MAGNETISM

3.1 CONCEPT OF MAGNETISM

Magnetism is a phenomenon produced by the

motion of electric charges those results in an

attractive and repulsive force between objects

Magnet is an object or material that produces a

magnetic field and has the ability to attract

certain materials, especially iron, steel, nickel

and cobalt.

NB: In many atoms these magnetic

effects cancel each other because

electrons are paired. Example in Helium,

electrons is paired so the atom has no net

magnetic effect. In other atoms electrons

are not paired, hence the magnetic effect

does not cancel.

Example of magnet is Lodestone and a bar

magnet

NB:

(i)

The lodestone is an example of a

Example: Iron (Fe) and oxygen (O)

magnetized piece of the mineral magnetite.

These atoms have unpaired electrons, so

their magnetic effect does not cancel

Magnetic force

(ii)

Chemically, it consists of iron oxide

with the formula 퐅퐞_ퟑ퐎_ퟒ

(iii)

discovered

This was the earliest magnet to be

Magnetic force is the force of attraction or

repulsion that arises due to a magnetic field

Magnetic force can be explained in two

common ways:

Causes of magnetism

(i) Between magnets

Magnetism arises from two types of electron

motion, such are:

Magnetic force causes magnets to attract each

other (unlike poles) or repel each other (like

poles)

(i) Motion of electron around the nucleus of

an atom

(ii) On moving electric charges and current

carrying conductor

(ii) The spin of electrons about their axis

These motions of charged particles

independently impart a magnetic effect,

causing an atom to behave like a tiny

magnet.

A magnetic force acts on moving electric

charges or current – carrying wire placed in a

magnetic field, cause it to experience a push or

pull

Forces that a magnet experience

Originates from

magnetic fields

Originate from

gravitational fields

A magnet can experience several types of

forces depending on what it can interact with.

These are;

Does not depend on

mass

Depends on masses

of objects

(i) Magnetic force due to another magnet

When two magnets are near each other, they

exert forces on each other, like poles repel

while unlike poles attract each other

Points to note

(ii) Forces on a magnet due to electric

current

(i)

When a magnet is brought near

different objects, some will be attracted or

repelled by the magnetic force acting on the

object and other will not.

When a magnet is placed near current –

carrying conductor, it experiences a force. This

happens because; magnetic field of current

interacts with a magnetic field due to a magnet

(ii)

Objects that are attracted are said to

possess induced magnetism

(iii) Magnetic force due to a magnetic

material

Magnetic induction

A magnet attracts magnetic materials such as

iron, steel, nickel and cobalt. The force pulls

the material towards the magnet near to the

poles

Induction is the process by which a material

becomes a magnet due to its closeness or

contact with a magnet.

Example: If an iron nail is brought close

to a bar magnet, the nail becomes

magnetized and attract small pins. This

magnetization occurs due to magnetic

induction.

Forces due to a magnet and that due to

gravity

Forces due to a

magnet

Forces due to

gravity

Acts only on magnetic Acts on all objects

materials like iron,

nickel, cobalt or other

magnets

that have mass

Can be attractive or

repulsive

Always is attractive

Depends on magnetic

strength and distance

Depends on mass

of objects and

distances between

them

It shows the induced poles nearest the

magnet are of opposite sign to that of the

inducing pole.

Can be produced or

removed (by

magnetization and

demagnetization)

Cannot be switched

off

The induced magnetism can be temporary or

permanent upon the removal of magnet in

contact

Note

Note: Any materials which contain, iron,

nickel or cobalt are ferromagnetic materials.

The nature of material being magnetized

determines how long the induced magnetism

lasts. For example: Iron exhibits temporary

magnetism while steel retains its magnetism

for long time.

Ferromagnetic materials are strongly

magnetized because their magnetic domains

align easily when placed in magnetic field.

Example 02

How do you know that a certain material is

magnetic in nature?

Example 01

From the arrangement shown in the figure

below, mark the polarities of the iron nails

forming a magnetic chain

Answers

A material is known to be magnetic in

nature if it shows any of the following

behaviors (properties)

(i) Attraction to a magnet

If the material is attracted when brought

near a magnet, it is magnetic. Example;

Iron, Nickel and Cobalt

(ii) Can be magnetized

Answers

If the material can be turned into a

magnet by rubbing it with a magnet or

placing it in a magnetic field, it is a

magnet.

From the top to bottom: NS, NS, NS

(iii) Produces magnetic field when

magnetized

MAGNETIC AND NON-MAGNETIC

MATERIALS

After magnetization, if can attract iron

fillings or small iron objects, it confirms

magnetic nature

Magnetic materials are those which can be

magnetized even by weak magnets. Example

Iron, nickel and cobalt

(iv) It shows alignment in magnetic fields

Ferromagnetic materials

When freely suspended, a magnetic material

aligns itself in the North – south direction

Ferromagnetic materials are materials that

are attracted to a magnet and can be

strongly magnetized.

