1. Overview…
• Properties of magnets
• Magnetic & non-magnetic materials
• Testing of a magnet
• Induced magnetism
• Theory of magnetism
• Methods of magnetisation & demagnetisation
• Plotting / Drawing of magnetic field lines
2. Testing of Magnet
One end of specimen The other end of
Specimen brought near to N specimen brought
pole near to N pole
Bar Magnet Attraction occurs Repulsion occurs
(or repulsion occurs) (or attraction occurs)
Soft Iron Rod Attraction occurs Attraction occurs
(magnetic material)
Wooden Rod Nothing happens Nothing happens
3. Testing of magnet
Thus
• If attraction occurs for both ends of the
material, then it must be a soft iron (or any
magnetic material)
• If repulsion occurs for one side of its end,
then it must be a magnet.
• Only repulsion between a specimen and a
magnet allows us to conclude that the
specimen is a magnet
4. Induced Magnetism
• When a piece of unmagnetised magnetic material
(iron or steel) touches or is brought near to a
permanent magnet, it is attracted and becomes a
magnet itself. The material is said to have
magnetism induced into it.
• Materials that has magnetism induced into it are
called induced magnet.
• The process by which the unmagnetised magnetic
materials become magnets is called magnetisation.
6. Induced Magnetism
Temporary. They lose their
magnetism once the permanent
magnet is drawn away from it.
7. Background
knowledge
Theory of Magnetism
• In an unmagnetized state, the domains
all point in different directions.
• Their magnetic effects cancel each other.
8. Background
knowledge
Theory of Magnetism
• All the domains of a fully magnetized
material point in the same direction.
10. Method of magnetization
• The process of magnetization
converts a piece of steel into a
permanent magnet.
• Note: Methods of magnetisation
produces permanent magnet, unlike
induced magnets. Thus, steel must be
used for all magnetisation and not iron
11. Method of magnetization
• Magnetization by stroking
Single stroke method
Double touch method
• Magnetization using an electric current.
12. SS S
SS
NN
NNN
N Steel Bar S
The pole produced at the end of the
magnetized steel is of the opposite
polarity of the stroking pole.
13. N S
S N
Steel Bar
What are the poles induced in the
steel bar above?
Click on either of the diagrams below.
N S S N
14. The steel bar to be
N S magnetized is placed
inside a solenoid.
When a direct current
(d.c.) is passed
through the solenoid
and then turned off,
Use the Right-Hand the steel bar becomes
Grip Rule to determine magnetized when
the polarity of the
magnetized steel.
removed from the
solenoid.
15. Ways of increasing the Magnetism
• 1) Increase the current (ie Use a stronger
battery)
• 2) Increase the number of coils / turns in the
solenoid
16. Methods of Demagnetization
• The methods of demagnetisation removes all
magnetism from a permanent magnet.
– Heating
– Hammering (east-west direction)
– Using an A.C current.
• Note:
We only demagnetised permanent magnets.
So all specimen described here are either
permanent bar magnets or magnetised steel
and not iron
17. Hammer magnet
vigorously.
By heating, the molecules vibrate vigorously
thus causing the magnetic alignment to be lost.
This is a very quick way to remove magnetism
18. Heat magnet to red-hot.
Then cool.
As in heating, the molecules are set to vibration,
causing the magnetic alignment to be lost.
19. solenoid
• Place a magnet or
magnetized steel inside a
12 V
a.c.
solenoid connected to an
alternating current (a.c)
• Turn on the current and slowly remove the
magnet 2 to 3 metres away from the solenoid.
20. Magnetic Field
Magnetic field lines were introduced
by Michael Faraday (1791 – 1867)
who named them lines of force.
22. Magnetic Field
It is a region around a magnet where
other magnetic objects experience a
magnetic force.
Lines used to represent
the direction of
magnetic field
pattern.
23. Magnetic Field Lines
Magnetic field lines are similar to electric
field lines
1) The lines can never cross each other.
2) The more closely spaced the lines, the
stronger the force and the further apart
the lines, the weaker the force.
25. How would the magnetic field pattern
differ if there are two magnets with
like poles facing each other?
26. It is the region between two magnets
where there are no magnetic field lines.
lines
This is because the field due to one
magnet cancels out that due to the other.
X is known as the neutral point.
No magnetic field lines at X.
29. Magnetic Properties of Iron and Steel
Iron Steel
It gets easily magnetized It is hard to get
magnetized
It gets easily It is hard to get
demagnetized demagnetized
Used for temporary Used to make permanent
magnets such as induced magnets
and electromagnets
Known as soft-magnetic Known as hard magnetic
material material
Editor's Notes
To explain the principles of magnetism
How do we see the field? How can we plot the magnetic field lines? In the last lesson, you have learned about the use of compass as a means of telling direction. One can interpret these lines as indicating the direction that a compass needle will point if placed at that position.
Are there finite nos of magnetic lines? Would the pattern differ when another magnet is brought near? Is the pattern identical for all magnets? No, depending on the strength of the magnet but the general pattern would be the same. What assumption(s) has/have been made? Ignore earth’s magnetic field.
