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NCERT Class XII Chapter
Electric Charges and Fields
Questions With Solutions

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Question : 12 of 34
Marks: +1, -0
(a) Two insulated charged copper spheres A and B have their centres separated by a distance of 50 cm. What is the mutual force of electrostatic repulsion if the charge on each is 6.5 × 10−710^{-7} C ? The radii of A and B are negligible compared to the distance of separation.
(b) What is the force of repulsion if each sphere is charged double the above amount, and the distance between them is halved?
Solution:  
(a) F = 14πε0q1q2r2\frac{1}{4\pi\varepsilon_0} \frac{q_1 q_2}{r^2}
or F = 9 × 109(6.5×10−7)2(0.5)2\frac{10^9 (6.5 \times 10^{-7})^2}{(0.5)^2}
or F = 1.520 × 10−210^{-2} N
Electric Charges and Fields
(b) q1′q_1' = 2q12q_1 , q2′q_2' = 2q22q_2 and r' = r2\frac{r}{2}
So , F' = 14πε0\frac{1}{4\pi\varepsilon_0} q1′×q2′(r′)2\frac{q_1' \times q_2'}{(r')^2} = 14πε02q1×2q2(r2)2\frac{1}{4\pi\varepsilon_0} \frac{2q_1 \times 2q_2}{\left(\frac{r}{2}\right)^2}
or F' = 16 × 1.520 × 10−210^{-2} = 24.32 × 10−210^{-2} N or F' = 0.240.24 N
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