Note: If a material is attracted by a magnet and

can be magnetized, it is a magnetic in nature

Example:

(i) Iron

HARD AND SOFT MATERIALS

Ferromagnetic materials are grouped as

magnetically hard or soft depending on how

they retain their magnetism when magnetized.

(ii) Nickel

(iii) Cobalt

(iv) Steel

(vi) In magnetic recording devices

Hard magnetic materials

Magnetically hard materials are those which do

not readily lose their magnetism, though they

are difficult to magnetise.

Used in hard disks, magnetic tapes, and

memory storage systems

(vii) To make magnetic separators

Examples:

Separate materials from non – magnetic

ones in industries (mining, recycling)

(i) Steel.

(ii) Alnico alloys

(iii) Neodymium magnets

Notes:

Soft magnetic materials

Soft magnetic materials are those which

can be easily magnetized, but they tend to

lose their magnetism easily.

 Hard magnetic materials are those

which are difficult to magnetise but

once magnetized, can retain the

magnetism for long.

 Steel consists of iron and small

percentage of carbon

 Hard magnetic materials are used when

permanent and strong magnetism is

required

Examples:

(i) Soft Iron

(ii) Silicon steel

(iii) Mild steel

(iv) Nickel – iron alloy

Uses

(v) Soft Ferrites

(i) In making permanent magnets.

Notes:

Hard materials are used to make bar magnets,

horse shoe magnets and magnetic toys and

holders

 Soft magnetic materials are those

which can be easily magnetized,

but do not retain their magnetism.

(ii) In electric motors and generators

 These materials have high

permeability and low coercivity,

making them ideal for applications

where magnetism needs to be

switched on and off

Provide a constant magnetic field without

needing electric current

(iii) In making magnetic compasses

Used for navigation because they keep their

magnetism for long periods

Uses

(i) To make cores of electromagnets

(iv) In loud speakers and headphones

Used in electric bells, relays, lifting magnets

and solenoids

Creates steady magnetic field that

interacts with electric current to

produce sound (v) In making

measuring instruments

They produce strong magnetic field when

current flows and lose magnetism quickly

when the current is switched off (ii) In

transformer cores

Moving coil galvanometers and ammeters

require strong permanent magnets

Used in power transformers and signal

transformers

PARAMAGNETIC AND DIAMGNETIC

MATERIALS

Soft iron or silicon steel reduces energy loss

and improves efficiency of a transformer

Non – magnetic materials are classified as

paramagnetic or diamagnetic materials

depending on their behavior when

subjected to magnetic fields

(iii) In making electric motors and generators

Used in stators and rotors to allow rapid

reversal of magnetization during rotations (iv)

Making inductors and chokes

Paramagnetic materials

Paramagnetic materials are substances that

are weakly attracted by an external

magnetic field

Used in electronic circuits. Help to store energy

in magnetic field and filter electrical signals

(v) In magnetic shielding

Examples:

Protects sensitive electronic equipment from

unwanted magnetic fields

(i) Aluminium

(ii) Brass

(vi) Platinum

(vii) Chromium

(viii) Manganese

(ix) Magnesium

(iiii) Tin

NON – MAGNETIC MATERIALS

(iv) Zinc

Non- magnetic materials are those which are

(v) Gadolinium

(x) Oxygen

not attracted by a magnet Examples:

(푂₂)

Note:

(i) Brass

(ii) Copper

(iii) Tin

When placed in an external magnetic

field, they become slightly magnetized in

the same direction to the field, but they

lose magnetism as soon as the field is

removed

(iv) Aluminium

(v) Zinc

These materials cannot be magnetized. Non

– metals also are non – magnetic materials.

Characteristics of paramagnetic materials

(i) Are weakly attracted by a magnet

(ii)

Magnetism exists only in the presence

of external magnetic field

Examples:

(i) Plastics

(ii) Rubber

(iii) Water

(iii)

They do not retain magnetism after the

field is removed

Uses

(iv) Wood

(i)

Magnetic field measurement: uses

in magnetic sensors because are

weakly attracted by magnetic fields

(v) Ceramics

(ii)

In medical imaging (MRI); example

gadolinium compounds are used as

contrast agents to improve MRI

images

Uses

(i)

Magnetic levitation: used in

experiments to demonstrate

magnetic levitation due to

repulsion from magnetic field

example. Bismuth and graphite

(iii)

In research and laboratories: used to

study magnetic properties of

materials and atomic structures

(ii)

Magnetic shielding: used to

reduce or shield weak

magnetic field in sensitive

instruments

Diamagnetic materials

(iii)

