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Magnetism
All magnets have two poles: The North pole and the South pole. They are called so
because if you hang a magnet by a thread, it will rotate so that the two ends will point to
the north and south poles of the earth respectively. The force which causes this is called
the magnetic force and it is a non-contact force. This means that the force is exerted
even if magnets are not in contact with each other.
If two south poles or two north poles are brought near each other, repulsion will occur. If
a north pole is brought near a south pole, attraction will occur.
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Like poles repel each otherUnlike poles attract each other
Making a magnetA piece of steel becomes permanently magnetized when placed near a magnet but its
magnetism is weak. It can be magnetized more strongly by stroking it with one end of a
magnet. However the best way of magnetizing it is to place it in a solenoid carrying a
current ( seen later).
Magnetic and non-magnetic materials
A magnetic material also called a ferromagnetic material is one which can be
magnetized and is attracted to magnets. All strongly magnetic materials contain iron,
nickel or cobalt. Steel is made mostly of iron. Strong magnetic materials like this are
called ferromagnets. They are described as hard or soft depending on how well they
keep their magnetism.
Hard magnetic materials such as steel and alloys called Alcomax and Magnadur are
difficult to magnetize but do not readily lose their magnetism. They are used for
permanent magnets.
Soft magnetic materials such as iron and Mumetal are relatively easy to magnetize, but
their magnetism is only temporary. They are used in electromagnets and transformers
because their magnetism can be turned off or reversed easily.
Non-magnetic materials include metals such as brass, copper, zinc, tin and aluminium
as well as non-metals.
Magnets are made up of tiny groups of dipoles or atomic magnets called domains.
These are represented in diagrams by arrows. In a magnet, these arrows all point
towards the North pole. In an unmagnetised piece of iron or steel, the arrows (domains)
point in all directions. However, when a piece of iron or steel is magnetized, the domains
will align themselves and all point towards the north pole. In non-magnetic materials like
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copper, zinc and aluminum, the dipoles can never be lined up, so they are not attracted
by magnets.
Demagnetization
If a magnet is hammered (or dropped continuously), its atomic magnets are
thrown out of line. It becomes demagnetized.
Heating it to a high temperature has the same effect.
Placing the magnet in a changing magnetic field
Magnetic fields
The area around a magnetic where it has a magnetic effect is called the magnetic field.
The lines which show the direction of the magnetic field are called lines of flux. The
lines are always coming out of the North pole and go back in through the South pole.
These can be found by a simple experiment using iron filings. The iron filings which are
very light pieces of iron are attracted to these lines of flux and will settle down on the
lines thus forming the same pattern as the magnetic field.
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The field lines run from the N pole to the S pole of the magnet. The field
direction, drawn by an arrowhead, is defined as the direction in which the force
on a N pole would act.
The magnetic field is strongest where the field lines are closest together.
If two magnets are placed near each other, their magnetic field combine to produce a
single field. ( see book)
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Earth’s magnetic field
The earth has a magnetic field which acts as though there was a giant bar magnet in its
centre, lined up approximately between its geographic north and south poles, although
the angle is constantly changing. The north pole of a compass points towards a point
called the magnetic north and its south pole towards the magnetic south.
Magnetic effect of a current
If a compass ( which detects magnetism) is placed under a wire carrying a current, the compass needle will move. This is because a wire carrying a current has a magnetic field around it.
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To find the direction of the lines of flux, we use a method called the
Right Hand Grip Rule:
imagine gripping the wire with your right thumb pointing the same way as the current,
then your fingers are curling the same way as the magnetic field.
The magnetic field produced has these features:
The magnetic field lines are circles
The field is strongest close to the wire
Increasing the current increases the strength of the field
Magnetic field near a coil
A coil whose length is large compared to its diameter is called a solenoid. The right
hand grip rule can be used to find the direction of the magnetic field.
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This shows that the magnetic field round a solenoid has the same shape as the field
round a bar magnet. Since the inside of the solenoid has a very strong field, objects can
be magnetized by putting them inside a solenoid.
Two ways to find where the north and south pole are :
When the coil is curled clockwise the pole will be the south pole. When the coil is
called a N ticlockwise , the pole will be the N orth pole.
Use your right hand to grip the wire with your fingers curling in the direction of
current flow. The thumb will point towards the north pole.
