capacity of the capacitor in two states what is the difference between the capacity of the capacitor when we ignore the effect of the sides and in reality?(with regarding side effcts)
this question is from"fundamental of physics by 'rex neleson'".
I couldn't solve it because I knew just guass's law for calculating the capacity but with regarding side effects,it doesn't work...
thank you
 A: I expect that the book question is asking for a qualitative answer, as provided by @Unnikrishnan in his comment, rather than a formula.  The capacitance increases because the fringing fields effectively increase the area of the plates.  You can see this in diagrams which show the field lines extending onto the back of the capacitor plates.  These fields are present also when the gap between the plates is much smaller than their width or diameter, but their effect on capacitance is much less significant.   
A formula for a circular parallel plate capacitor is given in the first link below.  Details of an experiment to verify the formula are included.  A formula for a capacitor with square plates is much more difficult to obtain because of the lack of circular symmetry.
The second link discusses various approximate formulae for the circular capacitor.
http://www.santarosa.edu/~yataiiya/UNDER_GRAD_RESEARCH/Fringe%20Field%20of%20Parallel%20Plate%20Capacitor.pdf
http://meeng.technion.ac.il/Research/TReports/2004/ETR-2004-02.pdf
