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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