Plates of a parallel plate capacitor of capacitance 4μF is connected to the terminals of  a battery of 12 volt. Now the battery is disconnected from the capacitor and the plates of the capacitor are connected to the plates of a  1μF parallel plate uncharged capacitor. If a dielectric slab of dielectric constant k = 2 is introduced into the space between the plates of 1μF capacitor, The potential difference across the combination of capacitors is