A parallel plate capacitor with air as dielectric is charged to a potential V. It is then connected to an uncharged parallel plate capacitor filled with wax of dielectric constant K. The common potential of both capacitor is

Before connection, the charge Q on the plates of air filled capacitor is C V. After it is connected with an uncharged capacitor, let y' be the common potential. Let Q1 and Q2 be the charges on charged and uncharged capacitor respectively. ThenQ1=V′C and Q2=V′(KC)∴ CV=V′C+V′(KC)=(1+K)V′C or  V′=V1+K