An ideal parallel-plate capacitor having circular plates of diameter D that are a distance d apart stores energy U when it is connected across a fixed potential difference. If you want to triple the amount of energy stored in this capacitor by changing only the size of its plates, the diameter should be changed to.

Explanation:

Given

Diameter of Plates is D

distance between Plates is d

Energy Stored in the Plates is U

Suppose fixed Potential is V

Energy is given by

where C=capacitance

where A=area of Plates

Thus

Keeping all other factors as same

For triple the Energy

divide 1 and 2 we get

Thus the diameter should be change to times of original value          

be subject to a force andchange its motion