The fundamental law of magnetism is also

called the first law of magnetism

This law states “like poles repel, unlike

poles attract”

Attraction is not a sure test because,

attraction can occur between:

(i)

Two unlike poles

(ii)

A magnet and unmagnetized

(neutral) material

Therefore, if attraction occurs, no firm

conclusion can be drawn.

Testing for polarity of a magnet

Another way of testing the polarity of a

magnet is by suspending it freely using a

cotton thread.

The polarity of any magnet may be tested by

bringing both its poles, in turn, near to the

known poles of a suspended magnet.

The magnet comes to rest in N – S direction.

The end facing north geographic pole is the

N – pole while the end facing the south

geographic pole is the S – pole of the

magnet

Repulsion will indicate similar poles

If attraction occurs, no firm conclusion can

be drawn. This is because, attraction can

occur between:

(i)

Two unlike poles

(ii)

A pole and a piece of

unmagnetized material

Example 04

Give a reason, why attraction in magnetism

is not considered as a reliable method of

testing for polarity?

Therefore, repulsion is a sure test for

polarity of a magnet because, repulsion

occurs only between two like poles.

Answers

Attraction is not a sure test because, all

magnetic materials are attracted to the

magnet but repulsion occurs only on similar

poles.

Attraction can occur between:

(i)

Two unlike poles

(ii)

A magnet and unmagnetized

(neutral) material

Therefore, if attraction occurs, no firm

conclusion can be drawn.

Placing an object in external magnetic field

results in some degree of alignment.

Vibration or heating the object can increase

the amount of alignment by causing the

atomic dipoles to move and eventually

become aligned.

Example 05

Explain how you would identify the

polarities of a magnet whose poles are not

marked?

Note:

Many natural magnetic starts out as part of

lava (molten rock). If the lava contains

ferromagnetic materials, the atom aligns

with the earth’s magnetic field which is

subjected to vibrations while the rock is still

liquid. As the rock cools and solidifies, the

alignment becomes permanent

Answers

(i) By suspending a bar magnet freely using

a cotton thread

The magnet comes to rest in N -S direction

of the Earth. The end pointing in the north is

N – pole of a magnet while the end pointing

in the south of the Earth is S – pole of the

magnet.

Example 06

Using the domain theory of magnetism

explain why:

(a) The strength of a magnet cannot be

increased beyond a certain point

Answers

(ii) By using another magnet

A north pole of a magnet is brought nearby

the unknown magnet.

Because, after all the magnetic

domains are completely aligned, no

further increase in magnetization is

possible, no matter how strong the

external field is:

If repulsion occurs, then the end of unknown

magnet facing the bar is N – pole while the

end facing away from the magnet is S – pole

(b) Temperature increase weakens or

destroys the magnetism of a magnet

Methods of magnetization

Answers

The alignment of domains in materials can

be achieved through on of the following

methods.

Because, rising the temperature

increases the thermal energy as well

as the kinetic energy of the atoms

causing them to vibrate vigorously.

(i)

Heating or vibrations in external

magnetic field

(ii)

(iii)

Stroking method

Electric method

This disrupts the alignment of

magnetic domain, reducing or

eliminating the net magnetic field.

HEATING OR VIBRATIONS

In this method of magnetization, an

external magnetic field is required.

STROKING (TOUCH) METHOD

This is a method of a magnetization

whereby an existing magnet is moved

from one end of the material being

magnetized to the other end repeatedly

in the same direction

(i)

The pole produced at the end of

the stroke is always opposite of

that the end of magnet used for

stroking

After completing stroking, care

has to be taken not to move the

magnet a magnet close to the

needle because it is taken as a

repeated stroke. It disorganizes

the already aligned dipoles

(ii)

In this method of magnetization, a piece

of unmagnetized material is stroked

using a magnet

Between two successive strokes,

the pole is lifted high above to

avoid to disturb the already

aligned dipoles which may cause

weakening of the magnetism

already indued in it.

Types of stroking

Stroking (touch) method is divided into

two:

(i)

(ii)

Single stroke method

Double stroke method

Advantages of single stroke method

(i)

(ii)

It is a simple method

Requires only one magnet

Single stroke method

This is the stroking method whereby only

one magnet is used.

Disadvantages of single stroke method

(i)

The magnet produced has one

pole near the end of the material

than the other

It is also called single – touch method

Double stroke method

Action

This is the stroking method whereby two

magnets are used

An existing magnet is moved from one

end of the material to the other end in

the same direction. This process is

repeated several times.

It is also called double – touch method.

In this method, the material is magnetized

by using two permanent magnets

Action

The ends of two magnets of different

polarities are positioned at the middle of the

material to be magnetized. The two magnets

are then moved in the opposite directions.

The steel needle becomes magnetised

with end A becoming an N – pole while

end B becoming a S – pole

Note

From the diagram above, similar poles (N –

poles) appear at each end of the steel needle.

The middle of the needle becomes south

pole

It is observed that, the end C becomes an N

– pole while end D becomes the S – pole

Note:

(i)

The double stroke method is also

called the divided touch (stroke)

method

(ii)

(iii)

For each stroke, a number of

dipoles align themselves in one

direction

Stroking is repeated to ensure

that a maximum number of

dipoles have aligned themselves

to such a level that the end

product is a magnet

A state when no further

alignment takes place is known

as saturation

During stroking, a wide loop is

advised so that the alignment of

the dipoles is not disturbed.

Note:

(i)

It is possible to have a magnet

with like poles on both ends.

These are called consequent

poles

One should never assume,

without prior taste, that a bar

magnet always has opposite

poles at its ends. This is because,

a magnet can possess consequent

poles

(ii)

(iv)

(v)

Example 07

Consequent poles

Explain why double stroking is more

effective than single stroking method?

Consequent poles are like poles at the ends

of the magnet formed by divided touch

using two similar poles of permanent

magnets.

Answers

Double stroking is more effective because it

applies a stronger magnetic field, aligning

more magnetic domains than single stroking

Action

When a material is magnetised by divided

touch method using two S – poles, the N –

pole is formed at bot ends of the material

and double S – pole in the centre

Electric method

This method of magnetization is done by

passing direct current through the material.

The electric current produces magnetic field

Action