Three identical capacitors C 1 , C 2 and C 3 have a capacitance of 1.0 µF each and they are uncharged initially. They are connected in a circuit as shown in the figure and C 1 is then filled completely with a dielectric material of relative permittivity ε r . The cell electromotive force (emf) V 0 = 8V. First the switch S 1 is closed while the switch S 2 is kept open. When the capacitor C 3 is fully charged, S 1 is opened and S 2 is closed simultaneously. When all the capacitors reach equilibrium, the charge on C 3 is found to be 5 µC. The value of ε r =____________.