Magnetism Slides

Unit 20

Contents20.1 Materials and Magnetism20.2 Magnetic Induction20.3 Magnetisation and Demagnetisation20.4 Magnetic Field20.5 Magnetic properties of Iron and Steel

N

E

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SESW

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Chapter 20At the end of this chapter you should be able to:

• state the properties of magnets

• describe induced magnetism

• distinguish between magnetic & non-magnetic materials

• describe electrical methods of magnetisation and demagnetism.

• describe the plotting of magnetic field lines with a compass.

• distinguish between the properties and uses of temporary magnet.(e.g. iron) and permanent magnets (e.g steel)

Unit 20.1

Discovery of the phenomenon

About 900 years ago, the Chinese found that a dish carrying a certain type of rock known as magnetite would constantly float in water in a North-South direction.

Unit 20.1

1. Magnetic materials

Magnetite consists of an iron oxide.

Natural magnet attracts certain materials:• cobalt• nickel• iron• steel• alloys of any of the above

These materials are called Magnetic materials.

Unit 20.1

Non-Magnetic materials

Natural magnet cannot attract other materials.These include:• copper• brass• wood• plastics• materials other than iron, steel, cobalt, nickel

These materials are called Non-Magnetic materials.

Unit 20.1

2. Properties of magnets

All magnets exhibit the following properties:

Unit 20.1



a. Attract magnetic materials

Unit 20.1



b. They have 2 magnetic poles; the North and South seeking poles. These are the strongest parts of the magnets. The poles are found very near (but not at) the ends of the magnet.

Unit 20.1



c. If allowed to swing freely a magnet will come to rest with one end pointing towards the Earth’s North pole, the other end pointing towards the Earth’s South pole.

cylindrical rod made of non-magnetic material

Unit 20.12. Properties of magnets


c. Hence, a magnet can be used as a compass for navigational purposes. A free hanging magnet will rest in a north-south direction.

cylindrical rod made of non-magnetic material

NNEESS

WW

NENE

SESE

SWSW

NWNW

Properties of magnets


d. Law of magnetic poles:Take a look at the following actions taken during an experiment. What can you conclude ?

(Step 1) (Step 2)

Unit 20.1



d. Law of magnetic poles:Take a look at the following actions taken during an experiment. What can you conclude?

(Step 4)(Step 3)

NS S

NN

SN S

Unit 20.1Properties of magnets


d. Law of magnetic poles:

Conclusion: Like poles repel, Unlike poles attract.** (Similar to the law of electrostatic: Like charges attract and unlike

charges repel.)** (Similar to the law of electrostatic: Like charges

repel and unlike charges attract.)


Eg.1:

In an experiment conducted to test if an object is a magnet,one end of this object (A) is brought near one end (X) of a suspended bar magnet.Attraction occurs.Can you conclude that the object is a magnet?

A X YY


Eg.1:


Answer:Not yet. The object could have been a magnet with end A an opposite poleto that of end X of the magnet;

A X YY


Eg. 1:


Answer:Not yet. The object could have been a magnet with end A an opposite poleto that of end X of the magnet; orThe object could just have been an ordinary magnetic material(unmagnetised yet).

A X YY


Eg. 2:

In the same experiment,The same end of this object (A) is brought near the other end (Y) of a suspended bar magnet.

(i) If attraction occurs again, can you conclude now that the object is a magnet?(ii) If repulsion occurs instead, can you conclude now that the object is a magnet?

A XYY


Eg 2:


Answer:(i) If attraction occurs again, can you conclude now that the object is a magnet?

No, the object is just an unmagnetised magnetic material asno repulsion between the object & bar magnet was observed.

(ii) If repulsion occurs instead, can you conclude now that the object is a magnet?

A XYY


Question 2:


Answer:(i) If attraction occurs again, can you conclude now that the object is a magnet?

No, the object is just an unmagnetised magnetic material asno repulsion between the object & bar magnet was observed.

(ii) If repulsion occurs instead, can you conclude now that the object is a magnet?Yes, it is a magnet.Since only like poles repel (A repels Y)& unlike poles attract (A attracts X),the object is indeed a magnet.

A XYY

Unit 20.1


Remember:

Repulsion is the only test to confirm that an object is a magnet i.e. Repulsion is the only true test for polarity.

Unit 20.1: Magnets and MaterialsTest Yourself 20.11. Give four examples each of magnetic and

non-magnetic materials.

Answer:

Magnetic: cobalt, nickel, iron and steel

Non-magnetic: copper, aluminum, wood and plastics

Unit 20.1: Magnets and MaterialsTest Yourself 20.12. State the properties of magnets.

