Plate A of parallel plate air filled capacitor is connected to a spring having force constant k and plate B is fixed. They are held on a frictionless table top as shown in Fig. If charge `+q` is placed on plate A and a charge `-q` on plate A and a charge `-q` on plate B, by how much does the spring expand ?

Explanation

Consider equilibrium of plate `A`, let `x` is extension of spring.
In equilibrium
`kx=F=(Q^(2))/(2Ain_(0))`
`x=(Q^(2))/(2Ain_(0)k)`