Past exam of the physics 2 paper of the University of Oxford / 2018 / 7 / ii / Solution

For $b<r<a$, a coaxial Gauss surface of length $l$ encloses net charge $Q$: the sphere and the cavity-wall charge cancel, leaving $+Q$ on the outside of the inner conductor. In the long-cylinder approximation,The electric field points radially outward for $V>0$. End effects and axial nonuniformity near the openings are excluded by the ideal coaxial cylindrical capacitor approximation.