TS EAMCET Medical 2018 May 2 Shift 1 Paper

Consider two coils of radii r1 and r2 placed at a distance 'd' away as shown in the figure. Assuming the length of the coils l1,l2 ≫ d with n1,n2 the number of turns per unit length of the two coils respectively, the mutual inductance of the two coils is