[meteorite-list] A bit OT: magnets and conductors

From: Pat Brown <radio_ranch_at_meteoritecentral.com>
Date: Tue Sep 12 02:15:34 2006
Message-ID: <20060912061532.75687.qmail_at_web51304.mail.yahoo.com>

Hi Kevin and Robert,

Another excellent way to demonstrate this same effect
is to remove the magnets from a hard disc drive,
remount the magnets so that they are in the same
configuration as they were in the HDD. This has the
two magnets facing each other with a gap of about 3mm
between the magnets. You can drop a coin made of
aluminum (a Japanese 1 Yen coin works great) and see
the slow motion free fall of the coin while in the
gap. I have used a demonstration of this effect as an
interview question when I am interviewing engineers
for a job position. The point is not to try to trick
or fool an engineer, but rather to see if they go back
to first principals to try to explain the observed
effect.

The strong static magnetic field in the gap causes
circulating currents in the coin as it drops through
the field. These circulating currents are commonly
called eddy currents. These eddy currents circulating
in the coin make their own magnetic field that bucks
the magnetic field caused by the permanant magnets.
This in turn cause the coin to fall very slowly.

This effect is visable with a copper coin, but the
higher mass makes a copper coin fall through the gap
more rapidly.

With Best Regards,
                Pat

--- "Matson, Robert" <ROBERT.D.MATSON_at_saic.com> wrote:

> Hi Kevin,
>
> > Hey, meteorite collectors, go and get your rare
> earth magnets
> > (mine is 3/4 inch dia, 1 inch long) and a small
> bit of flat
> > Aluminium. Hold the magnets N or S end (doesn't
> matter which)
> > about 2 or 3 mm above the Aluminium and move the
> magnet quickly
> > from one end to the other, DaDaaaaaaa, the
> Aluminium follows
> > the magnet. I tried this with gold and the same
> thing happens.
>
> > Wowee was I surprised, all my life I thought
> Aluminium, Copper,
> > Gold were not affected by a magnet. It is
> apparently called
> > Lenz's law and involves magnetic coupling to
> usually non-
> > magnetic material.
>
> What you're seeing is an excellent demonstration of
> the relationship
> between electricity and magnetism. Non-magnetic
> materials that
> are nevertheless good conductors of electricity
> (gold, silver,
> copper, platinum, aluminum, etc.) are affected by
> changing
> magnetic fields because those fields will cause
> currents to
> flow in the conductors. And whenever you have
> current flowing
> in a conductor, that current itself produces a
> magnetic field.
>
> In Kevin's example, the current-created magnetic
> field opposes
> the field of the rare earth magnet. An experiment
> that shows
> this effect even more dramatically is to drop a
> small disk-shaped
> neodymium magnet through a hollow copper pipe held
> vertically.
> (Use a copper pipe that has an inner diameter just a
> tiny bit
> larger than the magnetic disk's diameter.) You will
> be shocked
> how slowly the magnet freefalls through the pipe!
>
> --Rob
>
>
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Received on Tue 12 Sep 2006 02:15:32 AM PDT