When a magnet is broken in half, the

two portions both have two poles.

Unlike poles appears on opposite sides

of the break

A magnet is cut into two equal pieces.

What happens to the magnetic poles of

each piece, and why?

Answers

This is because, every molecule of a

magnetic substance is itself a permanent

magnet having two poles

When a magnet is broken in half, the

two portions both have two poles.

Unlike poles appears on opposite sides

of the break

This is because, every molecule of a

magnetic substance is itself a permanent

magnet having two poles

Example 02

Note:

Explain what would happen if you cut a

bar magnet into half?

(i)

No matter how small the pieces

are, there will always be two

poles N – pole and S – pole

There are two poles of equal

strength though opposite to each

other

The unlike poles appear on

opposite sides of the break. No

matter how many times the

magnet is subdivided, each piece

is found to posses two poles

Answers: see your notes.

(ii)

(iii)

DOMAIN THEORY

Domain theory states that “inside a

magnet there are small regions in which

the magnetic direction of all dipoles are

aligned in the same directions”

Magnetic domains are minute regions

in a ferromagnetic material with millions

of magnetic dipoles coupled together in

a preferred direction

(iv)

A magnet cannot be divided to

have only one end which is

stronger than the other. This is

because, every molecule of a

magnetic substance is itself a

permanent magnet having two

poles at its ends

Magnetic domain is a region within a

magnetic material in which

magnetization is in a uniform direction

Magnetic dipole is a tiny subatomic

magnet equivalent to a flow of electric

charge around a loop

Example 01

Note:

(i)

If we were able to cut the pieces

of a magnet over possible, we

would see the smallest magnets

(molecules)

In ferromagnetic materials, these

small molecules are called

molecular magnets magnetic

dipoles) with two poles.

(i)

Beyond saturation, the material

acts like free space

(ii)

Magnetic saturation is a

condition whereby all magnetic

domains are aligned in only one

direction and cannot be aligned

beyond.

(ii)

(iii)

(iv)

These magnetic dipoles occupy

the region called domains

In each domain, separated by the

wall, the molecular magnets

(magnetic dipoles) are aligned in

one particular direction but the

axis of the domain points in

different and random directions.

The resultant magnetism is

therefore zero

Unmagnetized material

Unmagnetized material is the one in

which the magnetic dipoles are

randomly aligned, forming magnetic

domains.

Magnetic saturation

Magnetic saturation is the condition in

a magnetic material where an increase in

the applied magnetic field no longer

produces significant magnetization.

In partially magnetised material, most of

the domain’s axes point in the same

direction.

In the “unmagnetized” state the axis of

the domain points in all directions at

random and so the bar as a whole shows

no polarity.

3.2 Magnetization and

demagnetization

When the ferromagnetic material is

placed in strong magnetic field, all the

domains align in one direction with the

field and the material is said to be

magnetically saturated.

The magnetic properties of the material

can be created as well as can be

destroyed by the process of

magnetization and demagnetization

respectively

This forms the free atomic poles at the

ends of the bar which gives the poles of

the magnet

Note

Magnetization

Magnetization is a process of aligning

the magnetic dipoles in a material in one

direction to produce a net effect of

attraction or repulsion

Properties of a magnetised material

(i)

When suspended always points N

– S direction of the Earth

Fig: A fully magnetized material

Note

(i)

When a magnetic material is

fully magnetized, the domain

wall disappears and the magnetic

dipoles align themselves in one

particular direction

(ii)

If atomic dipoles arrange in such

a way that N – poles of all

dipoles face in one common

direction, the matter is said to be

magnetised

(ii)

When brought near a magnet,

experiences a force of attraction

or repulsion

(iii)

The domains may show a net

magnetic behavior in the absence

of magnetic field. On applying an

external magnetic field, all

dipoles align themselves in the

direction of applied magnetic

field.

In this way, the material is strongly

magnetised in the direction parallel to the

magnetic field.

Magnetised material: - is the material in

which all the magnetic dipoles align in one

direction, and hence can attract magnetic

materials like iron

In a magnetised material, all the magnetic

domains orient themselves in the same

direction, producing a strong overall

magnetic effect

It has been observed that, when a magnet

is freely suspended so that it can swing

in a horizontal plane it oscillates to and

fro for a short time and then comes to

rest in an approximate N – S direction.

Explain how you would identify the

magnetised bar without using another

magnet?

Answers

Magnetised bar when suspended, always

points N – S direction of the Earth

The magnet comes to rest with its axis in

a vertical plane called magnetic meridian

Example 02

Using domain theory, explain the

process of magnetization

Answers

Magnetization is a process of aligning

the magnetic dipoles in a material in one

direction to produce a net effect of

attraction or repulsion

Magnetic meridian is a vertical plane

containing the magnetic axis of a freely

suspended magnet of rest under the

action of the action of the Earth’s

magnetic field

Example 03

What is the difference between ceramic

and a bar magnet?

Note:

(i)

Materials which are possible to

cause the alignment are

ferromagnetic or paramagnetic

Ferromagnetic materials such as

steel, nickel and cobalt can form

permanent magnets

Paramagnetic materials such as

aluminium and chromium are

weakly magnetised and do not

retain their magnetism once the

external magnetic field is

removed

Answers

Ceramic

Bar manet

(ii)

(iii)

It is made from

iron oxide (ferrite)

mixed with barium such as steel,

or strontium

It is made from

many materials

Alnico,

Neodymium

It has moderate

magnetic strength

The strength

depends on the

materials used

Can be of any

shape (disc, ring,

block)

Always rectangular

(bar shaped)

Example 01

You have been provided with two iron

bars P and Q of which one is a

magnetised bar and the other is not.

THE BASIC LAW OF MAGNETISM