Getting a ring to spin ** A nonconducting thin ring of radius a carries a static charge q. This ring is in a magnetic field of strength B0, parallel to the ring's axis, and is supported so that it is free to rotate about that axis. If the field is switched off, how much angular momentum will be added to the ring? Supposing the mass of the ring to be m, show that the ring, if initially at rest, will acquire an angular velocity ω=qB0/2m. Note that, as in Problem 7.5, the result depends only on the initial and final values of the field strength, and not on the rapidity of change.