Magnetism
Magnets are made from magnetic materials. These are metals that can be
magnetised or will be attracted to a magnet. Most materials are not
magnetic, but iron, cobalt and nickel are magnetic. Steel is mostly iron,
so steel is magnetic too. If you bring two bar magnets together, there
are two things that can happen. If you bring a north pole and a south
pole together, they attract and the magnets may stick together.
If you bring two north poles together, or two south poles together, they
repel and the magnets push each other away. We say that unlike poles
attract, and like poles repel.
The magnetic field of the earth
In fact , the earth acts as if there is a
bar magnet inside it (although such a
magnet cannot really exist because
the center of the earth is too hot).
Notice in the diagram that the S-pole
of the imaginary magnet is in the
northern hemisphere so as to attract
the N-pole of a compass.
Theory of magnetism
In iron and steel , there are atomic magnets which line up with each other
groups, called domains. In an un magnetised piece of iron, the magnetic
domains are pointing in all directions and so cancel out each other.
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If the magnetic domains can be turned round to point the same way, then
the piece of iron becomes magnetized. This is because all the tiny N-poles
add up at one end and all the S-poles add up at the other end.
Magnetic materials
Iron is called a soft magnetic material because it is easy to magnetize
and also loses its magnetism easily. Iron
is
used
in
electromagnets
and
transformers. iron is also used as the
magnetic material in a reed switch.
Steel
Steel
is
called
a
hard
magnetic
material because it is harder to magnetize and also does not lose its
magnetism so easily. It is used to make permanent magnets.
Fridges and freezers have magnetic strips all round the door to keep the
door completely closed and so keep the cold air inside.
A permanent magnet is placed in the oil sump tank of a car engine so that
it collects any bits of steel that are worn off the engine.
Iron oxide is another hard magnetic material. It is used to make computer
discs , videotapes, and tape for tape-recorders.
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Magnetic ink and paints
It is possible to make
magnetic
mixing
inks
very
by
small
particles of a magnetic
substance with a liquid.
The mixture ( a dark
brown color) can be
used as a paint to make magnetic tape for tape-recorders and hard discs
for computers.
Banks use magnetic ink on checks so that the checks can be stored
automatically by a machine which detects
the magnetic field round each number.
Automatic cash-cards also use magnetic
ink to store information on the card.
Magnetic liquids
Magnetic ink can be made with oil so that
it does not dry out. This magnetic liquid can be used to detect very small
cracks in the surface of steel pipelines. These cracks can be very
dangerous if left untreated.
The pipe is magnetized by a simple coil and painted with magnetic liquid.
If there is a tiny crack, some of the magnetic lines of force leak out of the
pipe at the crack. Then the magnetic liquid shows up the shape of the
crack.
Worksheet
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1. Copy out and complete:
a. Like poles ___repel ____ ; unlike poles ___attract __ .
b. The earth's magnetic field is rather like that of a __ bar _ magnet
with its ___south __ pole in the northern hemisphere.
c. Iron is easily ___m agnetised __ and easily __dem agnetised __ .
d. Steel is used to make __ perm anent _ magnets.
e. If the N-pole of a magnet is brought near an unmagnified nail, then
magnetism is __transferred __ into the nail. The nail is then
___attracted ___ to the magnet.
2. Explain the following:
a) Magnets are often fitted to the doors of refrigerators and cupboards.
to keep the door com pletely closed and so keep the cold air
inside.
b) The sump plug on a car is usually magnetized.
so that it collects any bits of steel that are w orn off the engine.
c) A magnetized screwdriver can help to place screws in holes that are
hard to reach.
I t holds the screw and keeps it from falling dow n.
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3. you are given a small plotting compass and three grey
metal bars. one is aluminum, one is un magnetised iron, one
is a magnet. how can you identify each one?
 P lace the com pass near the m etal bars , the one w ho don’t
attract or affect the pointer of the com pass is the
alum inum .
 The one w ho changes the direction of the com pass pointer
is the m agnet.
 The one w ho is attracted and stuck to the com pass is the
un m agnetised iron.
4. Professor Messer says that 'like poles must attract' because
the North pole of the Earth attracts the N-pole of a
compass. Explain why he is wrong.
because like poles repel and un like poles attract. The north
pole of the earth attracts the south pole of the m agnet.
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Resources:
 New Physical or you.
Nelson Thornes
ISBN 0-7487-8328-8
Chapter 34 – page 280
 Websites:
http://www.google.com.qa/imgres?q=Hires+bar+magnet&hl=en&sa=X&qscrl=1&nord=1&rlz=1T4RNRN_enUS444US445&bi
w=1280&bih=513&tbm=isch&prmd=imvns&tbnid=b5xwvgohOTYtNM:&imgrefurl=h
ttp://physicsbuzz.physicscentral.com/2009_09_01_archive.html&docid=Db_eNFYLZ
EbgxM&imgurl=http://upload.wikimedia.org/wikipedia/commons/thumb/d/d8/Bar
_magnet.jpg/800pxBar_magnet.jpg&w=800&h=600&ei=sZfVT6rXNNHQrQe8_Z38Dw&zoom=1&iact=hc
&vpx=539&vpy=2&dur=4094&hovh=194&hovw=259&tx=152&ty=88&sig=109337
572700176211273&page=2&tbnh=144&tbnw=237&start=11&ndsp=18&ved=1t:42
9,r:2,s:11,i:112
http://www.google.com.qa/imgres?q=bar+magnets&hl=en&qscrl=1&nord=1&rlz=
1T4RNRN_enUS444US445&biw=1280&bih=513&tbm=isch&tbnid=LajmKv8KkKylJM
:&imgrefurl=http://www.universetoday.com/48178/barmagnet/&docid=B9ZGWzmYG9pizM&imgurl=http://www.universetoday.com/wpcontent/uploads/2009/12/Bar_magnet.jpg&w=580&h=435&ei=V5vVT_OOMI_jrAfG
vtj7Dw&zoom=1&iact=hc&vpx=216&vpy=183&dur=1781&hovh=194&hovw=259&t
x=130&ty=100&sig=109337572700176211273&page=1&tbnh=134&tbnw=173&st
art=0&ndsp=13&ved=1t:429,r:1,s:0,i:110
http://www.bbc.co.uk/schools/ks3bitesize/science/energy_electricity_forces/magn
ets_electric_effects/revise2.shtml
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