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PHYSICS – Simple phenomena of magnetism
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LEARNING OBJECTIVESCore •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
Magnets
N S
Properties Have magnetic fields around them.
Attracted?
.. or not?
Magnets
N S
Properties Have magnetic fields around them.
Have two opposite poles (N & S) – like poles repel, unlike poles attract.
Attracted?
.. may be?
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?
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?
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?
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.
NIron loses
magnetism – it was only a temporary
magnet
S
N
Steel retains magnetism – it became a permanent
magnet
Attracted?
YES!!!
Magnets – make your own!
N SS
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.
Magnets – make your own!
N
S
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
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
Magnets – how do they work?
N SJust what is happening inside the magnet to make it
magnetic?
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.
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.
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.
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.
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.
Magnetic and non-magnetic
Magnetic and non-magneticMagnetic material – can be magnetized, and is attracted to magnets. Strongly magnetic materials contain iron, nickel or cobalt (eg. Steel is mainly iron).
Magnetic and non-magneticMagnetic 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.
Magnetic and non-magneticMagnetic 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.
Magnetic and non-magneticMagnetic 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.
Magnetic fields
Magnetic fields
Iron filings sprinkled around a magnet
Magnetic field lines around the magnet
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.
Magnetic fieldsUsing a plotting compass to
find the field lines.
N S
Magnetic fieldsUsing a plotting compass to
find the field lines.
N S
Magnetic fieldsUsing a plotting compass to
find the field lines.
N S
Magnetic fieldsUsing a plotting compass to
find the field lines.
N S
Magnetic fieldsUsing a plotting compass to
find the field lines.
N S
Magnetic fieldsUsing a plotting compass to
find the field lines.
N S
Magnetic fieldsUsing a plotting compass to
find the field lines.
N S.
..
.
Magnetic fieldsUsing a plotting compass to
find the field lines.
http://www.physbot.co.uk/magnetic-fields-and-induction.html
Magnetic fieldsInteractions 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.
Magnetic fieldsInteractions 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
The Earth’s magnetic field
The Earth’s magnetic field is like that around a very large, but very weak, bar magnet.
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 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.
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’.
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.
ElectromagnetsDistinguish between the design and use of permanent magnets and electromagnets
ElectromagnetsDistinguish 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.
ElectromagnetsDistinguish 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.
ElectromagnetsDistinguish 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
coilSoft 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.
ElectromagnetsDistinguish 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
coilSoft 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
ElectromagnetsDistinguish 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
coilSoft 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
turnsUses: scrapyard electromagnets, circuit breakers, relays, electric bells.
LEARNING OBJECTIVESCore •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
PHYSICS – Simple phenomena of magnetism