A parallel plate air capacitor of capacitance C is connected

A parallel plate air capacitor of capacitance C is connected to a cell of emf V and then disconnected from it. A dielectric slab of dielectric constant K, which can just fill the air gap of the capacitor, is now inserted in it. Which of the following is incorrect ?

1. The energy stored in the capacitor decreases K times.
2. The chance in energy stored is 1 CV² 1 - 1
  K
3. The charge on the capacitor is not conserved.
4. The potential difference between the plates decreases K times.

Capacitance of the capacitor, C = Q/V
After inserting the dielectric, new capacitance C¹ = K.C
New potential difference V¹ = V/K

U_i =	1	CV² =	Q²	(∵ Q = CV)
	2		2C

U_f =	Q²	=	Q²	=	C²V²	=		u_i
	2f		2KC		2KC			K

∆u = u_f – u_i =	1	CV²		1	- 1
	2			K

As the capacitor is isolated, so change will remain conserved p.d. between two plates of the capacitor

L =	Q	=	V
	KC		K

The chance in energy stored is	1	CV²		1	- 1
	K