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 N/C is also applied. Net electric force on this charge is 15N. away from the center of the sphere as shown. Find magnitude of net force exerted on the charge placed at point A by the induced charges on the sphere.

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20N

15N

35N

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 the induced charges on outer surface should be equal and opposite to that applied by electric field. This is equal $q E = 1 \times 20 = 20 N$ toward left. 15N force is only due to Induced charge on the inside surface of the hollow sphere. Hence net force by induced charges is 15+20=35N

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