Are we justified in writing ρ(r’)=Q[δ^3(r’)] to designate the charge density of a charge Q at origin?I say this because the standard expression is
ρ(r)=Q[δ^3(r-r’)].
However,I believe the first expression will do as well.

Hi neelakash,

Your first expression just says that you have a charge Q at r' = 0 in a certain coordinate system. If you have only one charge it is clearly always possible to choose your coordinates so that the charge lies at the origin of coordinates. The second expression is useful if you have multiple charges at different points in space so that they can't all be at the origin.

Hope this helps.