6 MAGNETISM
I.
Tick (3) the most appropriate answer.
1. When a bar magnet is suspended freely, it comes to rest :
(a) east-west direction
(b) north-east direction
(c) north-south direction
(d) north-west direction
2. A bar magnet is cut into four pieces. Each piece is :
(a) complete magnet (b) some pieces have only north pole
(c) some pieces have only south pole
(d) all pieces lose magnetism
3. A bar of steel can be permanently magnetised by :
(a) rubbing a bar magnet with it along the length
(b) rubbing a bar magnet at its ends
(c) rubbing a bar magnet at its centre
(d) none of these
4. Similar poles of bar magnets :
(a) attract each other
(b) repel each other
(c) neither attract nor repel each other (d) none of these
5. Lodestone is an example of :
(a) artificial magnet
(b) electromagnet
(c) natural magnet
(d) none of these
6. An artificial magnet which is used for finding geographical
directions is :
(a) bar magnet
(b) electromagnet
(c) horseshoe magnet
(d) magnetic compass
7. The surest test for magnetism is :
(a) attraction
(b) repulsion
(c) both attraction and repulsion
(d) none of these
8. Very strong magnets are :
(a) natural magnets
(b) permanent magnets
(c) electromagnets
(d) magnetic needle
9. Making a nail behave like a magnet by bringing a magnet near
it but not touching it is :
(a) single-touch method
(b) magnetic induction method
(c) double-touch method
(d) none of these
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10. The attraction of iron filings by the poles of a magnet is :
(a) maximum (b) zero
(c) medium (d) minimum
11. In a magnet :
(a) north pole is stronger than south pole
(b) south pole is stronger than north pole
(c) both the poles are of equal strength
(d) none of these
12. The earth behaves like a huge bar magnet with its magnetic
north pole situated near the :
(a) geographical north pole (b) geographical west pole
(c) geographical south pole (d) geographical east pole
13. Ferrite magnets are made up of a mixture of :
(a) iron oxide and aluminium oxide
(b) iron oxide and barium oxide
(c) iron oxide and calcium oxide
(d) potassium oxide and barium oxide
14. Which of the following is an example of a natural magnet?
(a) Lodestone (b) Iron
(c) Steel
(d) Cbalt
15. Artificial magnet used in the laboratory to detect the direction
is a :
(a) horseshoe magnet
(b) U-shaped magnet
(c) bar magnet
(d) magnetic needle
16. In an unmagnetized substance, the molecular magnet forms :
(a) open chains
(b) straight chains
(c) closed chains
(d) none of these
17. A permanent magnet :
(a) attracts all substances (b) attract only magnetic substances
(c) repels all substances (d) none of these
18. Which of the following is a magnetic substance?
(a) Cobalt
(b) Aluminium (c) Copper
(d) Wood
19. Temporary magnets are made up of :
(a) steel
(b) copper
(c) soft iron (d) aluminium
20. Permanent magnets are made up of :
(a) steel
(b) copper
(c) soft iron (d) aluminium
21. Presence of magnetism in a magnet, is due to :
(a) basic structural property
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(b) due to molecular arrangement in the form of a closed chain
(c) due to molecular arrangement in a particular order
(d) none of these
22. Effective length of a magnet is :
(a) equal to its geometric length
(b) greater than its geometric length
(c) smaller than its geometric length
(d) either greater or smaller depending on the nature of the
material
23. Which of the following is a non-magnetic substance?
(a) Iron
(b) Steel
(c) Nickel
(d) Copper
Ans. 1. (c) 2. (a) 3. (a) 4. (b) 5. (c) 6. (d) 7. (b) 8. (c) 9. (b)
10. (a) 11. (a) 12. (c) 13. (a) 14. (a) 15. (d) 16. (c) 17. (b)
18. (a) 19. (c) 20. (a) 21. (c) 22. (c) 23. (d).
II. Fill in the blank spaces using correct words from the list given
below
[List: magnetised, natural, broken, powerful, magnet, surest
magnetic, electromagnets, telegraph.]
1. Artificial magnets are preferred to _______ magnets because
they are more _______ .
2. A substance which is strongly attracted by a _______ is called
_______ substance.
3. _______ are used in telephones and _______ .
