A very long, hollow cylinder is formed by rolling up a thin sheet of copper. Electric charges flow along the copper sheet parallel to the axis of the cylinder. The arrangement is, in effect, a hollow tube of current I. Use Ampère's law to show that the magnetic field
(a) Is μ0I / (2πr) outside the cylinder at a distance r from the axis and
(b) Is zero at any point within the hollow interior of the cylinder.
(Hint: For closed paths, use circles perpendicular to and centered on the axis of the cylinder.)