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Re: [Phys-l] drop a metal cylinder through a solenoid



Motion strictly along a magnetic field line doesn't do
anything. Look at the form of the Lorentz force law.

Thanks, I had forgotten it's easier to analyze these kinds of things using the Lorentz law. So scrap the solenoid. Let's instead think about a horizontal field, as uniform as we can make it. Perhaps between the poles of a C-shaped magnet.

That reminds one of the demo of a swinging paddle. Let's do that, but make the paddle and arm short enough that it's always inside the poles.

Okay so here's my comparison now:

1. A cylindrical magnet is a pendulum bob on a toothpick arm swinging entirely between two parallel Al plates. The two flat faces of the magnet are its poles and they are parallel to the plates.

2. A cylindrical Al bob on the same arm swinging entirely within two poles of a large C-shaped magnet. For simplicity, assume the pivot of the toothpick is at the center of a pole face, so the bob always sees the same strength magnetic field as it swings. That strength is the same as that of the magnet bob in case 1. Size of bob and plates are about the same in both cases.

Which pendulum will have a shorter period and why? -Carl