4. Repulsion is the _______ test of magnetism.
5. A piece of iron can be _____ by rubbing a bar magnet over it.
6. When a magnet is _______ every piece is a complete magnet.
Ans. 1. natural, powerful 2. magnet, magnetic 3. Electromagnets,
telegraph 4. surest 5. magnetised 6. broken.
III. Fill in the blanks.
1. The Chinese sailors used natural magnets as _______ .
2. Magnet is a term derived from _______ .
3. Hammering a magnet violently destroys its _______ property.
4. Temporary magnets are usually made of _______ .
5. Magnets are used in the construction of certain toys to give
_______ like effect.
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A freely floating magnet points in the _______ direction.
In a magnet, _______ have the greatest magnetic force.
_______ is a sure test of magnetism.
There is a very large _______ around the earth.
Artificial magnets can be made into different _______ .
Effective length of a bar magnet is always _______ than
geometric length.
12. A freely suspended bar magnet always points towards _______
and _______ .
13. Similar poles _______ each other.
14. Lodestone is a _______ magnet.
15. Electromagnet is an _______ magnet.
16. The imaginary line joining north and south pole is called
magnetic _______ .
Ans. 1. compass 2. magnetite 3. magnetic 4. soft iron 5. magic
6. north-south 7. poles 8. Repulsion 9. magnetic field 10. shapes
and sizes 11. less 12. north and south 13. repel 14. natural
15. artificial 16. lines.
IV. State whether the following statements are true or false (T/F).
1. If a substance is attracted by a magnet, then the substance is
also a magnet.
2. Natural magnets are not used because they have weak magnetic
force.
3. Magnetic force can easily pass through non-magnetic substances.
4. A mixture of iron and brass powder can be separated with
magnet.
5. Electromagnets can be prepared by passing current around any
metal.
6. Repulsion is the surest test of magnetism.
Ans. 1. F 2. T 3. T 4. T 5. F 6. T.
V. Classify the following into magnetic and non-magnetic
substances:
Wood, iron, copper, air, brass spoon, plastic ruler, soap cake, water
paper, button, cobalt, nickel, aluminium.
6.
7.
8.
9.
10.
11.
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Ans. Magnetic substances: Iron, cobalt, nickel.
Non-magnetic substances: Wood, copper, air, brass spoon,
plastic ruler, soap cake, water, paper, button, aluminium.
VI. Statements given below are incorrect. Write the correct
statements.
1. Natural magnets are stronger than artificial magnets.
Ans. Artificial magnets are stronger than natural magnets.
2. A mixture of iron and aluminium powder can not be separated
with a magnet.
Ans. A mixture of iron and aluminium powder can be separated with
a magnet.
3. Attraction is the surest test of magnetism.
Ans. Repulsion is the surest test of magnetism.
4. Alnico is used in the coils of transistors.
Ans. Magnadur is used in the coils of transistors.
5. For magnetising a magnetic substance, single-touch method is
better than double-touch method.
Ans. For magnetising a magnetic substance, double touch method is
better than single-touch method.
6. A magnet has no effect even if it is heated to a very high
temperature.
Ans. A magnet gets demagnetised if it is heated to a very high
temperature.
7. Permanent magnets lose their magnetism after the magnetising
force is removed.
Ans. Temporary magnets lose their magnetism after the magnetising
force is removed.
8. Magnetic poles always occur in single.
Ans. Magnetic poles always occur in pairs.
9. The north pole of a freely suspended bar magnet will always
face towards geographic south.
Ans. The north pole of a freely suspended bar magnet will always
face towards geographic north.
10. Iron is a non-magnetic substance.
Ans. Iron is a magnetic substance.
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11.
Ans.
12.
Ans.
13.
Ans.
14.
Ans.
15.
Ans.
16.
Ans.
17.
Wood is a magnetic substance.
Wood is a non-magnetic substance.
Similar poles attract each other.
Similar poles repel each other.
An electromagnet is a permanent magnet.
An electromagnet is a temporary magnet.
Aluminium is a magnetic substance.
Aluminium is a non-magnetic substance.
Steel is a non-magnetic substance.
Steel is a magnetic substance.
Unlike poles of magnets repel each other.
Unlike poles of magnets attract each other.
Electromagnets are used for removing brass pieces from a
scrap of brass and copper.
