A dielectric is inserted between the plates of a capacitor. The capacitor is then charged and the dielectric is removed. The electric potential energy stored in the capacitor is...

A) greater than it would have been if the dielectric were left in place.

B) the same as it would have been if the dielectric were left in place.

C) less than would have been if the dielectric were left in place.

D) indeterminate.

Solution: A. The dielectric is polarized when it is between the charged plates. The positively charge dielectric surface is attracted to the negatively charged plate, and the negatively charged dielectric surface is attracted to the positively charged plate. Thus, you must do positive work to remove the dielectric. As a result, the electric potential energy stored in the capacitor increases when the dielectric is removed.

Another way to look at it would be with the formulas for capacitance and energy of a capacitor. The capacitance decreases with the removal of the dielectric. With a fixed charge, the formula U = Q^2/(2C) indicates that the electric potential energy increases (just as it did when pulling apart the plates.