Three long conducting plates `A, B` and `C` having charges `+q, -2q`, and `+q` as shown in figure. Here plate `A` and `C` are fixed. If the switch `S` is closed. The middle plate `(B)` will start moving in

Options

A. leftward direction
B. rightward direction
C. will not move
D. first move leftward and then rightward

Answer

B

Explanation

Before clsoing switch, net force on `B` due to `A` and `C` is zero. Potential of `A` is more than `C`. So as the switch is closed, charge will flow from `A` to `C` through switch.
Charge on `C` increases and on `A` decreases. So attraction of `C` on `B` increases and that of `A` decreases. So net force on `B` will act towards right.