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4   MAGNETISM

Important points to remember : 4. M

AGNETISM

1. Natural magnet : Some iron ores possess the property of

magnetism in their natural state. A lodestone is a natural

magnet. It was discovered about 800 to 600 BC by the people

living in Magnesia in Asia Minor. The properties of the

lodestone (magnetite) were studied since then. This study

developed into the science of magnetism.

2. Magnetic materials : Materials which are attracted by a

magnet are called magnetic materials. Iron, cobalt and nickel

are magnetic materials.

3. Nonmagnetic materials : Materials which remain unaffected

by a magnet are called nonmagnetic materials. Plastic, rubber,

copper, etc. are nonmagnetic materials.

4. Characteristics of a magnet :

(1) Directional : When a magnet is suspended freely, it comes

to rest in the north-south direction.

(2) Attraction : A magnet attracts magnetic materials.

(3) Non-separation of poles : The two poles of a magnet cannot

be separated from each other.

5. Shapes of magnets : Magnets are of varied shapes. A horseshoe

magnet, a bar magnet, a disc magnet, a magnetic needle are some

magnets of different shapes in common use.

6. Properties of magnets : (1) Like magnetic poles repel each

other. (2) Unlike magnetic poles attract each other.

7. Uses of magnets : Magnets are used in magnetic compass,

magnetic door closers, refrigerators, pin holders, etc.

Questions and Answers

Q. 1 Answer the following questions :

(1) State a common feature / characteristic of gravitational

force, static electric force and magnetic force.

Ans. Gravitational force, static electric force and magnetic

force act without physical contact between two bodies.

43

S N

N S S

N

N S

Horseshoe

magnet

Bar

magnet Disc magnet Magnetic needle

N S

(2) How did the name magnet come into existence ?

Ans. Around 800 to 600 BC a special kind of mineral was found

by people living in Magnesia in Asia Minor. This mineral had the

property of attracting small pieces of iron. As the mineral was

found in Magnesia, it came to be known initially as magnetite and

later as magnet.

(3) Name various types of magnets on the basis of their

shapes.

Ans. Magnets of different shapes are used according to the

requirements of a particular device.

A magnet in the shape of a horseshoe is called a horseshoe

magnet. A magnet in the shape of a rectangular strip is called a bar

magnet. A magnet in the shape of a disc is called a disc magnet.

Fig. 4.1 : Types of Magnet

A magnet with sharp pointed ends and supported so as to rotate

freely in a horizontal plane is called a magnetic needle.

(4) What is a magnetic material ? Give two examples.

Ans. A material that is attracted to a magnet is called a

magnetic material.

Examples : Iron, cobalt.

(5) How can we tell whether a particular substance is

magnetic or nonmagnetic?

Ans. Take a powerful magnet. Bring it near the given substance.

If the substance is attracted by the magnet, it is a magnetic

substance. If the substance remains unaffected by the magnet,

the given substance is nonmagnetic.

Answer to the question given on page 38 of the textbook

* A bar magnet was placed deep inside a sack of coal.

Fine particles of coal got stuck to it. From this, can we

infer that coal is attracted towards a magnet?

Ans. In the example given above, fine particles of coal

stick to the magnet due to the physical contact between them

and not due to the magnetic attraction.

Note : Whenever two bodies are in physical contact, there is a

force of attraction between them.

(6) What is a nonmagnetic material ? Give two examples.

Ans. A material that remains unaffected by a magnet is called

a nonmagnetic material.

Examples : Plastic, rubber.

(7) Pieces of iron, aluminium, copper, nickel, cobalt, zinc,

rubber and plastic are kept on a table. How will you separate

them into magnetic and nonmagnetic substances?

Ans. Bring a powerful magnet near each of the pieces. If a

piece is attracted by the magnet, the substance is magnetic. If the

piece remains unaffected by the magnet, the substance is

nonmagnetic.

It is found that iron, nickel and cobalt are magnetic substances

while aluminium, copper, zinc, rubber and plastic are nonmagnetic

substances.

* (8) Classify the following substances as magnetic or

nonmagnetic :

copper, phosphorus, iron, cobalt, soil, water, silver, mercury,

nickel, wood, oil.

Ans. Magnetic substances : Iron, cobalt, nickel.

Nonmagnetic substances : Copper, phosphorus, soil, water,

silver, mercury, wood, oil.

(9) State the characteristics of a magnet.

Ans. Characteristics of magnet :

(1) Directional characteristic : When a magnet is freely

suspended, it always comes to rest in the north-south direction.

(2) Attraction characteristic : When a magnet is brought

near a piece of iron, the latter is attracted towards the magnet.

(3) Non-separation of poles : The two poles of a magnet

cannot be separated from each other.

