Re: [Phys-l] Figuring Physics in the March TPT The Physics Teacher

[Robert Carlson <rcarlson@physicstoolkit.com>]

Ok, N on top and S on bottom, the B field from the magnet points up.  Looking down on the conducting loop from above, and using Lenz's law, the current in the loop will be clockwise as the magnet enters the loop and counterclockwise as the magnet leaves the loop (the currents in the loop are in a direction to oppose the changing magnetic field through it).  Think of the loop as creating its own magnetic field to repel the magnet when it is entering and attract the magnet when it is leaving.  Like many of us, it opposes any change.  Hence, when the magnet passes through the loop, an ammeter will show a change in the direction of the current. 

This relates to the detection of magnetic monopoles.  If, for example, a south magnetic monopole were to pass through the same loop, then looking down on the loop, the current would be clockwise as the S pole enters the loop and also clockwise as the S pole leaves the loop.  Hence, an ammeter would detect no change in the direction of the current through the loop.



--- On Wed, 2/18/09, Paul Lulai <plulai@stanthony.k12.mn.us> wrote:

> From: Paul Lulai <plulai@stanthony.k12.mn.us>
> Subject: Re: [Phys-l] Figuring Physics in the March TPT The Physics Teacher
> To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
> Date: Wednesday, February 18, 2009, 9:27 PM
> Hi.
>
> So I have a loop of wire parallel to Earth.  I drop a
> magnet through the loop.  The magnet is lined up with N on
> top and S on bottom (so S goes through the loop first).
> If I were to qualitatively graph the flux vs time, and
> current vs time I'd get (would I get?):
>
> Part 1: S is far away to S / N magnet bisect the loop.
> Flux goes from zero (or really really small) to a maximum
> (when S pole is in center of loop) and then back to zero
> when the magnet is bisected by loop (weakest part of
> magnetic field).
>
> Part 2: S / N bisect loop to N is far away.
> Flux goes from zero to maximum (when N pole is in center of
> loop) to zero when N is really far away.
>
> When this is graphed, is this like a 2 humped camel with
> both humps up, or is this more like a sine curve with one
> hump up and one hump down? 
> How would we get the up vs down (+ vs -) from Flux = BA
> (where would the negative come from)?
> When we rotate a magnet in the place of a coil, the angle
> changes and rotates through the 2rpi.  The cosine would give
> us +/-.  Since we are not rotating, it would appear that we
> could choose if we want it to be + or - the entire fall.
>
> So-
> Would flux be a two humped camel?
> Would the two humps be both up, or could you explain how to
> make one up (+) and one down (-)?
>
> Sorry to bother, I'm ashamed to admit I don't know
> the answer.  I've loaned out my favorite references (no,
> not Feynman yet... this summer).
>
> Thanks for your help.
>
>
>  
> Paul Lulai
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