A)
The energy stored in a capacitor is given by:
where
Q is the charge stored
C is the capacitance
The capacitance of a parallel-plate capacitor is
where
is the vacuum permittivity
is the area of each plate
is the distance between the plates
Substituting,
The charge stored on the capacitor is
So, the energy stored is
B)
This time, the separation between the plates is
d = 1.7 mm = 0.0017 m
So, the new capacitance is
And so, the new energy stored is
C)
Energy must be conserved, so the difference between the initial energy of the capacitor and its final energy is just equal to the work done to increase the separation between the two plates from 1.5 mm to 1.7 mm (in fact, the two plates of the capacitor attract each other since they have opposite charge, so work must be done in order to increase their separation)