How to magnetize and demagnetized a metal?

1. PHYSICS – Simple phenomena of
magnetism

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3. LEARNING
OBJECTIVES
Core
•Describe the forces between magnets,
and between magnets and magnetic
materials
• Give an account of induced magnetism
• Distinguish between magnetic and non-
magnetic materials
• Describe methods of magnetisation, to
include stroking with a magnet, use of
d.c. in a coil and hammering in a
magnetic field
• Draw the pattern of magnetic field
lines around a bar magnet
• Describe an experiment to identify
the pattern of magnetic field lines,
including the direction
• Distinguish between the magnetic
properties of soft iron and steel
• Distinguish between the design and
use of permanent magnets and
electromagnets
Supplement
Explain that magnetic forces are due to
interactions between magnetic fields
• Describe methods of demagnetisation, to
include hammering, heating and use of a.c. in
a coil

4. Magnets
N S
Properties
Have magnetic
fields around
them.
Attracted?
.. or not?

5. Magnets
N S
Properties
Have magnetic
fields around
them.
Have two opposite poles (N & S)
– like poles repel, unlike poles
attract.
Attracted?
.. may be?

6. Magnets
N S
Properties
Have magnetic
fields around
them.
Have two opposite poles (N & S)
– like poles repel, unlike poles
attract.
Exert little or
no force on a
non-magnetic
material.
Attracted?
.. possibly?

7. Magnets
N S
Properties
Have magnetic
fields around
them.
Have two opposite poles (N & S)
– like poles repel, unlike poles
attract.
Exert little or
no force on a
non-magnetic
material.Attract magnetic
materials by
inducing magnetism
in them.
N
Iron Steel
Attracted?
.. hopefully?

8. Magnets
N S
Properties
Have magnetic
fields around
them.
Have two opposite poles (N & S)
– like poles repel, unlike poles
attract.
Exert little or
no force on a
non-magnetic
material.Attract magnetic
materials by
inducing magnetism
in them.
N
Poles induced in both iron and steel.
S
N
S
N
Attracted?
.. mmmm?

9. Magnets
N S
Properties
Have magnetic
fields around
them.
Have two opposite poles (N & S)
– like poles repel, unlike poles
attract.
Exert little or
no force on a
non-magnetic
material.Attract magnetic
materials by
inducing magnetism
in them.
N
Iron loses
magnetism – it was
only a temporary
magnet
S
N
Steel retains magnetism
– it became a permanent
magnet
Attracted?
YES!!!

10. Magnets – make your own!
N S
S
N
How strong is it?
Not very.
Placing a piece of steel near a magnet
makes it permanently magnetised,
but its magnetism is usually weak.

11. Magnets – make your own!
N
How strong is it?
Getting stronger.
The magnet can be magnetized more
strongly by stroking it with one end
of a magnet
S
Wide sweep away
from the steel
Induced poles

12. Magnets – make your own!
How strong is it?
Strongest!
The best way of magnetizing is to
place the steel bar in a long coil of
wire and pass a large, direct (one
way) current through the coil. The
coil has a magnetic effect which
magnetizes the steel.
Coil
Steel

13. Magnets – how do they work?
N SJust what is
happening inside
the magnet to
make it
magnetic?

14. Magnets – how do they work?
N SJust what is
happening inside
the magnet to
make it
magnetic?
We need to look closely at what
is happening to the particles
(electrons) inside the magnet.

15. Magnets – how do they work?
N SJust what is
happening inside
the magnet to
make it
magnetic?
We need to look closely at what
is happening to the particles
(electrons) inside the magnet.
In an unmagnetized material,
the tiny electrons, or atomic
magnets point in random
directions.

16. Magnets – how do they work?
N SJust what is
happening inside
the magnet to
make it
magnetic?
We need to look closely at what
is happening to the particles
(electrons) inside the magnet.
When the material becomes
magnetized, more and more
of the tiny atomic magnets
line up with each other. They
act as one BIG magnet.

17. Magnets – how do they work?
N SJust what is
happening inside
the magnet to
make it
magnetic?
We need to look closely at what
is happening to the particles
(electrons) inside the magnet.
If a magnet is hit with a hammer,
the tiny atomic magnets get
thrown out of line again, so the
material becomes demagnetised.

