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GAHAYR SECONDARY SCHOOL

FORM TWO

PHYSICS

CHAPTER THREE

Magnetism

Prepared: Farah yasin (F.khadad)

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Magnets

The word “magnet” comes from the name “magnesia” of an ancient city in Asia

Minor when the first magnetic rocks were found.

These rocks able to attract certain substances like iron, steel, etc,

Also when these rocks suspended freely, they always turned with one end pointed

north, and the other south.

This property of rocks made them very useful as direction-finders and they came to

be called leading-stone or lodestone.

Magnets can be Natural or Artificial. The iron ore magnetite is part of naturally

occurring rock,

Artificial magnets are magnets that produced by human. They are usually made of

steel and other iron alloys.

Artificial magnets have different shapes such as bar-magnet, horse-shoe magnet,

and u-shaped magnet.

Magnetic materials

Elements which react strongly to a magnet are called “ferromagnetic materials”

of elements such as iron, steel, cobalt and nickel,

Ferromagnetic materials can be divided into two types

1. Soft materials: is magnetic material which magnetizes easily but do not keep their

magnetism, like iron.

2. Hard materials: are magnetic materials when they are magnetized keep their

magnetism for a long time like steel.

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Magnetic poles

There are two parts of magnet where the attraction forces are especially strong

these parts are called magnetic poles every magnet has two poles, and these

poles are found at the ends of the magnet.

How indicate the poles of magnet

a. Take bar magnet tie it with a string and suspended it in air, if it is free to

move, it will swing around and finally comes to rest pointing north and south.

b. Place a strong magnet on a cork floating in a dish of water, the cork rotates

until the magnet comes to rest pointing in a north-south direction.

Law of magnetic

If the N pole of a magnet is brought near the N pole of a magnet is brought near

the N pole of another magnet, repulsion occurs,

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Two S poles also repel,

By contrast N and S poles always are attract,

The law of magnetic poles summarizes these facts and states

( Like poles repel, unlike poles attract )

Permanent magnets and electromagnets

Some materials when they are magnetized keep their magnetism for a long time

others lose the effect more quickly,

Steel for example keeps its magnetism but pure iron loses it quickly,

Magnets which do not lose their magnetism with time are called “permanent

magnets “

The most power full permanent magnets are made of alloy of iron with other

elements.

Magnetism can also be produced by winding a coil of insulated wire around an iron

rod, and passing an electric current though the coil,

A magnet made in this way is called “electromagnets or temporary magnets”.

Permanent magnets and electromagnets

Some materials when they are magnetized keep their magnetism for a long time

others lose the effect more quickly,

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Steel for example keeps its magnetism but pure iron loses it quickly,

Magnets which do not lose their magnetism with time are called “permanent

magnets “

The most power full permanent magnets are made of alloy of iron with other

elements.

Magnetism can also be produced by winding a coil of insulated wire around an iron

rod, and passing an electric current though the coil,

A magnet made in this way is called “electromagnets or temporary magnets”.

Uses of electromagnets

The electromagnet is an essential part of many electrical devices,

1. The electric bell

A simple electric bell is shown in (fig: below) when someone pushes the button,

current flows and the armature is attracted to the electromagnet,

The moving armature causes the hammer to strike the bell.

At the same time the circuit broken at the contact screw and the current is cut

off,

The armature returns to its original position and so closes the circuit again,

repetition of this make-and-break process causes the ringing.

2. The telephone

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A telephone contains a microphone at the speaking end and a receiver at the

listening end,

a. Carbon microphone

When someone speaks in to a carbon microphone (fig : 5) sound waves

causes the diaphragm to move backward and forward,

This varies the pressure on the carbon granules between the movable carbon

dome which is attached to the diaphragm and fixed the carbon cup at the

back.

When the pressure increases the granules are squeezed closer together and

their electrical resistance decreases.

A decrease of the pressure has the opposite effect the current passing

through the microphone varies in a similar ways to the sound wave

variations.

b. Receivers

The coils are wound in opposite direction on the two S poles of a magnet as

in (fig: below) if the current goes round one in a clockwise direction, its goes

the other anticlockwise,

So making one S pole stronger and the other weaker,

This causes the iron armature to rock on its pivot towards the stronger S

poles,

When the current reverses, the armature rocks other way due to the S pole

which was stronger before becoming the weaker,

These armature movements are passed on to the diaphragm machining its

vibrate and produce sound of the same frequency as the alternating current

in the coil( received from the microphone )

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3. Loudspeaker

The inside of the loudspeaker, at the center of the paper cone is a wire coil the

coil has flexible leads connected to the terminals outside.

