Biot-savart law states that, the magnetic field induction dB at a point due to current element is                 dB = μo4π I dl sinθr2 where,I.dl is the current element,r is the distance of point from the current elemnt and,θ is the angle between dl ⇀and r⇀.Expression for magnetic field at the centre of a circular coil :Consider a circular coil of radius r with center O. Let, I be the current flowing in the circular coil. Assume, that the current coil is made of a large number of current elements each of length dl .current element = I.dl Now, as per Biot Savart law we have,                 dB = μo4π I dl sinθr2Since, the angle between dl⇀ and r⇀ is 900.Therefore,                     dB = μo4π I dlr2 Thus, B= ∫dB = ∫μo4π I dl r2 = μo4π Ir2∫dlBut, ∫dl = total length of circular coil        = circumference of current loop = 2π r Therefore,                  B = μO4π 2πnIr  where,n is the number of turns in the coil.