2. We start this unit by looking at naturalWe start this unit by looking at natural
magnetism, which is any solid objectmagnetism, which is any solid object
which exhibits permanent magneticwhich exhibits permanent magnetic
properties.properties.
Most of this unit will deal withMost of this unit will deal with
electromagnetism. This is magnetism thatelectromagnetism. This is magnetism that
has been created by thehas been created by the movementmovement ofof
charge (orcharge (or direct currentdirect current))
Natural magnetism has been known forNatural magnetism has been known for
thousands of years (600 BC). Historically,thousands of years (600 BC). Historically,
all rocks that had magnetic propertiesall rocks that had magnetic properties
were named “lodestone”. These rockswere named “lodestone”. These rocks
contained iron oxide Fecontained iron oxide Fe33OO44. Today,. Today,
magnetic iron ore is called “magnetite”.magnetic iron ore is called “magnetite”.
3. Magnetic PolesMagnetic Poles
If a bar magnet is suspended by a string it willIf a bar magnet is suspended by a string it will
spin until the north magnetic pole pointsspin until the north magnetic pole points
__________ . This tells us that the north end of a__________ . This tells us that the north end of a
compass must have _________ magneticcompass must have _________ magnetic
polarity.polarity.
The names of the ends of a compass came fromThe names of the ends of a compass came from
navigation where it was observed that one endnavigation where it was observed that one end
of a magnet, the “north seeking end” alwaysof a magnet, the “north seeking end” always
points north.points north.
N S
C E IL IN GU p
W E
4. Metals which are strongly magnetic are calledMetals which are strongly magnetic are called
ferromagnetic.. These materials are: nickel (Ni), ironThese materials are: nickel (Ni), iron
(Fe) and cobalt (Co). These metals have(Fe) and cobalt (Co). These metals have domains
which are 1 mm long regions that have a specificwhich are 1 mm long regions that have a specific
polarity. In a permanent magnet (usually steel) all ofpolarity. In a permanent magnet (usually steel) all of
the domains are alignedthe domains are aligned (domain theory). In a non-In a non-
magnetic piece of iron, all the domains are random inmagnetic piece of iron, all the domains are random in
nature. (Note: The arrowhead is the “north” polarity.)nature. (Note: The arrowhead is the “north” polarity.)
M A G N E T I C D O M A I N S
O F I R O N B A R A R E
N O W A L I G N E D
M A G N E T I C D O M A I N S
O F I R O N B A R A R E
N O T A L I G N E D
5. Magnetic Properties:
Like poles repelLike poles repel
Opposite poles attractOpposite poles attract
The magnitude of the force of attraction orThe magnitude of the force of attraction or
repulsion, between permanent magnets is asrepulsion, between permanent magnets is as
follows:follows: F α d-2
, ( an inverse square relationship, ( an inverse square relationship
) and) and d is the separation between the twois the separation between the two
magnets.magnets.
All permanent magnets have a south and northAll permanent magnets have a south and north
pole. Breaking a magnet still results in bothpole. Breaking a magnet still results in both N
&& S poles –poles – why?
Only magnetic materials can be attracted toOnly magnetic materials can be attracted to
each other. Metals such as copper (Cu) andeach other. Metals such as copper (Cu) and
aluminum (Al) are not magnet and will not bealuminum (Al) are not magnet and will not be
attracted to a permanent magnet –attracted to a permanent magnet – why?
6. What happens when aWhat happens when a
permanent magnet is broughtpermanent magnet is brought
close to a non-magnetizedclose to a non-magnetized
piece of iron?piece of iron?
As seen from the diagram,As seen from the diagram, the
domains of the iron bar realign
with the field from the strong
magnet and the iron bar is nowand the iron bar is now
magnetic and is attracted tomagnetic and is attracted to
the strong magnet.the strong magnet.
What happens to the domains
when the strong magnet is
removed?
Ans. The domains will go back
to being random (point in all
directions).
N
S T R O N G M A G N E T
S
M A G N E T I C D O M A I N S
O F I R O N B A R A R E
N O W A L I G N E D
7. The magnetic field of a barThe magnetic field of a bar
magnet is represented inmagnet is represented in
2D by “lines of force” -2D by “lines of force” -
these are the green lines.these are the green lines.
But the field is actually 3DBut the field is actually 3D
with the field beingwith the field being
somewhat football shaped.somewhat football shaped.
The arrows show theThe arrows show the
polarity and also representpolarity and also represent
the direction a compassthe direction a compass
would point if placed in thewould point if placed in the
magnetic field.magnetic field.
The stronger the field, theThe stronger the field, the
moremore lines of force that arelines of force that are
drawn. This is often calleddrawn. This is often called
field density..
N
S
8. Other Aspects of Permanent Magnets:
Usually made of steel (which is mainly iron)Usually made of steel (which is mainly iron)
since steel tends to better lock the domains insince steel tends to better lock the domains in
place. In iron, the domains quickly will go backplace. In iron, the domains quickly will go back
to a random state.to a random state.
If a magnet is dropped or hit, the domains willIf a magnet is dropped or hit, the domains will
likely lose their alignment and magneticlikely lose their alignment and magnetic
properties will weaken or be lost.properties will weaken or be lost.
Heating a magnet above its Curie pointHeating a magnet above its Curie point
(temperature) will result in all magnetic(temperature) will result in all magnetic
properties being lost (table page 673-McGraw).properties being lost (table page 673-McGraw).
The magnetic field tends to be strongest at theThe magnetic field tends to be strongest at the
poles (ends) of the magnet. This is where thepoles (ends) of the magnet. This is where the
field lines are closest together.field lines are closest together.
9. Practice Questions
Nelson Textbook
Page #552 #5, 6a, b, dPage #552 #5, 6a, b, d
Workbook Pages 66-67Pages 66-67
From McGraw-Hill TB (pg 675)From McGraw-Hill TB (pg 675)
1.1. If an iron bar is held in a north-south orientationIf an iron bar is held in a north-south orientation
and is then tapped gently with a hammer, it willand is then tapped gently with a hammer, it will
become a weak magnet. In light of domainbecome a weak magnet. In light of domain
theory, why should this occur?theory, why should this occur?
2.2. If a steel nail is continuously stroked along itsIf a steel nail is continuously stroked along its
length by one pole of a magnet it will become alength by one pole of a magnet it will become a
permanent magnet. Use domain theory topermanent magnet. Use domain theory to
explain this.explain this.
10. 3.3. Changes in magnetic field strength areChanges in magnetic field strength are
shown by increasing or decreasing theshown by increasing or decreasing the
number of field lines. What can benumber of field lines. What can be
concluded about a magnet that has 5concluded about a magnet that has 5
times as many field lines as another in atimes as many field lines as another in a
similar region of space?similar region of space?
It is useful to refer to textbook reference
pages for diagrams on magnetism..