Magnets are magnetized or demagnetized using coils. In audio and video cassette
recorders, tiny coils are used to put magnetic patters on tape. The patterns store sound
and picture information.
If an iron core is inserted inside the solenoid, the strength of the field increases. Other
things can be done to increase the strength of the magnetic field.
In general, the strength of a magnetic field can be increased by
1. using a larger current
2. using more turns of wire on the coil
3. using an iron core
4. bringing the poles closer together.
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When a soft iron core is inserted inside a solenoid, it becomes magnetized. Since iron
loses its magnetism easily, this magnetism is lost as soon as the current is switched off.
This is called an electromagnet. Its disadvantage is that for the magnet to work, the
power supply providing current must be continuously on.
Uses of electromagnets
1. Electromagnets are used to pick up large pieces of steel and iron such as in
scrap yards.
2. Electromagnets are used to remove splinters ( small pieces) of iron or steel from
injuries
3. In an electric bell, a switch is used to complete a circuit so that current can flow
through the solenoid. This magnetises the iron core and hence attracts the soft
iron armature which is stuck to the bell hammer. This movement of the armature
causes the hammer to hit the bell ringing it. It also causes the circuit to break so
the current stops, the iron loses its magnetism and the armature goes back to its
place.
4. When the security button is switched on, current passes through the solenoid
thus creating a magnetic field around it. The lock which is made of iron is
attracted to the magnetic field and moves towards it thus opening the door.
Which the button is released, the magnetic field disappears and the iron lock is
pulled back in place by springs attached to it.
5. In a circuit breaker, when current which is larger than usual flows through the
solenoid, the magnetic field will become stronger and will produce a powerful
attraction on the iron above it. When this iron moves towards the solenoid,
contact with the circuit is broken and the current stops. A reset button is used to
raise the iron back in its place after the fault has been seen to, to restore the
current.
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6. A relay switch can be closed by the action of an electromagnet. A relatively small
current in the coil can be used to switch on a large current without the circuits
being electrically linked.
The motor effect
When a piece of wire is placed in a magnetic field and a current is then passed through
it, the wire moves. If the direction of current is reversed, the wire moves in the opposite
direction. The wire moves because the magnetic field of the magnet and the magnetic
field of the wire (remember: a wire carrying a current has a magnetic field around it)
repel and the magnet tries to push the wire with its magnetic field away.
The direction of movement can easily be found using Flemings Left Hand Rule.
The First finger ( pointer) points towards the magnetic Field.
The seCond finger points in the direction of Current
Then the thuMb will show you the direction of Movement of the wire
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The moving coil loudspeaker is one use of this effect. As the current varies according to
the music, the loudspeaker cone will move accordingly and this movement is transmitted
as compressions and rarefactions of sound waves.
A piece of wire carrying current into or out of the paper can be difficult to draw. Symbols
can be used
Means wire carrying current out of the paper
Means wire carrying current into the paper
A coil pivoted in a magnetic field
Consider a coil connected to a battery suspended in a magnetic field produced by a
horseshoe magnet. The coil has a piece of wire going up and another going down.
Applying Fleming’s left hand rule to each wire we can find in which direction each one
will move. Since they are going in opposite directions, we will notice that they will move
in opposite directions. Also the two faces of the coil will become north and south poles
respectively. The North pole will be attracted to the magnet’s south and the south pole
will be attracted to the magnet’s north. These two combined movements will produce a
twisting effect on the coil. This principle is used in electric motors.
The Simple electric motor
In the above experiment, the twisting effect stopped as the poles of the coil moved
towards those of the magnet. In a motor this turning effect must be continued. The way
to do this is by changing the direction of the current so that the coil twists the other way.
By reversing the direction of current continuously, the coil turns continuously producing
the desired motor effect.
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If the current being used is a direct current ( current which flows in one direction only),
the current is reversed by a split ring commutator.
Examples of an electric motor in everyday use is the driller and the loudspeaker.
