1 \begin{problem*}{22.43}
2 Niobium metal becomes superconducting when cooled below 9K. Its
3 superconductivity is destroyed when the surface $B$ field exceeds
4 $B_{max} = 0.100\U{T}$. Determine the maximum current in a
5 $d=2.00\U{mm}$ diameter niobium wire can carry and remain
6 superconducting, in the absence of any external $B$ field.
7 \end{problem*} % problem 22.43
10 For long, cylindrical wires, the magnetic field a distance $r$ from
11 the center of the wire is
13 B = \frac{\mu_0 I}{2 \pi r}
15 As long as you are outside the wire.
17 Therefore, the magnetic field at the surface is maximized when
19 B_{max} &= \frac{\mu_0 I_{max}}{2 \pi r} \\
20 I_{max} &= (2 \pi r B_{max})/\mu_0 = \ans{500\U{A}}