A long metal tube having a section of radius r and a section of radius r is
1 2
placed along a uniform magnetic field of induction B. How much will the
energy of a superconducting projectile of radius r and length l ≫ r , r fly-
0 1 2
ing along the axis of the tube change when it crosses the boundary between
sections of the tube of different radii? The effect of metal resistance on the
process of interaction of the projectile with the tube is ignored.