A system consists of charges +q and +q at the opposite corners of a square of sides 2a and `-q` and `-q` at the other two corners . Calculate the potential and field at a distance r from the centre of the square along a line parallel to the two sides of the square . Assume a `lt lt `r .

Answer

`V = (1)/(4 pi epsi_(0)) xx (3qa^(2))/(r^(3)) , E = (1)/(4 pi epsi_(0)) xx (9 q a^(2))/(r^(4))`