 (10) How will you find out by an experiment which of the

two poles of a magnet is the north pole ?

Fig. 4.2 : Finding out

the north pole

of a magnet

Ans. Take a thread without any twist. Tie

a bar magnet at its middle with this thread.

Suspend the magnet from a wooden stand in

such a way that it can turn freely in the

horizontal plane. Allow it to come to rest. Mark

the end pointing the north direction with N to

indicate the north pole.

[Note : The south pole is near the other end of the

magnet.]

Answers to the questions given on page 39 of the textbook

* (1) What will you see if the above experiment is done

using a coir rope ?

Ans. If the above experiment is done using a coir rope,

the rest position of the magnet can be different each time

we rotate it and leave it. It will not be possible to locate the

north pole of the magnet.

* (2) Where are the poles of a disc magnet located ?

Ans. The poles of a disc magnet are arranged in circles;

one face of the disc behaving as the south pole and the other as

the north pole.

(11) Describe an experiment to show that it is the poles

of a magnet that have the greatest strength for attracting

magnetic substances.

Fig. 4.3 : Strength of a

magnet to attract

magnetic substances

Ans. Take some iron filings in a china

bowl. Place a bar magnet in it. Lift the

magnet holding it at the centre. It is

observed that iron filings do not stick to all

parts of the magnet in equal proportions.

More iron filings are found to stick to the

magnet near its ends than in the middle.

Very few iron filings are found sticking to

the middle of the magnet. This shows that

the portion of the magnet around the poles

possesses more magnetism.

 (12) What are the poles of a magnet?

Ans. When a bar magnet is placed in iron filings, they stick to

it. It is observed that a lot of iron filings cling to the bar magnet near

its ends. These points of the magnet where there is maximum

attraction are called the poles of a magnet. In reality, the magnetic

poles are located a little inside the ends of a magnet.

(13) State the properties of magnets regarding attraction

and repulsion.

Ans. (1) Like magnetic poles repel each other. (2) Unlike

magnetic poles attract each other.

(14) With neat labelled diagrams, describe an experiment

to demonstrate properties of magnets regarding attraction

and repulsion.

Ans. Suspend a bar magnet from a wooden stand by a thread as

shown in the figure. When it comes to rest in the north-south

position, bring the north pole of another magnet close to its north

pole.

You will find that the north pole of the suspended magnet

moves away from the north pole of the other magnet due to

magnetic repulsion [Fig. 4.4 (a)].

S

N

N

N

S

S

Wooden

stand

(a)

Wooden

stand

S

N

N

S

S

N

(b)

Fig. 4.4 : Repulsion and attraction of magnet

Now, take the other magnet far away so that the suspended

magnet returns to its north-south position of rest. Bring the south

pole of the other magnet close to the north pole of the suspended

magnet. You will find that the north pole of the suspended magnet

4. MAGNETISM 47

moves towards the south pole of the other magnet due to magnetic

attraction [Fig. 4.4 (b)].

This shows that (1) like magnetic poles repel each other.

(2) unlike magnetic poles attract each other.

Answers to the questions given on page 40 of the textbook

* (1) In the above experiment, what will you observe if

the poles were interchanged?

Ans. In the above experiment, even if the poles are

interchanged, it will be observed that like magnetic poles repel

each other and unlike magnetic poles attract each other.

* (2) What difference will you see if an iron bar were

suspended in place of the bar magnet?

Ans. If in the above experiment (see Q. 14), an iron bar

were suspended in place of the bar magnet, it will be seen that

in both parts of the experiment, there is attraction between the

iron bar and the other bar magnet.

(15) With a neat labelled diagram, describe an experiment

to show that two poles of a magnet cannot be separated from

each other.

Fig. 4.5 : Breadthwise cut

of a magnet

Ans. As shown in the figure,

cut a bar magnet breadthwise

with shears. We get two pieces of

the magnet.

Observe that a lot of iron

filings stick to both the ends of

both the pieces. If each piece is

freely suspended, it will be

observed that the right end of part ‘A’ of the magnet has become a

south pole and the left end of part ‘B’ of the magnet has become a

north pole.

Thus, by cutting a magnet, we get two magnets, the poles are

not separated from each other.

* (16) In olden times, how was a magnet placed in a

compass?

Ans. In olden times, a magnetic needle was supported on a

wooden block and floated on water. The needle was free to rotate.

Hence, it used to come to rest in the north-south direction. It was

used as a compass to determine the direction.

(17) State four uses of a magnet.

Ans. (1) A magnet is used in a compass to determine the

direction. (2) A thin round magnet is fitted in the mouth of a plastic

pin holder. Iron pins stick to the lid and can be easily taken out.

(3) The bottom of the shutter of a cupboard is fitted with a magnet.

