SIMPLE PHENOMENA OF MAGNETISM

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


Have two opposite poles (N & S) – like poles repel, unlike poles attract.

Attracted?

.. may be?

Magnets

N S



Exert little or no force on a non-magnetic material.

Attracted?

.. possibly?

Magnets

N S




Attract magnetic materials by inducing magnetism in them.

N

Iron Steel

Attracted?

.. hopefully?

Magnets

N S





N

Poles induced in both iron and steel.

S

N

S

N

Attracted?

.. mmmm?

Magnets

N S





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.


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


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?



magnetic?

We need to look closely at what is happening to the particles (electrons) inside the magnet.



magnetic?


In an unmagnetized material, the tiny electrons, or atomic magnets point in random directions.



magnetic?


When the material becomes magnetized, more and more of the tiny atomic magnets line up with each other. They act as one BIG magnet.



magnetic?


If a magnet is hit with a hammer, the tiny atomic magnets get thrown out of line again, so the material becomes demagnetised.



magnetic?


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).


Ferromagnets

Hard magnetic materials, eg. Steel, alloys (Alcomax, Magnadur). Difficult to magnetise, but do not lose their magnetism. Used for permanent magnets.


Ferromagnets


Soft magnetic materials, eg. Iron, Mumetal. Relatively easy to magnetise, but magnetism is temporary. Used in electromagnets and transformers.


Ferromagnets


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



N S



N S



N S



N S



N S



N S.

..

.



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.



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.



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’.




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


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.





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.





microphones.

switch battery

coilSoft iron core


Strength increased by:- Increasing the current- Increasing number of

turns





microphones.

switch battery

coilSoft iron core


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

SIMPLE PHENOMENA OF MAGNETISM

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