Question

When the switch is opened the electromagnet does not instantly lose quite all of its magnetism. What effect does this have on the results?

Answer 1

1. The magnetic field was created by the current. Since the original power is gone, the magnetic field begins collapsing (fields don't just disappear, they collapse).
2. As the field collapses, it induces a current in the windings in the same direction it was flowing before. Since the coil now acts as a source, the voltage is reversed. There is no discontinuity in current flow, the effect is that it keeps flowing uninterrupted as the switch is opened.
3. Because power is conserved, it must go somewhere. Most will arc across the switch contacts as they are opening and be given off as heat, light, and radio waves. If the core is conductive and not laminated, a lot of it will cause eddy current losses in the iron core.
4. The magnetic field decays, how fast depends on where and how the main losses are going, but in any case it will happen very quickly, on the order of milliseconds, give or take an order of magnitude or two.
5. Some residual magnetism will remain due to a property called magnetic hysteresis. Essentially, the magnetic domains that were twisted to line up partly with the electromagnetic field will stay a little bit aligned, not quite twisting back to a scrambled nonmagnetic state.