A current is set up in a wire loop consisting of a semicircle of radius 4.00cm, a smaller concentric semicircle and two radial straight lengths all in the same plane. Figure ??a?? shows the arrangement but is not drawn to scale. The magnitude of the magnetic field produced at the center of curvature is 47.25μT. The smaller semicircle is then flipped over (rotated) until the loop is again entirely in the same plane (Figure b). The magnetic field produced at the (same) center of curvature now has magnitude 15.75μT and its direction is reversed. What is the radius of the smaller semicircle?

(b) (a)