ELECTROSTATICS

The neutral hydrogen atom in its normal state behaves in some respects like an electric charge distribution which consists of a point positive charge of magnitude +e surrounded by a charge distribution of negative charge whose density is given by

-p ( r) = C exp(-2 r / a )

Here a is the Bohr radius, 0.53E-8 cm and C is a constant with the value required to make the total charge e.

(a) What is the net electric charge inside a sphere of radius a?

(ANS : 0.68e)

please help & thanx in advance....