An isolated air$-$filled parallel$-$plate capacitor that is no longer connected to anything

has been charged up to Q $=2\mathrm{.}9$ nC$.$ The separation between the plates initially is $1\mathrm{.}20$ mm$,$ and

for this separation the capacitance is $31$ pF$.$ Calculate the work that must be done to pull the

plates apart until their separation becomes $5\mathrm{.}30$ mm$,$ if the charge on the plates remains constant.

$($$0=8\mathrm{.}8510-12$ C$2/$N $$ m$2)$

Answer: $0\mathrm{.}46$ J