In a conducting hollow sphere of inner and outer radii 5 cm and 10 cm, respectively, a point charge 1 C is placed at point A, that is 3 cm from the center C of the hollow sphere. An external uniform electric field of magnitude 20 NC−1 is also applied. Net electric force on this charge is 15 N, away from the center of the sphere as shown. Find the magnitude of net force exerted on the charge placed at point A by the induced charges on the sphere.
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a
15 N
b
20 N
c
5 N
d
35 N
answer is D.
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Detailed Solution
External applied field's effect is neutralized by charge induced on outer surface, i.e., their combined effect is zero. Hence, force applied by induced charges on outer surface should be equal and opposite to that applied by electric field. This is equal to qE=1×20 N toward left. 15 N force is only due to induced charge on the inside surface of the hollow sphere. Hence, net force by induced charges is 15 + 20 = 35 N.