Consider an infinitesimally small section of the disc located at polar coordinates
that subtends an infinitesimally small angle 
and whose width spans an infinitesimally small length dr. The area of the small section will be 
. Hence the charge contained in this infinitesimally small section of the disc will be 
. The electric potential at point O located at the edge of the disc due to this infinitesimally small section of the disc will be,
Sir, why you first integred r, then theta.
ReplyDelete4no line from last ,there is a silly mistake,costheta then natural logarithm.
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