Question

A parallel-plate capacitor, whose plates are kept horizontal, is charged from a cell and then isolated from it. A dielectric slab which can just fit in the gap between the plates is now inserted to fill exactly half of the gap and then left alone. Neglect gravity and friction. The slab will 

(a) remain stationary 

(b) move further into the gap 

(c) move out of the gap 

(d) either (b) or (c) depending on whether its dielectric constant is greater or less than 2

Answer 1

Correct Answer is: (b) move further into the gap

Energy = Q2/2C .Here, Q is constant.

As the slab moves into the gap, C increases and hence energy decreases. A system tends to move towards lower energy.