When a varying electric current passes through the coil it moves to and fro in

the radial magnetic field of the tubular magnet.

The cone thus sets the surrounding air vibration and sound is transmitted, the

varying current could come from a microphone, a radio, a television and so on,

different variations in the current produce different sounds.

4. Recovering scrap iron

Cans are made from aluminum or iron, cans for recycling are passed on a belt

below an electromagnet ‘

The iron cans are removed and the aluminum cans pass on

Steel scrap can be lifted in a scrap-yard by a powerful electromagnet

The crane moves the scrap to its new place and when the current is turned off,

the scrap falls from the electromagnet.

Q: why would such an electromagnet need an iron core rather than a steel core?

A: a steel core would keep some magnetism when the current was turned off,

and scrap might not fall off.

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5. Relay

A relay is a switch based on the principle of an electromagnet, it is useful if we

want one circuit to control another, especially if the current and power are larger

in the second circuit

When switched on the current in the first circuit, the soft iron core is magnetized

and attracts the L-shaped iron armature, this rock on its pivot and closes the

second switch.

The relay is then switched on or off.

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Magnetic domains

Any small piece from a magnet is itself a little magnet it seems, that a magnetic

material is made up of many tiny magnets, each tiny magnet called “domain”

In an un-magnetized substance there millions of these domains, but they are

pointing in random directions

In magnetized substance the domains are arranged in order so as to produce

internal effect and directed in one direction.

Making magnets

a. Mechanical method

i. Hammering

As early as 1600 it was known that if an iron bar is heated red hot, placed so

that it points N-S and hammered vigorously as it cools, it will become a

magnet.

ii. Stroking

It is a kind of rubbing a magnet to an iron bar in one direction repeating it

and lifting the magnet well a part from the iron the action marks the iron

rod a magnet.

b. Electrical methods

If a wire coil is wound over a magnetic substance from one end to the other and its

two ends are connected to a battery, when a current flows through the coil,

The rod will be magnetized, such a magnet is called “electromagnet “

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Demagnetization

Most magnetic materials lose their magnetism over a period of time

Magnetism can destroyed

1. By heating the metal to red heat and letting it cool while it points in an E-W

direction

2. By laying it pointing E-W and hammering vigorously

3. The best way is to place the object inside a solenoid lying E-W and passing A.C

current through the solenoid.

Magnetic fields

The space surrounding a magnet where it produces a magnetic force is called a magnetic field.

The magnetic field is made up of lines of force

The direction of magnetic field lines starts from a north end to a south end

The magnetic field is strongest where the lines are closest together .

Where the field lines are far apart the field is weak. No two lines ever to cross ,

The magnetic field lines of two similar poles facing each others opposes and the

magnetic field lines of two dissimilar poles facing each other help.

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How to plot the lines of force

a. Plotting compass method

Lay a bar magnet on a sheet of paper, place the plotting compass at a point

such as X near one pole of a magnet mark the position of the pole N,S of the

compass by pencil dots Y,X, move the compass so that pole S is exactly over

Y mark the new position of N by dot Z.

Continue this process until the S pole of the bar magnet is reached. Join the

dots to give one line of force,

And show its direction by putting an arrow on it.

Plot other lines by starting at different points round the magnet.

b. Iron flings method

Place a sheet of paper on top of a bar magnet and sprinkle iron fling thinly

and evenly on the paper from a “pepper pot “

Tap the paper gently with a pencil and flings should form patterns of the lines

of force.

This method is quick but no use for weak fields.

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Magnetic flux

Magnetic field lines are sometimes called lines of magnetic flux magnetic flux

is vector quantity which has both strength and direction,

The strength of of the field is defined in terms of flux density.

A high flux density means that the lines of force are close together.

Neutral point

This figure represents the field pattern of two like poles.

Between two like poles where the field lines are bent away from each other,

Is a point where the magnetic field is zero this is called “Neutral Point”

Earth’s magnetic field

The earth has a magnetic field no one is sure of its cause, although it is

thought to come from electric currents generated in the earth’s core.

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If the lines of force are plotted on a sheet of paper with no magnets near, a

set of parallel straight lines is obtained.

