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 through the outer wire and counterclockwise through the inner wire. Part A What is the magnitude, B, of the magnetic field at the center of the semicircles? Express B in terms of any or all of the following: I, a, b, and mu_0.