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

["Jeffrey Schnick" <JSchnick@Anselm.Edu>]

As an undergraduate, we had a lab course in which each laboratory exercise was in a different laboratory room.  Over the course of the semester we went from room to room, doing one laboratory exercise in one room each week.  In one of the laboratory rooms, in addition to our lab apparatus and many other things, there was a large C magnet with pole faces about 5 cm in diameter.  While I was working diligently on the lab exercise one of my lab partners was playing with the magnet.  He dropped an aluminum meter stick, oriented so that it extended vertically along its longest dimension, through the space between the poles.  The pole faces were separated from each other by a distance that I would estimate to have been a couple of millimeters greater than the thickness of the meter stick.  The meter stick fell quite slowly--I would estimate the rate to have been about 1 meter per 5 seconds.

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of Carl Mungan
> Sent: Thursday, March 22, 2012 2:05 PM
> To: phys-l@carnot.physics.buffalo.edu
> Subject: 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
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