Answer:1. A freely suspended magnet always point

in the North-South direction.2. They have magnetic poles (i.e. N-pole

and S-pole).3. They obey the law of magnetism, i.e.

like poles repel and unlike poles repel.

Unit 20.2

3. Induced MagnetismSN Far apart

Soft-iron barPermanent magnet

Induced Magnetism

When a non-magnetised magnetic material is brought near to (or touches) a magnet, the material itself will become a weak magnet. This is called induced magnetism(which means the material has magnetism induced in it).

SN

S S NN

Soft-iron bar becomes an induced magnet

Permanent magnet brought near to soft-iron bar

Far apart

Soft-iron bar Permanent magnet

Induced Magnetism

Notice that magnetic induction, an opposite pole is always induced.In other words, 2 unlike poles facing each other is observedduring magnetic induction.

SN

SS NNSoft-iron bar becomes an induced magnet

Permanent magnet brought near to soft-iron bar

Far apart

Soft-iron bar Permanent magnet

Induced Magnetism

If placed sufficiently near to each other, attraction occurs between the permanent & induced magnets.

SN SN

permanent magnet induced magnet

Unit 20.2

Induced Magnetism

Induced magnetism in magnetic materials is the reason that these non-magnetised objects are able to be attracted to magnets.

SN SN

permanent magnet induced magnet

Unit 20.2

Induced Magnetism

Here is another example of induced magnetism.

Unit 20.2

Induced MagnetismUsing the theory of induced magnetism,explain how it is possible to get several iron nails to stick together(as shown in the diagram below).

Fig. 20.11 Safety pins attracted by a magnet become induced magnets and can even attract other pins. Do you notice that the pins at the lower ends tend to fan apart? Why?

Unit 20.2: Magnetic InductionMagnetic Induction

Magnetic induction is the process of inducing magnetism in ferromagnetic materials.It can also occur without any contact with the magnet.

Fig. 20.12 Magnetic induction can happen at a distance.

Unit 20.2: Magnetic InductionTest Yourself 20.21. In an experiment with a bar magnet, a piece of

wood is held between the N pole of a magnet and two iron nails (Fig. 20.15).(a) Although wood is a non-magnetic material, the two nails are attracted when the wood is held between the magnet and the nails. Suggest a reason for this.(b) It is observed that the pointed tips of the iron nails point away from each other. Why is this so? Fig. 20.15

Answer:(a) The magnet has a very strong magnetic field that is able to pass

through the wood.(b) The iron nails become induced magnets. The pointed tips of the

iron nails are like poles (N pole) and thus they will repel each other.

Unit 20.3: Magnetisation and Demagnetisation4. Theory of Magnetism

When a bar magnet is cut into many small pieces, every piece becomes a magnet itself.The bar magnet is made up of many such ‘tiny magnets’, or magnetic domains.

Fig. 20.16 Each resulting piece of the cut bar magnet is a magnet itself.

Unit 20.3: Magnetisation and DemagnetisationWhat are magnetic domains?

The orbiting motion of electrons in a magnetic material makes each atom an atomic magnet.

A group of atomic magnets pointing in the same direction is called a magnetic domain.


In an unmagnetised bar, the magnetic domains point in random directions.The magnetic effects of the atomic magnets cancel out so there is no resultant magnetic effect.

Fig. 20.17 An unmagnetised bar – the magnetic domains point in random directions.


In a permanent magnet, magnetic domains point in the same direction.The atomic magnets at the ends of the bar magnet fan out due to repulsion between like poles.

Fig. 20.18 In a permanent magnet, the magnetic domains point in the same direction.

Unit 20.3: Magnetisation and Demagnetisation

The theory of magnetic domains can be used to explain the following phenomena:

a. Magnetic saturationWhen all the magnetic domains point in the same direction, the magnet is magnetically saturated and cannot be any stronger.

Unit 20.3: Magnetisation and Demagnetisationb. Demagnetisation of magnets

Demagnetisation is the process of removing magnetism from a magnet (e.g. heating and hammering).

Cause the atoms of the magnet to vibrate vigorously, mixing up the directions of the magnetic domains.

Unit 20.3: Magnetisation and Demagnetisationc. Storage of magnets using soft iron

keepersOver time, magnets placed side by side will become weaker.

The magnetic domains will be altered due to the repulsion between the ‘free’ poles.

To prevent this, bar magnets are stored in pairs with soft iron keepersacross the ends of the bar magnets.

The poles of the bar magnets are in closed loops with no ‘free’ poles.

Fig. 20.19 Soft iron keeps help permanent bar magnets stay strongly magnetised.

Unit 20.3

5. Magnetisation

There are several ways to magnetise materials.

