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Transcript
Magnetism
Name: ________________
Class: _________________
N
Index: ________________
NW
NE
E
W
SW
SE
S
Objectives
--state properties of magnets
--describe induced magnetism
--describe electrical methods of magnetisation and demagnetisation
--describe the plotting of magnetic field lines with a compass
--draw the magnetic pattern around a bar magnet and between the
poles of two bar magnets
--distinguish between the magnetic properties and uses of temporary
magnets (e.g. iron) and permanent magnets (e.g. steel)
Discovery of the phenomenon
About 900 years ago, the Chinese found that a dish
carrying a certain type of rock known as magnetite would
constantly float in water in a North-South direction.
Magnetite
Magnetic materials
Magnetite consists of an iron oxide.
Natural magnet attracts certain
materials:
• cobalt
• nickel
• iron
• steel
• alloys of any of the above
These materials are called
Magnetic materials.
Alloy of Nickel and
Cobalt
Non-Magnetic materials
Natural magnet cannot attract other
materials.
These include:
• copper
• brass
• wood
• plastics
• materials other than iron, steel,
cobalt, nickel
These materials are called NonMagnetic materials.
Copper/Brass plated
pipes
Properties of magnets
All magnets exhibit the following properties:
1. Attract magnetic materials
Properties of magnets
All magnets exhibit the following properties:
2. They have 2 magnetic poles;
the North and South seeking poles.
These are the strongest parts of the magnets.
The poles are found very near (but not at) the ends of the magnet.
Properties of magnets
All magnets exhibit the following properties:
3. If allowed to swing freely a magnet will come to rest with one
end pointing towards the Earth’s North pole, the other end
pointing towards the Earth’s South pole. Hence, a magnet can
be used as a compass for navigational purposes.
cylindrical rod made of non-magnetic material
Properties of magnets
All magnets exhibit the following properties:
4. Law of magnetic poles:
Take a look at the following actions taken during an
experiment. What can you conclude ?
(Step 1)
(Step 2)
Properties of magnets
All magnets exhibit the following properties:
4. Law of magnetic poles:
Take a look at the following actions taken during an
experiment. What can you conclude?
(Step 3)
(Step 4)
Properties of magnets
All magnets exhibit the following properties:
4. Law of magnetic poles:
Conclusion: Like poles repel, Unlike poles attract.
Properties of magnets
Question 1:
In an experiment conducted to test if an object is a magnet,
one end of this object (A) is brought near
one end (X) of a suspended bar magnet.
Attraction occurs.
Can you conclude that the object is a magnet?
A
X
Y
Properties of magnets
Question 1:
In an experiment conducted to test if an object is a magnet,
one end of this object (A) is brought near
one end (X) of a suspended bar magnet.
Attraction occurs.
Can you conclude that the object is a magnet?
Answer:
Not yet.
The object could have been a magnet with end A an opposite pole
to that of end X of the magnet; or
The object could just have been an ordinary magnetic material
(unmagnetised yet).
A
X
Y
Properties of magnets
Question 2:
In the same experiment,
The same end of this object (A) is brought near
the other end (Y) of a suspended bar magnet.
(i) If attraction occurs again, can you conclude now that the
object is a magnet?
(ii) If repulsion occurs instead, can you conclude now that
the object is a magnet?
A
YY
X
Properties of magnets
Question 2:
In the same experiment,
The same end of this object (A) is brought near
the other end (Y) of a suspended bar magnet.
Answer:
(i) If attraction occurs again, can you conclude now that the object is a
magnet?
No, the object is just an unmagnetised magnetic material as
no repulsion between the object & bar magnet was observed.
(ii) If repulsion occurs instead, can you conclude now that the object is
a magnet?
A
YY
X
Properties of magnets
Question 2:
A
In the same experiment,
The same end of this object (A) is brought near
the other end (Y) of a suspended bar magnet.
YY
X
Answer:
(i) If attraction occurs again, can you conclude now that the object is a
magnet?
No, the object is just an unmagnetised magnetic material as
no repulsion between the object & bar magnet was observed.
(ii) If repulsion occurs instead, can you conclude now that the object is a
magnet?
Yes, it is a magnet.
Since only like poles repel (A repels Y)
& unlike poles attract (A attracts X),
the object is indeed a magnet.
Properties of magnets:
Repulsion is the only true test for polarity.
Floating magnets
Induced Magnetism
N
S
Far apart
Permanent magnet
N
Soft-iron bar
S
Permanent magnet brought
near to soft-iron bar
N
S
Soft-iron bar becomes an
induced magnet
When a non-magnetised magnetic material is brought near to (or
touches) a magnet, the material itself will become a weak magnet.
This is called induced magnetism
(which means the material has magnetism induced in it).
