So I came across this question in Griffith's Electrodynamics which described a long co-axial cable which carried a current flowing through its inner cylinder of radius $\mathbf a$ to outer words cylinder of radius $\mathbf b$. (a<b)
It was given that, to find the magnetic field, Ampere's law has to be used which gives:
$B(2$ $\pi$ $s)$=$\mu_0$$\mathbf I$
But how is the enclosed current in some arbitrary circular loop of radius '$\mathit s$', $\mathbf I$ ? (Magnetic field is circumferential) but $\mathbf I$ is distributed over the entire cylindrical volume between the two co axial cylinders. Also, it is also said that the magnetic field inside the smaller cylinder of radius $\mathbf a$ will be zero. But isn't there's a current flowing inside it as well to produce some magnetic field? Please help!!