Unit 20.3ai. Magnetisation by Stroking (Single-Touch)

This method is derived from applying the lessons learnt on magnetic induction.Note the polarities of both the permanent magnet & steel bar that is to be magnetised.This form of magnetism gained is weak but permanent.

Unit 20.3

aii. Magnetisation by Stroking (Double-Touch)This method is also derived from applying the lessons learnt on magnetic induction.2 permanent magnets are used in this method,as compared to one being used in the single-touch stroking method.Note the polarities of both permanent magnets.Once again, do take note of the polarities of the permanent magnets & their induced ends of the steel bar.This form of magnetism gained is also weak but permanent.

Unit 20.3

b. Magnetisation by Heating & Hammering

A magnet can be made by first placing a steel bar in a magnetic field, then heating it to a high temperature and then finally hammering it as it cools.

This can be done by laying the magnet in a North-South direction in the Earth’s magnetic field. However, the magnet produced is not very strong but permanent.

Unit 20.3c. Magnetisation by the use of an Electrically-generated magnetic field of a Solenoid

Place the steel object inside a coil of wire (a solenoid).Pass a direct current (d.c.) through the solenoid for a few seconds.A magnetic field is produced on the solenoid.As such, the steel rod is now placed inside a magnetic field.When the current is turned off the steel rod is found to be magnetised.Note: the d.c. flows through the solenoid. It does not flow through the steel rod.

steel rod

direct current

Unit 20.3Magnetisation by the use of an Electrically-generated magnetic field of a Solenoid

The polarity of the newly-formed magnet can be determined using the Right-hand Grip Rule. (aka Maxwell Corkscrew Rule)(Fingers coiled round & following the direction of the flow of d.c. in the solenoid;the thumb will point in a direction indicating the end which becomes the N-pole).

Unit 20.3Magnetisation by the use of an Electrically-generated magnetic field of a SolenoidAn alternative method of determining the polarity:The polarity of the newly-formed magnet can also be determined using the method:Take a look at which way the d.c. is flowing at each end.If the direction of flow is anticlockwise, the end is a N-pole.If the direction of flow is clockwise, the end is a S-pole.

steel rod

direct current

aNticlockwisedirection flow of d.c.

Looking throughend BLooking through

end A

clockwiSedirection flow of d.c.

Unit 20.3Magnetisation by the use of an Electrically-generated magnetic field of a Solenoid

The magnetism produced using this method is strong & permanent.

steel rod

direct current

Unit 20.36a. Demagnetisation by Heating & Hammering

Heat a magnet.

Then hammer it as it is allowed to cool in the absence of a magnetic fieldi.e. facing East-West .

Unit 20.3b. Demagnetisation by the use of an Electrically-generated magnetic field of a Solenoid

(700 turns)

magnetwithdrawnto a few metres

Unit 20.3Demagnetisation by the use of an Electrically-generated magnetic field of a SolenoidPlace magnet in a solenoid.

Pass an alternating current (a.c.)through the solenoid (not through the magnet).

Slowly remove the magnet from the solenoid with the a.c. supply still on.Remove to a great distance.

Repeat the procedure for as many times as it is necessary.Each time it is done, the magnet’s strength weakens.

Finally, it is completely demagnetised.

(700 turns)

magnetwithdrawnto a few metres

Unit 20.46. Magnetic Field

A magnetic field is the region where a magnetic force is exertedon any magnetic objects placed within the influence of the field.

Showing the Magnetic Field Using Iron a FilingsOne method to observe the shape of the magnetic field is by sprinklingiron filings onto a piece of paper placed on top of the magnet.

Unit 20.4Plotting CompassA compass is a freely suspended. A compass is normally drawn with the N-pole shown as an arrowhead. It can be used to find the direction of a magnetic field.

Remember the N-pole of the compass points to the Earth’s N-pole.

The Earth’s magnetic field is produced by electric currents at its core. It is similar to the field that would be due to an imaginary large bar magnet in the Earth’s centre.

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Unit 20.4

What do you think are the directions that the compass would point in if placed in theten different points around a strong permanent magnet.

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Unit 20.4

Did you get them all correct?

NS

Unit 20.4Magnetic Field Lines

Magnetic field lines are imaginary & represent the direction of the magnetic field.

Magnetic field lines are also known as lines of forcebecause if magnetic objects are placed in the region of the field lines,the magnetic objects will experience a magnetic force directed along the same lines.

By convention, the magnetic field line is the path along whichan imaginary “free” N-pole will move if placed along this line.

magnetic fieldsmagnetic fields

photographs of magnetic field patterns formed by iron filings for various arrangements of magnets

Unit 20.4Neutral Point

Whenever a point in space has no magnetic fieldthe magnetic field due to one magnet cancels out that due to another magnet,this point is known as a Neutral Point.

neutral point

Unit 20.4Plotting Magnetic Field Lines With A Plotting CompassThe lines can be investigated to find their path and directionusing a plotting compass.

