I have only one small magnet shaped like a disk. I know one side is
positive and the other side is negative. How can I tell which side is
negative?

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Request for Question Clarification by josh_g-ga on 13 Nov 2002 21:14 PST

Do you have any other form of a magnet which would be usable as a
reference?  A compass, perhaps?

As far as I know at the moment, "positive" and "negative" are just
arbitrary names for the two opposite poles of a magnet.  If you don't
have a compass around, I could dig out ye olde physics textbook and
doublecheck for you.

Hi dickfeynman, 

There are a number of ways that you can determine which pole is which
on a magnet.

1. Colours. 
***********
The standard method for colouring a magnet are to colour the North
Pole Red and the South Pole Blue. Therefore, if the magnet has colours
on it, the job is made infinitely easier.

2. Using another magnet. 
************************
At most hardward stores, you can buy magnets, If you can get one with
the poles either coloured and/or marked North and South then you can
use this with your mystery magnet in the following way.

One rule of magnetism is that "like poles repel and unlike poles
attract"

Therefore if you hold the marked magnet in one hand with the North
(blue) pole leading and move it close to your mystery magnet held in
the other hand you will notice one of two things.

a) You will feel resistance in which case the mystery magnet is
repelling the known magnet. Therefore the outward facing side of your
mystery magnet is also a North pole.

b) They will pull together in which case the outward facing side of
the known magnet is the South Pole.

3. Using a compass
******************
Again compasses can be purchased cheaply from gift shops, newsagents,
etc.

The red end of the needle, being a North Pole itself, will be repelled
by your mystery magnets north pole and attracted to your mystery
magnets south pole, so this is a quick and easy way to determine it's
polarity.


Sources - Additional Links
**************************

Summer Institute
http://isaac.exploratorium.edu/~pauld/summer_institute/summer_day16magnetism/%20Magnetic_poles_1.html

Integrated magnetism FAQ
http://www.intemag.com/designtools/faq.asp


Search Strategy
***************

"find north pole" "magnetism" "polarity of a magnet" Magnet and FAQ

I hope that this helps


Johnny Phoenix

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Request for Answer Clarification by dickfeynman-ga on 14 Nov 2002 10:54 PST

My magnet is neither colored or marked in any way.
I do not have another magnet ("I have only one"). If I did, it would
be easy, as you point out.
I was hoping to get a solution which does not require a compass, or
anything else.
However, you jogged my memory so I made a compass out of the magnet
which is only 3/8" diameter and 3/16" thick. I sliced two disks off
the end of a wine cork, sandwiched the magnet in between, and held all
three pieces together with a rubber band. I then filled a 10" plastic
dish with water, set it away from any metal, and dropped it in the
water. It immediately lined up north/south, thus solving my question.

My question is answered. Your answer was invaluable.

---

Clarification of Answer by johnny_phoenix-ga on 15 Nov 2002 01:24 PST

Glad I could help. People of all abilities ask questions on here and
as such, I hope that my answer was not too simplistic as you obviously
know a thing or two about magnetism from your last clarification.

Take care.

Not a great answer, but jogged my mind to find a very good solution.
This is my first question submitted to Google Answers and I'm
definitely going to do it again

Dear dickfeynman,

I would like to add the following:
1) The terms used in magnets to differentiate (depending on the shape)
the ends/sides are “north” and “south”, not “positive” and “negative”.
2) The north end/side of a magnet that points/faces towards the
Earth's north pole is actually the magnet's south pole. This is
because of the fact that unlike poles attract while like poles repel.
3) That because of this, and if you choose north as the "positive"
end/side of your magnet, the true "positive" end/side will actually be
the south pointing/facing part of it.

I hope this also helps.

Feilong

Additional reliable sources about magnets and how it works:

Great magnetic info from the Forcefield website
http://www.wondermagnet.com/dev/magfaq.html#q12
NASA Goddard Space Flight Center, Code 696
http://www-istp.gsfc.nasa.gov/Education/Imagnet.html

Interesting magnetic brain food from the Exploratorium
http://www.exploratorium.edu/snacks/iconmagnetism.html

Since the question specified "only one" magnet, I would tend to assume
that other manmade magnets (compasses included) should not be used in
the answer.  If this is the case, then the easiest way to decide which
end of your magnet is which is to turn your magnet into a compass. 
Suspend it from a thin thread, perhaps with a dab of superglue.  Then
(once any spinning/wobbling has ceased)the side that faces north is
the south pole.

The definitive way to determine the north & south poles of a magnet
is(in principle) with a moving electric charge.

An electrically charged particle passing through a magnetic field will
experience a force which will bend the particle's path. Let's say that
your disc shaped magnet is resting flat on a tabletop. A positive ion
then zips horizontally right over the magnet. Which way the ion's path
is bent, left or right, depends upon which magnetic pole is facing up
through the particle's path.

If the north pole is up, then the charge will bend to the right, or
clockwise as viewed from overhead. Counterclockwise if the south pole
is up. If the charge is negative, then the bending directions are
opposite those of the positive charge.

Of course, it's unlikely that you'll have the experimental facilities
for this test in your living room, so just use the colored
magnet/compass tests! >:)

Erm, isn't the compass polarity effect because the earth is a magnet? 
Then dickfeynman-ga's failed his own requirement of only using one
magnet.

The polarity of a magnet can tested with a coil an an ammeter. Pass
the magnet through the coil and observe the direction of the induced
current. Details are in an elementary physics testbook.