Solving a problem in physics section 60 paragraph 20 e

20. The thin wire (insulated) forms a flat spiral of N = 100 closely spaced coils, which current flow I = 8 mA. The radii of the inner and outer coils (fig.) Are a = 50 mm, b = 100 mm. To find:
a) induction of the magnetic field in the center of the spiral;
b) the magnetic moment of the spiral for a given current.