Induced Magnetism
N
S
Far apart
Permanent magnet
N
Soft-iron bar
S
Permanent magnet brought
near to soft-iron bar
N
S
Soft-iron bar becomes an
induced magnet
Notice that magnetic induction, an opposite pole is always induced.
In other words, 2 unlike poles facing each other is observed
during magnetic induction.
Induced Magnetism
permanent magnet
N
S
induced magnet
N
S
If placed sufficiently near to each other,
attraction occurs between the permanent & induced magnets.
Induced magnetism in magnetic materials is the reason that
these non-magnetised objects are able to be attracted to magnets.
Induced Magnetism
Here is another example of induced magnetism.
Induced Magnetism
Using the theory of induced magnetism,
explain how it is possible to get several iron nails to stick
together (as shown in the diagram below).
Magnetisation
Making a material permanently magnetic is called
magnetisation. There are several ways to magnetise
materials.
Magnetisation by Stroking (Single-Touch)
This method is derived from applying
the lessons learnt on
magnetic induction.
Note the polarities of both the
permanent magnet & steel bar that is to
be magnetised.
This form of magnetism gained is weak
but permanent.
Magnetisation by Stroking (Double-Touch)
This method is also derived from applying the lessons learnt on
magnetic induction.
2 permanent magnets are used in this method, as compared to one being
used in the single-touch stroking method.
Note the polarities of both permanent magnets.
Once again, do take note of the polarities of the permanent magnets &
their induced ends of the steel bar.
This form of magnetism gained is also weak but permanent.
Magnetisation by Heating & Hammering
A magnet can be made by first placing a
steel bar in a magnetic field, then heating
it to a high temperature and then finally
hammering it as it cools.
This can be done by laying the magnet
in a North-South direction in the Earth’s
magnetic field.
However, the magnet produced is not
very strong but permanent.
Magnetisation by the use of an
Electrically-generated magnetic field of a Solenoid
Place the steel object inside a coil of wire (a solenoid).
Pass a direct current (d.c.) through the solenoid for a few seconds.
A magnetic field is produced on the solenoid.
As such, the steel rod is now placed inside a magnetic field.
When the current is turned off the steel rod is found to be magnetised.
Note: the d.c. flows through the solenoid. It does not flow through
the steel rod.
steel rod
direct
current
Magnetisation by the use of an
Electrically-generated magnetic field of a Solenoid
The polarity of the newly-formed magnet can be determined using the
Right-hand Grip Rule.
(Fingers coiled round & following the direction of the flow of d.c. in the
solenoid; the thumb will point in a direction indicating the end which
becomes the N-pole).
steel rod
direct
current
Magnetisation by the use of an
Electrically-generated magnetic field of a Solenoid
The polarity of the newly-formed magnet can also be determined
using the method:
Take a look at which way the d.c. is flowing at each end.
If the direction of flow is anticlockwise, the end is a N-pole.
If the direction of flow is clockwise, the end is a S-pole.
steel rod
direct
current
Looking through
end B
aNticlockwise
direction flow of d.c.
Looking through
end A
clockwiSe
direction flow of d.c.
Magnetisation by the use of an
Electrically-generated magnetic field of a Solenoid
The magnetism produced using this method is strong & permanent.
steel rod
direct
current
Demagnetisation by Heating & Hammering
Heat a magnet.
Then hammer it as it is allowed to cool in the absence of a magnetic
field
i.e. facing East-West .
Demagnetisation by the use of an
Electrically-generated magnetic field of a Solenoid
(700 turns)
magnet
withdrawn
to a few metres
Demagnetisation by the use of an
Electrically-generated magnetic field of a Solenoid
Place magnet in a solenoid.
Pass an alternating current (a.c.)
through the solenoid (not through the magnet).
Slowly remove the magnet from the solenoid
with the a.c. supply still on.
Remove to a great distance.
Repeat the procedure for as many times as it is
necessary.
Each time it is done, the magnet’s strength
weakens.
Finally, it is completely demagnetised.
(700 turns)
magnet
withdrawn
to a
few metres
Magnetic Field
A magnetic field is the region where a magnetic force is exerted
on any magnetic objects placed within the influence of the field.
Showing the Magnetic Field Using Iron a Filings
One method to observe the shape of the magnetic field is by sprinkling
iron filings onto a piece of paper placed on top of the magnet.
Plotting Compass
A compass is a freely suspended. A compass is normally drawn with the
N-pole shown as an arrowhead. It can be used to find the direction of a
magnetic field.
Remember the N-pole of the compass points to the Earth’s N-pole. The
Earth’s magnetic field is produced by electric currents at its core. It is
similar to the field that would be due to an imaginary large bar magnet in
the Earth’s centre.
What do you think are the directions that the compass would point
in if placed in the ten different points around a strong permanent
magnet.