18. Magnets – how do they work?
N SJust what is
happening inside
the magnet to
make it
magnetic?
We need to look closely at what
is happening to the particles
(electrons) inside the magnet.
If a magnet is hit with a hammer,
the tiny atomic magnets get
thrown out of line again, so the
material becomes demagnetised.
A magnet will also
become demagnetized
if heated to high
temperature.

19. Magnetic and non-magnetic

20. Magnetic and non-magnetic
Magnetic material – can be
magnetized, and is attracted to
magnets. Strongly magnetic
materials contain iron, nickel or
cobalt (eg. Steel is mainly iron).

21. Magnetic and non-magnetic
Magnetic material – can be
magnetized, and is attracted to
magnets. Strongly magnetic
materials contain iron, nickel or
cobalt (eg. Steel is mainly iron).
Ferromagnets
Hard magnetic materials,
eg. Steel, alloys (Alcomax,
Magnadur). Difficult to
magnetise, but do not
lose their magnetism.
Used for permanent
magnets.

22. Magnetic and non-magnetic
Magnetic material – can be
magnetized, and is attracted to
magnets. Strongly magnetic
materials contain iron, nickel or
cobalt (eg. Steel is mainly iron).
Ferromagnets
Hard magnetic materials,
eg. Steel, alloys (Alcomax,
Magnadur). Difficult to
magnetise, but do not
lose their magnetism.
Used for permanent
magnets.
Soft magnetic materials,
eg. Iron, Mumetal.
Relatively easy to
magnetise, but magnetism
is temporary. Used in
electromagnets and
transformers.

23. Magnetic and non-magnetic
Magnetic material – can be
magnetized, and is attracted to
magnets. Strongly magnetic
materials contain iron, nickel or
cobalt (eg. Steel is mainly iron).
Ferromagnets
Hard magnetic materials,
eg. Steel, alloys (Alcomax,
Magnadur). Difficult to
magnetise, but do not
lose their magnetism.
Used for permanent
magnets.
Soft magnetic materials,
eg. Iron, Mumetal.
Relatively easy to
magnetise, but magnetism
is temporary. Used in
electromagnets and
transformers.
Non-magnetic materials.
Metals (brass, copper,
zinc, tin and aluminium);
non-metals.

24. Magnetic fields

25. Magnetic fields
Iron filings sprinkled
around a magnet
Magnetic field lines
around the magnet

26. Magnetic fields
Iron filings sprinkled
around a magnet
Magnetic field lines
around the magnet
Field lines run from the
north pole (N) to the
south pole (S). The
magnetic field is
strongest where the field
lines are closer together.

27. Magnetic fields
Using a plotting compass to find
the field lines.
N S

28. Magnetic fields
Using a plotting compass to find
the field lines.
N S

29. Magnetic fields
Using a plotting compass to find
the field lines.
N S

30. Magnetic fields
Using a plotting compass to find
the field lines.
N S

31. Magnetic fields
Using a plotting compass to find
the field lines.
N S

32. Magnetic fields
Using a plotting compass to find
the field lines.
N S

33. Magnetic fields
Using a plotting compass to find
the field lines.
N S
.
.
.
.

34. Magnetic fields
Using a plotting compass to find
the field lines.
http://www.physbot.co.uk/magnetic-fields-and-induction.html

35. Magnetic fields
Interactions between magentic
fields
http://www.homofaciens.de/technics-magnetic-field-energy_en_navion.htm
When unlike poles are placed near
each other, their magnetic fields
combine to produce a single field of
almost uniform strength.

36. Magnetic fields
Interactions between magentic
fields
http://www.homofaciens.de/technics-magnetic-field-energy_en_navion.htm
When unlike poles are placed near
each other, their magnetic fields
combine to produce a single field of
almost uniform strength.
When like poles are placed near each
other, their magnetic fields cancel
each other, and there is a neutral
point where the combined field
strength is zero.
Neutral point

37. The Earth’s magnetic field
The Earth’s magnetic field is like
that around a very large, but very
weak, bar magnet.

38. The Earth’s magnetic field
The Earth’s magnetic field is like
that around a very large, but very
weak, bar magnet.
A compass ‘north’ end points
north. But a north pole is always
attracted to a south pole, so the
Earth’s magnetic south pole must
actually be in the north.

