A long insulated copper wire is closely wound as a spiral of N turns. The spiral has inner radius a and outer radius b . The spiral lies in the XY-plane and a steady current I flows through the wire. The Z-component of the magnetic field at the centre of the spiral is

Step-by-step solution

If we take a small strip of d r at distancer from centre, then number of turns in this strip would be d N= N b-a d r Magnetic field due to this element at the centre of the coil will be aligned & d B= ₀(d N) I 2 r = ₀ N I 2(b-a) d r r & B= _ r=a ^ r=b d B= ₀ N I 2(b-a) b a aligned Correct answer is (a). Analysis of Question (i) If we see this problem independently, then I will rate this question moderately difficult. But the idea of this question is taken from question number 3.245 of IE Irodov. (ii) Interestingly