 cm = 0.1 m
E = 14πε0qr2\frac{1}{4\pi\varepsilon_0}\frac{q}{r^2} or 1.5 × 10310^3 = 9 × 10910^9 × q(0.20)2\frac{q}{(0.20)^2}
or q = 1.5×13×0.049×109\frac{1.5 \times 1^3 \times 0.04}{9 \times 10^9} = 6.67 × 10−910^{-9}
Since electric field points radially inwards, so charge is negative i.e.
q = –6.67 nC.
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