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Magnetism
Aurora
Where can we find a magnetic
field?
• Around a permanent magnet
• Around the conductor which
carry current
e.g. Electromagnets
All magnets have two poles, named the
“north pole”and the “south pole”.
What’s the effect for magnetic field?
• Attract certain types of
material(ferrous), like iron、 cobalt
or nickel alloys
• Attract or repel another magnet.
“like pole repel and unlike pole
attract.”
• Others……….
Magnetic Field
• It is a region in which a particle
with magnetic properties
experiences a force, and in which
a moving charge experiences a
force.
How to know the strength and the
direction of the magnetic field?
The forces increase as poles get closer together.
The greater the magnetic force experienced
by the test magnet, the stronger the
magnetic field strength.
the direction of the magnetic field at a point
is taken as the direction that a north pole
would tend to move if it were at that point
The Earth's magnetic field
• The Earth is a gigantic magnet with
its magnetic south pole is close to its
geographic north pole, and its
magnetic north pole is close to its
geographic south pole.
When a magnet is let to
move freely, it will align
itself with its north pole
pointing to the north.
This is how a magnetic
compass works.
How to describe a magnetic field?
•Field line：outside of the magnet,
start from N pole and terminate at S
pole
5 Describe an experiment to identify the
pattern of field lines round a bar magnet
Place a small compass (plotting compass) near
the north end of the bar magnet on a sheet of
paper ,Use a pencil to mark on the paper where
the arrow head points ,
Move the compass so that the tail of the arrow is
on the pencil mark ,Repeat steps 1-3 until you
reach the magnet again ,
Repeat the whole process from different starting
points until the field is complete as in the diagram
above .
Field line for a
bar magnet
Measuring Magnetic Field Strength
• Magnetic field strength, B
• Magnetic field strength is often called
magnetic flux density and is given the
symbol 'B' .
• Magnetic field strength is measured in
tesla, T.
Measuring Magnetic Field Strength
• The strength of a magnetic field
can be found using a device
called the Hall probe.
Magnetic field strength on Es about
0.00005Telsla / 5×10－6T
2 What is induced magnetism?
• A piece of iron or steel becomes
magnetised when a magnet is brought
near to it.
• Microscopic ‘domains’ inside the ferrous
material are all lined up by the nearby
magnet
25/05/2017
Why are Material MAGNETIC
Unmagnetised Material
Most materials look like this
Magnetised Material
Only magnetic materials can
do this
Breaking a magnet forms two new
magnets.
3 Describe how a piece of ferrous
metal can be magnetised
• Stroking Method:
• Stroke the metal with a magnet, repeating often, using
the same end of the magnet and always in the same
direction
• Electrical Method:
• Wind a wire round the metal and connect the wire to a
direct current source (e.g. a battery). This create an
electromagnet, but some magnetism remains after the
battery is disconnected
4 Describe how a magnet can be
demagnetised
•Hammering Method
Hit the magnet repeatedly with a hammer
•Heating
Heat the magnet in a Bunsen flame until it is red
hot
•Alternating Current Method
Wind a wire round the magnet and connect it to
an alternating current
6 Distinguish between the magnetic
properties of iron and steel
• Iron is known as a SOFT magnetic
material. It is easily magnetised and
easily loses its magnetism.
• Steel is known as a HARD magnetic
material. It is difficult to magnetise and
keeps its magnetism well
7 Describe the design and use of
permanent magnets and electromagnets
• Permanent magnets are made of hard
magnetic materials (steel) so that they do
not lose their magnetism. They are used in
compasses, electric motors, fridge doors, etc.
• Electromagnets are made of soft magnetic
materials (iron) so that they can be switched
on and off easily. They are used in junk yard
cranes to move old vehicles and also in
electronic relays, burglar alarms, etc.
MAGNETIC FIELDS DUE TO CURRENTS
MAGNETIC FIELDS DUE TO CURRENTS
• The ‘right-hand rule’ can be used to
give the direction of the magnetic
field lines produced by a current in
a long straight wire:
•
The wire is gripped with the righthand, with the thumb pointing in the
direction of the current, and the
fingers then curl in the direction of
the field lines .
Describe the magnetic field due to a
current in a straight wire
• The magnetic field round
a straight wire is in the
form of circles.
• The right hand grip rule
can be used to predict the
direction of the field.
• EXTENDED: the field is
strongest nearest to the wire
and stronger for bigger
currents
Direction of current
•Imagine a wire going directly into this screen.
•A dot would show a current coming outwards.
•A cross would show a current going inwards.
Draw out the direction of the
magnetic field line.
B
I
B
I
Draw out the direction of the current
by using“.”or “×” in the center.
B
I
B
I
Draw out the direction of the current
in the wire.
Draw out the direction of the
magnetic field line and the north
pole of the compass.
2 Describe the magnetic field due to a
current in a solenoid
• The field near a solenoid is
like that of a bar magnet
• The poles can be predicted
using the diagram above
• EXTENDED: the field is
strongest near the centre of
the solenoid, and stronger for
bigger currents
Right Hand Grip Rule
•Grasp the coil or
solenoid in your right
hand with the fingers
pointing in the
direction of the current.
The thumb points
towards the direction
of the magnetic field
inside the solenoid.
Compare magnetic field between
bar magnet and solenoid with
current.
• Inside the magnet, the field
lines continue from the
south pole to the north pole
to complete a loop.
1. Relay
An advantage of using a relay as a switch is that:
• a small current in one circuit
can safely control a larger
current in a second circuit
3 Draw a circuit for an electric bell and describe
how it works.
3 Draw a circuit for an electric bell and describe
how it works.
• When the circuit is completed, by
pressing the bell-push, the current
flows through the coil and creates an
electromagnet
• The electromagnet attracts the
armature, which causes the hammer
to hit the bell
• At this point the circuit is broken at X,
so the electromagnet is turned off
• As the armature is no longer
attracted to the electromagnet, the
springy metal strip pulls it back
• This creates contact at X again and
the whole cycle repeats until the
bell-push is released
4 List some other applications of the
magnetic effect of current
•
•
•
•
Electromagnetic Door Locks
Electromagnetic Relays
Loud Speakers
Ammeters and Voltmeters