ElectrostaticsHard

Question

Figure shows a system of three concentric metal shells A, B and C with radii a, 2a and 3a respectively. Shell B is earthed and shell C is given a charge Q. Now if shell C is connected to shell A, then the final charge on the shell B is equal to :

Options

A.−
B.−
C.−
D.−

Solution


(B)    Let q1 and q2 are charges on A and B respectively
From given conditions: Charge on A & C after connection with wire are q1 and Q − q1 on B, charge is q2
VA = VC and VB = 0
⇒  VB = + = 0
⇒  2Q + q1 + 3q2 = 0    .... (1)
Using VC = VA
+ =
⇒ q1 = −     .... (2)
Using it in (1),   q2 = − Q

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