Three identical capacitors C _ 1 , C _ 2 and C _ 3 have a...

Three identical capacitors ${C}_{1}, {C}_{2}and{C}_{3}$ have a capacitance of $1.0\muF$ 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 $5\muC$ . The value of ${ϵ}_{r}$ =____________.