39. The Earth’s magnetic field
The Earth’s magnetic field is like
that around a very large, but very
weak, bar magnet.
A compass ‘north’ end points
north. But a north pole is always
attracted to a south pole, so the
Earth’s magnetic south pole must
actually be in the north.
The Earth’s magnetic north is
actually over 1200km away from
the true geographic north pole.

40. The Earth’s magnetic field
The Earth’s magnetic field is like
that around a very large, but very
weak, bar magnet.
A compass ‘north’ end points
north. But a north pole is always
attracted to a south pole, so the
Earth’s magnetic south pole must
actually be in the north.
The Earth’s magnetic north is
actually over 1200km away from
the true geographic north pole.
Over a period of
time the Earth’s
magnetic pole will
‘flip’.

41. The Earth’s magnetic field
The Earth’s magnetic field is like
that around a very large, but very
weak, bar magnet.
A compass ‘north’ end points
north. But a north pole is always
attracted to a south pole, so the
Earth’s magnetic south pole must
actually be in the north.
The Earth’s magnetic north is
actually over 1200km away from
the true geographic north pole.
Over a period of
time the Earth’s
magnetic pole will
‘flip’.
In the last 10 million
years, there have been,
on average, 4 or 5
‘flips’ per million years.

42. Electromagnets
Distinguish between
the design and use
of permanent
magnets and
electromagnets

43. Electromagnets
Distinguish between
the design and use
of permanent
magnets and
electromagnets
Unlike bar magnets, which are
permanent magnets, the
magnetism of electromagnets
can be turned on and off.

44. Electromagnets
Distinguish between
the design and use
of permanent
magnets and
electromagnets
Unlike bar magnets, which are
permanent magnets, the
magnetism of electromagnets
can be turned on and off.
Permanent magnet uses:
1. Needles of compasses.
2. Fridge door seals, holding
the doors closed.
3. Loudspeakers and
microphones.

45. Electromagnets
Distinguish between
the design and use
of permanent
magnets and
electromagnets
Unlike bar magnets, which are
permanent magnets, the
magnetism of electromagnets
can be turned on and off.
Permanent magnet uses:
1. Needles of compasses.
2. Fridge door seals, holding
the doors closed.
3. Loudspeakers and
microphones.
switch battery
coil
Soft iron
core
When a current flows
through the coil it
produces a magnetic
field. This field is
temporary and is lost
when the current is
switched off.

46. Electromagnets
Distinguish between
the design and use
of permanent
magnets and
electromagnets
Unlike bar magnets, which are
permanent magnets, the
magnetism of electromagnets
can be turned on and off.
Permanent magnet uses:
1. Needles of compasses.
2. Fridge door seals, holding
the doors closed.
3. Loudspeakers and
microphones.
switch battery
coil
Soft iron
core
When a current flows
through the coil it
produces a magnetic
field. This field is
temporary and is lost
when the current is
switched off.
Strength increased by:
- Increasing the current
- Increasing number of turns

47. Electromagnets
Distinguish between
the design and use
of permanent
magnets and
electromagnets
Unlike bar magnets, which are
permanent magnets, the
magnetism of electromagnets
can be turned on and off.
Permanent magnet uses:
1. Needles of compasses.
2. Fridge door seals, holding
the doors closed.
3. Loudspeakers and
microphones.
switch battery
coil
Soft iron
core
When a current flows
through the coil it
produces a magnetic
field. This field is
temporary and is lost
when the current is
switched off.
Strength increased by:
- Increasing the current
- Increasing number of turns
Uses: scrapyard
electromagnets, circuit
breakers, relays, electric bells.

48. LEARNING
OBJECTIVES
Core
•Describe the forces between magnets,
and between magnets and magnetic
materials
• Give an account of induced magnetism
• Distinguish between magnetic and non-
magnetic materials
• Describe methods of magnetisation, to
include stroking with a magnet, use of
d.c. in a coil and hammering in a
magnetic field
• Draw the pattern of magnetic field
lines around a bar magnet
• Describe an experiment to identify
the pattern of magnetic field lines,
including the direction
• Distinguish between the magnetic
properties of soft iron and steel
• Distinguish between the design and
use of permanent magnets and
electromagnets
Supplement
Explain that magnetic forces are due to
interactions between magnetic fields
• Describe methods of demagnetisation, to
include hammering, heating and use of a.c. in
a coil

49. PHYSICS – Simple phenomena of
magnetism