State Biot-Savart law.Use it to derive an expression for the magnetic field at the centre of a circular loop of radius R carrying a steady current I.Sketch the magnetic field lines for such a current carrying loop.
iot-Savart law states that the magnetic field dB due to the current element dl at any point P is
(i) directly proportional to the current I, dB 
I
(ii) directly proportional to the length dl of the element, dB dl
(iii) directly proportional to sin
, whereis the angle between dl and r, dB sin
(iv) inversely proportional to the square of the distance r from the current element AB,
From (i), (ii) , (iii), and (iv), we obtain
At centre, x = 0
