21. A thin insulated wire forms a flat spiral of a large number N of coils disposed along which the constant current I. The radii of the inner and outer coils are equal to a and b (Fig.) Find: 1) The magnetic induction B in the center of the spiral - the point O; 2) the magnetic moment of the spiral for a given current.

Detailed solution. Decorated in Microsoft Word 2003. (Target decided to use formula editor)