The magnetic field at the centre of coil of n turns, bent in the form of a square of side 2l , carrying current i, is

Magnetic field due to one side of the square at centre OB1 =μ04π .   2isin 450l⇒ B1 =  μ04π .  2ilHence, magnetic field at centre due to all sidesB=4 B1 =  μ0 2iπlMagnetic field due to n turns,Bnet =nB=μ0 2 niπl = μ0 2 niπl