They run from South Pole to the North Pole geographically.

Q: why is this?

A: the north pole of a magnet is attract to the north’s pole

If the earth were regarded as a bar magnet its north pole would be the south of

that magnet

But the convention it is called the north poles

At most places on the earth’s surface a magnetic compass points slightly east or

west of true north

The earth’s geographical and magnetic north poles do not coincide, the magnetic

north is over 1200km from the its geographic north pole.

The angle between magnetic north and true north is called “declination “

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Magnetic effect of the current

In current carrying conductor there is magnetic field around the wire the field has

the following features

The field lines are circles with conductor at the centre

Reversing the current reverses the direction of the field

Increasing the current increases the strength of the field

The direction of the field is given by tight hand grip rule (RHGR)

Soliniod

A soliniod is a long cylinderical coil it produces a field similar to the that of a bar

magnet

The magnetic field produced by this way has these features.

One of the ends will behave like a North Pole and other as a south pole

Increasing the current increases the strength of the field

Increasing the number of turns on the coil increases the strength of the field

The direction of the north pole can be found by (RHGR) imagine griping the coil

with your right hand so that your fingers point in the current direction

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Magnetic force

In the figure below a length of copper wire has been placed in magnetic field

Copper is non magnetic so it is feels no force from the magnet

However with a current passing through it there is the force on the wire,

The forces arises because the current produces its own magnet field which acts on

the poles of the magnet, in this case the force on the wire is upwards(see the box

below ) it would be downwards if either the magnetic field or the current were

reversed

Whichever way the experiment is done, the moves across the field, is not attracted

to either pole

The force is increased if

The current is increased

A stronger magnet is used

The length of wire in the field is increased

Fleming’s left hand rule (FLHR)

We can find which way the wire will move by using Fleming’s left hand rule

This gives the direction of the force on a current-carrying conductor lying in a

magnetic field at right angles to the current.

In First finger indicates the direction of the Field, and the seCond finger the

direction of the Current,

The thuMb shows the direction of the Motion

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Magnetic flux density

Magnetic flux density(B) is defined as the force acting in a wire of length one meter

carrying a current of one ampere at right angles to the field

The unit of the magnetic flux density is the tesla (T)

B= F/IL

Example1

Rectangular coil of wire carrying a current of 0.26A that has 150turns the coil is parallel

to a magnetic field of flux density 0.36T

a. Will the coil rotate clockwise or anticlockwise to the observer?

b. What is the size of the force on side AB?

c. Find the toque which exerted on the coil?

Solution

a) Uses the FLHR on side AB shows that its moves upward so the coil rotates to the

observer.

b) Force on a single coil F= BIL

a. Given B= 0.36

i. I = 0.26

ii. L = 0.20m

b. F= 0.36×0.26×0.20

i. 0.0187N

Force on 150 turns = 150×0.0187

ii. 2.80N

c) torque = F ×d ere d is the width of the coil

i. 2.8N×0.12m

ii. = 0.34Nm w

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Exercises

Q1. a magnet attracts

a. Plastic

b. Any metal

c. Iron and steel

d. Aluminum

e. Carbon

Q2. Which one of the following is not true?

A) If a lodestone is suspended freely in air it points north-south

B) A compass needle is small freely suspended magnet

C) A nail made of an iron can be made to be a magnet by stroking it with a magnet

D) The poles of a magnet are found at the centre of the magnet

Q3 Three bar magnets are held together as you see above. What are the magnets going

to look like when they are released?

PART TWO: FILL IN THE BLANKS OF THE FOLLOWING

1. The force of a magnet is strongest at its _________________________

2. Magnetic field lines are always shown pointing away from a _______________ pole

3. ____________Poles of a magnets attract each other, and __________ poles of a magnets repel

each other

4. Elements which react strongly to a magnet are called ______________________________

The point between two like poles where the magnetic field is zero, this point is called

Part three: Do these problems

1. In the picture given above F pulls a box having 4kg mass from point A to B, find the work

done?

2. Three magnets are placed like given picture below. When the system released, magnet B

gets closer to the magnet A. Find the pole types of 1 and 4.assume that 2 is north pole?

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4. Draw the directions of magnetic field lines at point A and B in the picture given below

5. Identify A and B

aniga iyo maayirka burao ,sadax sano ka hor kusoo ban dhignay gabaya carabiya

magaalada hotel barwaaqo iyo marti sharaf kale oo badan

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