Capacitor problem with a beam balance -- balancing after changing the charge

[Victim]

The lower plate of a parallel plate capacitor is supported on a rigid rod.The upper plate is suspended from one end of a balance.The two plates are joined together by a thin wire and subsequently disconnected.The balance is then counterpoised.Now a voltage V= 5000V is applied between the plates.The distance between the plates is d=5mm and the area of each plate is A=100sq.cm. Then find out additional mass placed to maintain balance.All the elements other than plates are massless and non conducting.

Homework Equations

The Attempt at a Solution

 

[phyzguy]

I don't see an attempt at a solution. Hint - try using the principle of virtual work.

 

[Victim]

I don't see an attempt at a solution. Hint - try using the principle of virtual work.

I had done Q^2d/2A ×apsilonnot =mg

 

[Chandra Prayaga]

Where did that equation come from?