Answer
See answers.
Work Step by Step
a. Choose a circular Amperian loop of radius r, centered on the wire, whose radius is larger than the radius of the inner wire and less than the radius of the outer cylindrical braid. Symmetry demands a magnetic field that has the same strength anywhere along the path, and which is parallel (tangent) to the circular path.
The net current through the area enclosed by the loop is I, because the entire inner wire penetrates the area once. Use equation 20–9.
$$B(2\pi r)=\mu_o I$$
$$B=\frac{\mu_o I }{2\pi r}$$
b. Choose a circular Amperian loop of radius r, centered on the wire, whose radius is larger than the radius of the outer cylindrical braid. The net current through the area enclosed by this path is 0, because the current I penetrates the area twice: once going into the page via the inner conductor, and once coming out of the page via the cylindrical braid. Use equation 20–9.
$$B(2\pi R)=\mu_o (0)$$
$$B=0$$