S
N
S
N
Did you get them all
correct?
Magnetic Field Lines
Magnetic field lines are imaginary & represent the direction of the
magnetic field.
Magnetic field lines are also known as lines of force
because if magnetic objects are placed in the region of the field lines,
the magnetic objects will experience a magnetic force directed along
the same lines. By convention, the magnetic field line is the path along
which an imaginary “free” N-pole will move if placed along this line.
Neutral Point
Whenever a point in space has no magnetic field the magnetic field due
to one magnet cancels out that due to another magnet, this point is
known as a Neutral Point.
neutral point
Plotting Magnetic Field Lines With
A Plotting Compass
The lines can be investigated to find
their path and direction using a
plotting compass.
Place a plotting compass at point A.
Note the direction it points at.
Mark a 2nd point next to the N-pole
of the plotting compass.
2
Plotting Magnetic Field Lines With A
Plotting Compass
The lines can be investigated to find their path
and direction using a plotting compass.
Place a plotting compass at point A.
Note the direction it points at.
Mark a 2nd point next to the N-pole
of the plotting compass.
These steps are repeated as shown.
The points are all joined using
a pencil.
All these steps are repeated for other
points next to the N-pole of the magnet.
Examples of Magnetic Fields
(i) A permanent bar magnet.
(ii) 2 opposite poles facing
each other
Examples of Magnetic Fields
(iii) 2 like poles facing each other
(e.g. 2 N-poles)
Neutral Point
If a plotting compass is placed at the neutral point (i.e. X), how
will it point?
Earth’s
Magnetic
field
Neutral Point
It will point in the same direction
as that of earth’s magnetic field.
Earth’s
Magnetic
field
Properties of Field Lines
·
·
·
·
Lines always start and end on the magnet.
The lines travel from the N-pole to the S-pole.
The lines never cross or touch each other.
The closer the lines the stronger the field.
(More magnetic field lines do not necessarily
mean stronger magnetic field)
Try labelling the poles of the following magnets. (Some
books draw magnetic field lines as broken lines)
These are possible answers.
N
N
S
S
N
N
S
N
S
N
N
N
S
N
S
S
These are also possible answers.
S
S
N
N
S
S
N
S
N
S
S
S
N
S
N
N
Magnetic Shielding
Magnetic fields are sometimes not wanted and can damage
delicate equipment such as watches, televisions, computer disks,
etc..
Shielding is achieved by surrounding the object with soft iron. The
lines of magnetic force concentrate in the soft iron.
Although iron and steel are both magnetic materials,
their properties are different.
•Two unmagnetised rods have a magnet
placed on top of them.
•Iron filings are supported from the
induced magnets
•The permanent bar magnet is then
removed.
•Note that the iron bar no longer has any
iron filings attracted to it.
•The steel bar, however, still has some iron
filings attracted to it.
Magnetic properties of Iron & Steel:
1. Iron is easily magnetised, whereas Steel is not easily magnetised.
2. Iron is easily demagnetised, whereas Steel is not easily
demagnetised.
3. Iron is thus known as a soft magnetic material (easily magnetised
& easily demagnetised).
4. Steel is however known as a hard magnetic material (difficult to
magnetise & difficult to demagnetise).
5. Iron is thus used in making electromagnets & steel is thus used in
making permanent magnets.
Permanent magnets have many uses.
Refrigerator
Door catch
Permanent magnets have many uses.
Ammeter / Voltmeter
Loudspeaker
Electromagnets have many uses too.
The electric-bell
A simple magnetic-relay
References
http://zentropolis.com/log_images_2003/magnetite.jpg
http://www.mech-seal.com/images/3.jpg
http://i01.i.aliimg.com/photo/v0/200020993/Copper_Brass_Plated_Pipe_and_tube.jpg
http://user.uni-frankfurt.de/~scherers/blogging/Magnets/magnet1.jpg
http://image.wistatutor.com/content/magnetism/laws-of-magentism.jpeg
http://www.tarangscientificinstruments.com/magnetism&electricity/floating_magnet.gif
http://1.bp.blogspot.com/_TKYHYGtSzCU/StcQ4N4bqXI/AAAAAAAAAsU/rtc5avxWdU/s400/jeffrey1.jpg
http://4.bp.blogspot.com/_EObJM5duFU8/SOn8ii5KINI/AAAAAAAAC6Q/6_rBfwQaXbU/s400/RightHand-Grip-Rules2.png
http://img.youtube.com/vi/OODRqLwV5uI/0.jpg
http://www.educationalmodels.com/product/campass-needle--coils/ELP.110.133-Plotting-compas.jpg
http://www.school-for-champions.com/science/images/magnetic_detection-iron_filings.jpg
http://homofaciens.com/bilder/technik/magnetic-field05.gif