Prove that the increment of the angular momentum ∆M of a ring in the
problem 10.1.21 proportional to the increment of the magnetic induction flux
through the ring ∆Φ: ∆M = 1 Q∆Φ, where Q is the electric charge of the ring.
2π
To prove this, use the fact that the magnetic induction flux through the side
surface of the cylinder is equal to the difference of the fluxes through its ends.