THEORETICAL AND PRACTICAL CAPSULE
(This document is intended for adults)
Attraction!
November 2009
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What is magnetism?
A natural, invisible force, just like static electricity.
Since when have we known about magnetism?
The ancient Greeks had noticed that certain stones repelled or attracted one
another, depending on their direction.
More than two thousand years ago, the Chinese had noticed that a black rock,
suspended to a string always turned in the same direction. We now know it was a
rock called "magnetite".
What is a magnet?
« A magnet is a piece of iron or steel that attracts or repels other pieces of iron or
steel» (also cobalt and nickel).
Translated from "L’électricité". Wendy Baker et Andrew Haslam, collection Eurêka!, Éditions
Scholastic, 1992, page 26
The Antidote software gives the following definition of the word magnet: mineral
(magnetite) that naturally attracts iron or steel.
There are both temporary and permanent magnets. Temporary magnets are made
from soft iron and some iron alloys. The "gain" their magnetism easily when they
are put into contact with a magnetic field, even a weak field. As soon as they are
no longer in the magnetic field, they "lose" their magnetism.
Permanent magnets are made from other alloys and retain their magnetism longer.
Magnetite is a rocky mineral containing iron. Magnetite acts as a natural magnet.
How does an object become magnetised?
The metal is made up of molecules (miniscule particles). They are made up of
atoms. In magnetic material, a small group of atoms is called a "domain". These
domains are normally directed every which way. When approaching the material of
a magnetic field, the domains all become directed in the same direction and the
material then acts as a magnet. So, as soon as an object made of iron, steel, nickel
or cobalt touches a magnet, this object also becomes a magnet.
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Here is a representation of domains in a material.
Before magnetisation
After magnetisation
Images drawn from:
http://www.sciencetech.technomuses.ca/francais/schoolzone/basesuraimant.cfm#whatare
What is the magnetic field?
Every magnet has a magnetic field. The magnetic field is a space where the magnet
exerts its force of repulsion or attraction. This magnetic field can go through
wood, plastic, glass, water, etc.
How to visualise the magnetic field of a magnet?
The magnetic field of a magnet can be revealed by spreading iron filings on a
magnet. This must be done carefully to allow the filings to be well spread out. It is
preferable to place a white piece of paper or an acetate on the magnet before
shaking the filings on. Then it is easier to pick up afterwards. The magnet may
also be placed inside a resealable plastic bag or in a medication jar.
The following sites allow you to:
• traces the lines of magnetic fields on the computer screen;
http://www.walter-fendt.de/ph14e/mfbar_f.htm
•
see a video where iron filings interact with a U shaped and a straight magnet;
•
see iron filings before and during the interaction with the magnet.
http://www.intellego.fr/soutien-scolaire-Terminale-S/aide-scolaire-Physique/VIDEO23-PHYSIQUE-Spectre-magnetique-d-un-aimant-droit-ou-en-U/21185
http://www.sciences.univ-nantes.fr/physique/perso/gtulloue/Elec/Champs/topoB.html
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Which objects are attracted by magnets?
Objects made from iron, steel, nickel and cobalt are attracted by magnets.
This explains why some pennies are attracted while others may not be. It all
depends on the materials used in their fabrication. On the following page, you will
find various compositions of Canadian money.
For pennies, there has been one of these metals (cobalt, nickel and iron) in pennies
made only since 2000. That is why only pennies manufactured starting in the year
2000 are attracted by a magnet.
As to nickels, the 25% of nickel in them is too little as compared to the copper
they contain for those coins made between 1982 and 1999 to be attracted by the
magnet. Coins made since the year 2000 contain 94.5% steel (itself made up of
iron) and a nickel plating which allows attraction with the magnet.
Did you know that?
All magnets do not have the same force.
The Earth itself is an enormous magnet. Its magnetic field extends millions of
kilometres into space. Thus the attraction of a compass needle which points north.
If you cut a magnet in half, you wind up with two magnets, both of which have a
north and a south pole.
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COMPOSITION OF VARIOUS COINS
Technical file - penny
1982-1996
Composition: 98 % copper, 1.75 % tin, 0.25 % zinc
Mass (g): 2.5
Diameter (mm): 19.1
Thickness (mm): 1.45
1997-1999
Composition: 98.4 % zinc, 1.6 % copper plating
Mass (g): 2.25
Diameter (mm): 19.05
Thickness (mm): 1.45
2000 to today
Composition: 94 % steel, 1.5 % nickel, 4.5 % copper plating or copper plated zinc
Mass (g): 2.35
Diameter (mm): 19.05
Thickness (mm): 1.45
Technical file - nickel
1982-1999
Composition : 75 % copper, 25 % nickel
Mass (g) : 4.6
Diameter (mm) : 21.2
Thickness (mm) : 1.76
2000 to today
Composition: 94.5 % steel, 3.5 % copper, 2 % nickel plating
Mass (g) : 3.95
Diameter (mm) : 21.2
http://www.mint.ca/store/mint/learn/monnaie-de-circulation-1100028?lang=en_CA
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Mediography
BAKER, Wendy. HASLAM, Andrew. L’électricité. Collection Eurêka!, Éditions
Scholastic, 1992, page 26
PINCE, Robert. Copain des sciences. Éditions Milan. 1998. Pages 60-61
Électricité et magnétisme. Collection Petit Ingénieur. Éditions Gründ. 2004.
Texte original : Alicia Rodriguez, adaptation française : Olivier Malthet
HAMMOND, Richard. Question de forces! ERPI. 2007.
http://www.sciencetech.technomuses.ca/francais/schoolzone/basesuraimant.cfm
http://lamap.inrp.fr/?Page_Id=32&DomainScienceType_Id=10&ThemeType_Id=21
http://www.edu.gov.mb.ca/m12/progetu/sn/thematique.html - 3e année - Les forces
– Bloc 3 – 3C
http://www.usborne-quicklinks.com/fr/ - Avec le livre Lumière, son et électricité
http://www.recit02.qc.ca/vigie/journal/IMG/pdf/ZOOMuniversmat-2.pdf
http://www.onfjeunesse.ca/lamission/html/fr/labo_fla.php
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