The Problem: An empty capacitor is connected to a 12.0 V battery and charged up. The capacitor is then disconnected from the battery, and a slab of dielectric material (k=2.8) is inserted between the plates. Find the amount by which the potential difference changes, and state whether this change is an increase or a decrease. My attempt at a solution: Using the equations Q=CV and C=(k*A*ε)/d And considering that A, ε, d, and Q stay constant, And k is 1 to begin with since the space is empty, and 2.8 after the material is inserted, I get 1*12(volts)=2.8V Thus, V=4.3 Volts. However, the book gives the answer as 7.7 V. Help?