A parallel plate capacitor is half-filled with a

Solution

Let the parallel plate capacitor have plates of area A and separation d. The original capacitance without the dielectric is When a dielectric slab of thickness d/2 and dielectric constant K=4 is inserted, the capacitor can be considered as two capacitors connected in series: 1.     A capacitor with air (or vacuum, K=1) and plate separation d/2. Its capacitance 2.  A capacitor with the dielectric (K=4) and plate separation d/2. Its capacitance The effective capacitance Ceff of two capacitors in series is given by: Therefore the correct answer is option (A).