Electric meters are devices in which the movement of the pointer depends on the strength of the current
Electro-magnetic induction
In the same way that if a current is passed through a wire placed in a magnetic field,
movement of the wire is produced, it stands to reason that an electromotive force
(voltage) can be produced by a moving wire inside a magnetic field. This is in fact what
happens. The current produced is called induced current.If a wire is moved to cut across lines of flux( not in the direction of the magnetic field),
then a current is induced in the wire ( forming a complete circuit). This is
electromagnetic induction.This is the basic idea behind a dynamo or a generator
The current produced can be increased by
1. using a stronger magnetic field
2. moving the wire faster
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Point 1 comes from Faraday’s law which states that
The size of an induced electromotive force (voltage) in a conductor is proportional to the
rate at which the magnetic field changes.
To find the direction of the induced current we use Fleming’s Right hand rule.
Using the same idea, if a magnet ( magnetic field) is inserted (movement) in a solenoid
( wire), a current will flow during the motion. When the magnet is stopped, current will
stop. If the magnet is then removed i.e. movement is in the opposite direction, current
will flow in the opposite direction. Faraday performed this experiment and found that the
induced voltage ( and the current) can be increased by
1. using a stronger magnet
2. moving the magnet faster
3. increasing the number of turns on the coil.
Lenz’s law gives us a way of finding the direction of current
The direction of the induced current is such that it opposes the change producing it.
If the north pole of a magnet is inserted in a coil, the current induced will produce a north
pole to try to repel (oppose) the magnet coming in. A south pole will produce an induced
south pole.
If a north pole is pulled out of the coil, the induced current will produce a south pole to
attract (oppose) the magnet going out. A south pole going out will induce a north pole.
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Two coils linked by magnetis m
Consider two coils near each other. Coil A is connected to a battery and switch. Coil B
is connected to a galvanometer. When coil A is switched on, the current through the coil
will produce a magnetic field which will reach coil B. This field movement will induce an
e.m.f. in B. This is the same as inserting a magnet in coil B. As soon as the magnetic
field around A becomes stable, the e.m.f. in B stops. This is because movement of the
field has stopped. When A is switched off, the field will diminish and fade ( field
movement will be in the opposite direction). Hence e.m.f. will again flow through B in the
opposite direction.
If a soft iron core is inserted through A and B, the induced e.m.f. will be larger because
the iron will increase the strength of the magnetic field. If the current through A is an
alternating current ( flowing in two directions alternately), the current induced in B will
also be alternating.. this is the basic idea of a transformer
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The transformer
In a transformer, coil A is connected to the alternating voltage (ac) supply. In this way an
alternating voltage is induced in coil B. The number of turns on each coil is related to the
voltages present in the two coils. The relationship is
Secondary voltage = Number of turns on secondary coil or V2 = N2
Primary voltage Number of turns on primary coil V1 N1
This is also called the turns ratio
If the second coil has more turns than the first one, it means the secondary voltage will
be higher than the primary voltage. This is a step-up transformer because voltage has
gone up. If the second coil has less turns than the first coil, then the secondary voltage
will be less than the primary voltage. This is a step-down transformer because the
voltage has gone down.
If a transformer is 100% efficient, no power should be lost and the power going in the
primary coil should be the same as the power coming out of the secondary coil.
( remember P = I V )
Power supplied to primary coil = Power delivered in secondary coil
V1 x I1 = V2 x I2
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Step-up transformers increase the voltage and reduce the current.
Step-down transformers decrease the voltage and increase the current.
In reality, transformers although very efficient ( up to 99% efficient ) can lose some
energy.
Energy can be lost if the magnetic field coming out of the primary coil does not
pass completely through the secondary coil. To reduce this energy loss, coils are
usually wound on top of each other.
Energy can also be lost because of eddy currents induced in the iron core. The
changing magnetic field induces currents in the core itself. These called eddy
currents have a heating effect. To reduce them, the core is laminated (layered): it
is made from thin insulated sheets of iron, rather than a solid block.
Uses of transformers
The voltage supply of the mains power supply is 230 V. however, most things we use
require a lower voltage ( radios, computers, hi fi ) so a step-down transformer is used.
Certain things like the TV require a larger voltage so a step up transformer is used.
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The nationwide supply network
This network also called the grid, is responsible for distributing power from the power
stations to the substations and then distributed to the consumer.
Alternating current is used for the mains supply. One main advantage is that
transformers can be used. Remember: transformers do not work with direct current.Transmission cables are very good conductors, but being hundreds of kilometers long
means that they will heat up thus causing energy loss. Transformers are used to
increase the voltage in these cables, thus reducing the current and the heat produced. In
this way, the energy lost is reduced.
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