A solenoid (coil) of radius R has 10 turns per centimeter. It is
10cm long, and the coiled wire carries a current I. In addition to
the coil of wire, a second wire runs right down the central axis of
the solenoid, carrying a current of 2I. (The solenoid wire doesn't
intersect this second wire, because the former wraps around the
latter, always a distance R away.) A) Find the magnitude of the
total magnetic field at a distance R/2...