An isolated metal sphere of radius 
has a charge 
on it. The charge may be considered to act as a point charge at the centre of the sphere. Show that the capacitance 
of the sphere is given by the expression
where 
is the permittivity of free space.
In order to investigate electrical discharges (lightning) in a laboratory, an isolated metal sphere of radius 
is charged to a potential of 
. Calculate the capacitance of the sphere, stating the unit in which it is measured,
.