Place a plotting compass at point A.Note the direction it points at.Mark a 2nd point next to the N-poleof the plotting compass.

2

Unit 20.4Plotting Magnetic Field Lines With A Plotting CompassThe lines can be investigated to find their path and directionusing a plotting compass.

Place a plotting compass at point A.Note the direction it points at.Mark a 2nd point next to the N-poleof the plotting compass.

These steps are repeated as shown.The points are all joined using a pencil.

All these steps are repeated for other pointsnext to the N-pole of the magnet.

Unit 20.4Examples of Magnetic Fields

(i) A permanent bar magnet.


(ii) 2 opposite poles facing each other


(iii) 2 like poles facing each other (e.g. 2 N-poles)

Unit 20.4iv. neutral Point

If a plotting compass is placed at the neutral point (i.e. X), how will it point?

N Earth’sMagnetic

field

Unit 20.4iv. neutral Point

It will point in the same directionas that of earth’s magnetic field.

N Earth’sMagnetic

field

Unit 20.4Properties of Field Lines

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Lines always start and end on the magnet.•

The lines travel from the N-pole to the S-pole.. •

The lines never cross or touch each other.•

The closer the lines the stronger the field. (More magnetic field lines do not necessarily mean

stronger magnetic field)

Unit 20.4Try labelling the poles of the following magnets.

Unit 20.4These are possible answers.

N NN

N

N NN

S

S

S SS

S

SN

N

Unit 20.4These are also possible answers.

S SS

S

S SS

N

N

N NN

N

NS

S

Unit 20.5

Although iron and steel are both magnetic materials,their properties are different.

•Two unmagnetised rods have a magnet placed on top of them.

•Iron filings are supported from the induced magnets

Unit 20.5

•Note that the induced magnet made of iron attracts more iron filings than the induced magnet made of steel

Unit 20.5

•The permanent bar magnet is then removed.

Unit 20.5

•Note that the iron bar no longer has any iron filings attracted to it.•The steel bar, however, still has some iron filings attracted to it.

Unit 20.5

Magnetic properties of Iron & Steel:

a. Iron is easily magnetised, whereas Steel is not easily magnetised.

Unit 20.5


b. Iron is easily demagnetised, whereas Steel is not easily demagnetised.

Unit 20.5


Iron is thus known as a soft magnetic material(easily magnetised & easily demagnetised).

Unit 20.5Magnetic properties of Iron & Steel:

Steel is however known as a hard magnetic material(difficult to magnetise & difficult to demagnetise).

Thus iron is used in making electromagnets & steel is thus used in making permanent magnets.

Unit 20.5 MagnetisationMaking a material permanently magnetic is called magnetisation. Some materials are easier to magnetised than others.

Iron Steel

a. Soft magnetic material a. Hard magnetic material

b. Gains or loses its magnetism easily

b. Retains its magnetism longer. ‘Hard’ to magetise ; ‘hard’ to demagnetise.

c. Commonly used as a core of electromagnet.

c. Good for making permanent magnet.

Unit 20.5: Temporary and Permanent MagnetsUses of permanent magnets

Moving coil ammeterIt consists of a coil suspended in the magnetic field of a permanent magnet. When a current flows into and out of the coil, a turning effect is produced on the coil and the pointer attached to it will move.

Fig. 20.43 Cutaway view of a moving-coil ammeter

Unit 20.5: Temporary and Permanent MagnetsUses of permanent magnets

Magnetic door catchMagnetic strips are fitted to the doors of freezers and refrigerators to keep the doors closed.

Fig. 20.44 A magnetic door catch

Unit 20.5Electromagnets have many uses too.Electromagnets

The electric-bell

Unit 20.5Electromagnets have many uses too.Electromagnets

A simple magnetic-relay

Electromagnet:Electromagnet:

nail is magnetised when the switch is closed

nail is demagnetised when the switch is opened

A solenoid is made by winding many turns of insulated copper wire around a soft iron core (e.g. an iron nail).The core becomes a magnet temporarily whenever a current starts or stops to flow in the solenoid. An direct current supply works effectively.

ElectromagnetsElectromagnets

magnetic properties of matter

The magnetic strength of an electromagnet can be increased by The magnetic strength of an electromagnet can be increased by

passing a larger current through the solenoidincreasing the number of turns of the solenoidinserting a core of soft magnetic materials

passing a larger current through the solenoidincreasing the number of turns of the solenoidinserting a core of soft magnetic materials

nail is magnetised when the switch is closed

nail is demagnetised when the switch is opened

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Magnetism Slides