A pair of parallel plates,forming a capacitor,are charged.The plates are pulled apart to quintuple the original separation,the charges on the plates remaining the same.What is the ratio of the final energy stored to the original energy stored?
A)25
B)5
C)1
D)1/5
E)1/25

Question

Quizplus · Accepted Answer

The energy stored in a capacitor is given by $ U = \frac{1}{2} \frac{Q^2}{C} $. When the separation between the plates is increased, the capacitance $ C $ decreases because $ C \propto \frac{1}{d} $. If the separation is quintupled, the capacitance becomes $ \frac{1}{5} $ of its original value. Since the charge $ Q $ remains constant, the energy stored becomes $ 5 $ times the original energy.

A Pair of Parallel Plates,forming a Capacitor,are Charged