- Part A A loop of wire is in the shape of two concentric semicircles as shown. (Figure 1) The inner circle has radius a; the outer circle has radius b. A current I flows clockwise What is the magnitude, B, of the magnetic field at the center of the semicircles? through the outer wire and counterclockwise through the inner wire. Express B in terms of any or all of the following: I, d, b, and fly. View Available Hint(s) AED Do ? B= Submit Part B Complete previous part(s) Provide Feedback