The point exactly opposite to it on the frame of the cupboard is fitted

with an iron strip. When the shutter is brought close to the frame,

it shuts tightly due to the attraction between the magnet and the

iron strip. (4) A magnet is fitted in the door of the refrigerator

to keep the door shut tightly.

Answer to the question given on page 42 of the textbook

* What are the special advantages of a horseshoe

magnet?

Ans. In a horseshoe magnet, the two poles are close to

each other. Hence, a strong magnetic field exists between these

two poles. Horseshoe magnets are used when a strong magnetic

field is needed in a small space.

Electronic valves in a radar set need powerful horseshoe

magnets to work. Horseshoe magnets are also used in electric

motors and generators.

(18) Why is the magnetic needle of a compass pointed at

its ends?

Ans. The magnetic needle of a compass is pointed at its ends so

that the directions shown by the ends can be marked easily.

* Q. 2 Give scientific reasons :

(1) A magnetic needle is used in a mariner’s compass.

Ans. (1) A magnetic compass is useful for ascertaining

directions in aeroplanes, ships and in desert areas. A magnetic

needle mounted in such a way as to rotate freely in the horizontal

plane comes to rest with its north pole pointing north. (2) Thus,

a magnetic needle can be used to ascertain the directions. Hence,

a magnetic needle is used in a mariner’s compass.

 (2) If a bar magnet is suspended vertically, it does not

hang in the north-south direction.

Ans. (1) In the region near the equator, the earth’s magnetic

force acting on a freely suspended magnet is in the horizontal plane

in the north-south direction. (2) This force is much weaker than the

earth’s force of gravity on the magnet. Hence, if a bar magnet is

suspended vertically, it does not hang in the north-south direction.

Q. 3 Answer the following questions in one sentence

each :

(1) Name two materials which are attracted by a magnet.

Ans. Iron and cobalt are attracted by a magnet.

(2) Name two materials which are not attracted by a

magnet.

Ans. Rubber and copper are not attracted by a magnet.

(3) What is the test for identifying a magnet? Attraction or

repulsion?

Ans. Repulsion, not attraction, is the test for identifying a

magnet.

Q. 4 Find the odd one out and give the reason :

(1) Iron, Copper, Cobalt, Nickel.

(2) Plastic, Rubber, Cobalt, Copper.

Ans. (1) Copper. Copper is a nonmagnetic material; iron,

cobalt and nickel are magnetic materials. (2) Cobalt. Cobalt is a

magnetic material; plastic, rubber and copper are nonmagnetic

materials.

Q. 5 State whether the following statements are True or

False. If a statement is false, correct it and rewrite :

(1) Nickel is a magnetic material.

(2) Rubber is a magnetic material.

(3) Cobalt is a nonmagnetic material.

(4) A magnetic needle is used in a mariner’s compass.

(5) Like magnetic poles attract each other.

(6) Magnetism is maximum at the poles of a magnet.

(7) Repulsion is the real test to identify a magnet.

(8) Magnetic force acts even without physical contact between

two magnets.

Ans. (1) True. (2) False. Rubber is a nonmagnetic material.

(3) False. Cobalt is a magnetic material. (4) True. (5) False. Like

magnetic poles repel each other. (6) True. (7) True. (8) True.

Q. 6 Fill in the blanks :

* (1) There is magnetic ……… between like poles.

* (2) Stainless steel is a ……… substance.

* (3) There is mutual attraction between………poles of magnets.

* (4) There is maximum magnetic force near the ……… of a

magnet.

(5) Plastic is a ……… material.

(6) Cobalt is a ……… material.

(7) A ……… is used in a mariner’s compass.

Ans. (1) repulsion (2) nonmagnetic (3) unlike (4) poles

(5) nonmagnetic (6) magnetic (7) magnetic needle.

Q. 7 Choose the correct alternatives and complete the

following statements :

(1) ……… is a magnetic material.

(a) Rubber (b) Copper (c) Nickel (d) Plastic

(2) ……… is a nonmagnetic material.

(a) Rubber (b) Iron (c) Cobalt (d) Nickel

(3) A ……… is used in a mariner’s compass.

(a) horseshoe magnet (b) bar magnet

(c) disc magnet (d) magnetic needle

Ans. (1) Nickel (2) Rubber (3) magnetic needle.

* Q. 8 Match the following :

‘A’ ‘B’

(1) Compass

(2) Cupboard

(3) Repulsion

(4) Magnetic poles

(a) Maximum magnetic strength

(b) Like poles

(c) Bar magnet

(d) Magnetic needle

Ans. (1) Compass– Magnetic needle (2) Cupboard– Bar

magnet (3) Repulsion – Like poles (4) Magnetic poles –Maximum

magnetic strength.