A very long, hollow cylinder (radius 4.7cm) 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 3.8A. Use Ampère's law to find the magnetic field
(a) outside the cylinder at a distance 6.8cm from the axis and
(b) inside the cylinder at a distance 3.0cm from the axis.