Non – magnetic applications

such as wiring materials,

example copper

Diamagnetic materials are materials that are

weakly repelled by a magnetic field Examples:

(i) Bismuth

(ii) Copper

(iiii) Silver

(iv) Gold

(vi) Mercury

(vii) Glass

Example 01

(viii) Wood

Choose the most correct answer among the

alternatives given and write its letter in the

box provided

(ix) Human being

(x) Carbon

(v) Water

(i). The force that acts at a distance

causing attraction or repulsion due to a

magnetic field is referred to as:

(graphite) (xi) Animals and plants

Note:

A. magnet

B. magnetic field

When placed in an external magnetic field,

they become magnetised in the opposite

direction to the applied field. As a result, they

move away from regions of stronger magnetic

field.

C. magnetism D. Ferromagnet

(ii). Materials that are very sensitive to

magnetic field are said to be:

A. ferro magnetic B. diamagnetic

Characteristics of diamagnetic materials

C. paramagnetic

D. soft magnetic

(i)

They do not have permanent

magnetic dipoles

(iii). An electromagnet;

A. is made of steel

when current is off

bare copper

B. is a strong magnet

C. has a solenoid of

(ii)

When a magnetic field is

applied, it induces a magnetic

moment that opposes the

applied

D. is made of soft iron

switch in its circuit

E. has no

(iii)

They lose magnetism

immediately when the external

magnetic field is removed

(iv) In which A to E can both metals be made

strongly magnetic?

A. nickel and copper

C. cobalt and iron

B. steel and brass

D. cobalt and copper

E. steel and copper

Answers

Ferromagnetic materials such as soft iron

increases the strength of magnetic field

produced by the current – carrying coil

i

ii

iii

D

iv

C

A

Example 05

Example 02

You have been provided with three rods,

X, Y and Z of which one is a magnet, the

second is made of magnetic material and

the other made of nonmagnetic material.

Explain how you would identify them?

Is copper paramagnetic or diamagnetic

substance? Justify your answer. Give other two

examples of diamagnetic materials

Answer

Copper is diamagnetic material.

(i)

Magnet: When suspended,

always points N – S direction of

the earth

This is because, copper is weakly repelled by a

magnetic field and does not retain magnetism

when the external magnetic field is removed.

(ii)

Magnetic materials: Gets

induced to the opposite polarity

and is always attracted by the

magnet

Examples:

(i)

Bismuth

Human being

Gold

(iii)

Non – magnetic material:

cannot be attracted by the

magnet

(ii)

(iii)

(iv)

Mercury

Example 06

Example 03

Distinguish between soft and hard magnetic

materials

What happens if Bismuth is placed in magnetic

field?

Answers

Soft materials

Hard materials

Bismuth is repelled by the magnetic field.

are those which can

be easily

magnetized, but they

tend to lose their

magnetism easily.

are those which do

not readily lose their

magnetism, though

they are difficult to

magnetise.

This is because it becomes magnetised in the

opposite direction to the applied magnetic

field, as a result it tends to move away from a

region of stronger to weaker magnetic field

Example: Soft Iron

Example: Steel

Example 04

Why is the core of an electromagnet made of

ferromagnetic materials?

Example 07

What are the ferromagnetic materials? Give

two examples of such materials.

Answers

(iv)

Once magnetic field is withdrawn, they

lose their magnetic behavior

Ferromagnetic materials are materials that are

attracted to a magnet and can be strongly

magnetized.

(v)

they are made of soft materials like soft

iron

Example: iron and nickel

Examples

Example 08

(i)

Iron nails

(ii)

(iii)

Iron paper clips

Explain why temporary magnets are made of

iron materials?

An electromagnet used in magnetic

cranes

(iv)

Soft iron

Answers

Note: Iro is called a “soft” magnetic

material because, it is easy to magnetize

and also loses its magnetism easily.

Because, the induced magnetism in iron does

not last for so long

Uses

TYPES OF MAGNETS

(i)

In electromagnet

Magnets are categorized according to their

sources of magnetism. These are:

(ii)

(iii)

In transformers

As magnetic material in a read

switch

(i)

Temporary magnets

Permanent magnets

(ii)

Electromagnet

An electromagnet is a temporary magnet

created by an electric current flowing

through the coil, often with an iron core

to enhance magnetic field

Temporary magnet

Temporary magnets are those which acquire

magnetism due to an applied external magnetic

field but lose their magnetism when the

external field is removed

Construction

An electromagnet is magnet consisting a

core, often soft iron that is temporary

magnetized by passage of electric current

Properties

(i)

These magnets retain magnetism in a

short time (the time during which magnetic

field is present)

(ii)

These are materials which easily gain

and lose easily their magnetic properties

(iii)

When they are within a strong magnetic

field, the act like a permanent magnet repelling

and attracting substances