The current-carrying wire loop in Figure ??a?? lies all in one plane and consists of a semicircle of radius 10.0 cm, a smaller semicircle with the same center, and two radial lengths. The smaller semicircle is rotated out of that plane by angle θ, until it is perpendicular to the plane (Figure b). Figure ??c?? gives the magnitude of the net magnetic field at the center of curvature versus angle θ. The vertical scale is set by B a = 10.0μT and Bb, = 12.0μT. What is the radius of the smaller semicircle?

B, (a) R/4 n/2 (b) 0 (rad) (c)