Ans. Electromagnets are used for removing iron pieces from a scrap
of iron and copper.
18. When a soft iron rod is placed in coil carrying current, it
changes to permanent magnet.
Ans. When a soft iron rod is placed in coil carrying current, it
changes to temporary magnet.
VII. Match the following
Column A
Column B
1.
2.
3.
4.
5.
6.
7.
8.
Iron, cobalt, nickel
Wood, air, water
Used in large computers
Used in electric bell
Lodestone
Soft iron
Steel
Magnet made from a mixture
of iron oxide and barium oxide
9. Magnet made from a mixture
of aluminium, nickel and cobalt
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Natural magnet
Electromagnet
Non-magnetic substances
Magnetic substances
Ceramic magnet
Magnetic needle
Alnico
Ferrite magnet
(i) Magnetic toys
Ans. 1. (d) 2. (c) 3. (e) 4. (b) 5. (a) 6. (f) 7. (i) 8. (h) 9. (g).
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VIII. Find the odd-one out. Give reasons for your choice.
1. Plastic, brass, iron, copper, zinc
Ans. Iron. Only iron is magnetic substance but other are nonmagnetic substance.
2. Iron, nickel, cobalt, aluminium
Ans. Aluminium. Only aluminium is non-magnetic substance but
other are magnetic substance.
3. Magnetic induction, magnetic separation, single-touch method,
double-touch method.
Ans. Magnetic separation. Except-magnetic separation, other are
methods for magnetising a magnetic substance.
4. Horseshoe magnet, U-shaped magnet, bar magnet, lodestone,
cylindrical magnet.
Ans. Lodestone. Only lodestone is a natural magnet but other are
artificial magnets.
5. Steel, copper, aluminium, brass.
Ans. Steel. Only steel is magnetic substance but other are nonmagnetic substance.
IX. Give reasons for the following.
1. Artificial magnets are more used than natural magnets.
Ans. As artificial magnets are more powerful and can be made in
various shapes and sizes hence, artificial magnets are more used
than natural magnets.
2. A freely suspended magnet comes to rest in the north-south
direction.
Ans. It is because the earth itself behaves like a magnet such that the
north pole of the earth’s magnet is pointing at the geographic
south and the south pole pointing at the geographic north.
3. It is impossible to obtain a piece of magnet having only one
magnetic pole.
Ans. As the magnetic poles always occur in pairs, north pole and
south pole hence, it is impossible to obtain a piece of magnet
having only one magnetic pole.
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4. Most of the iron filings cling near the ends of the magnet.
Ans. Because the ends of the magnet have greater magnetic strength
than the centre of the magnet. So most of the iron filing cling
near the ends of the magnet.
5. When a magnet is heated strongly, it gets demagnetised.
Ans. Because the kinetic energy of the molecules increases when
magnet are heated strongly. Then the straight chains of the
molecules get disrupted and the molecules go back to the
closed chain arrangement.
6. Repulsion is the surest test of magnetism.
Ans. Attraction can be caused either by opposite poles of magnets
or between one pole of magnet and an unmagnetised bar of iron
whereas repulsion is possible only between the two similar
poles of magnets. Hence repulsion is the surest test of
magnetism.
X. Define the following:
1. Pole of a magnet 2. Effective length of a bar magnet 3. Magnetic
induction 4. Magnetic axis 5. Magnetic meridian 6. Magnetic keeper
Ans. 1. Pole of a magnet. The regions near the ends of a bar magnet
where the attractive power is most pronounced are called the
poles of the magnet.
2. Effective length of a bar magnet. It is the distance between
the magnetic north pole and magnetic south pole.
3. Magnetic induction. The phenomenon due to which
temporary magnetism is produced in a magnetic substance due
to the presence of a magnet is called magnetic induction.
4. Magnetic axis. The imaginary line passing through the
magnetic north (N) and south (S) poles of a bar magnet is
called magnetic axis.
5. Magnetic meridian. It is an imaginary vertical line passing
through the magnetic axis of a freely suspended magnet.
6. Magnetic keeper. It is a piece of soft iron. Two such soft iron
bars are placed at the either end of the pair of bar magnets. It
is used to avoid self-demgnetisation of magnets.
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XI. Differentiate between the following.
1. Ferrite magnets and alloy magnets.
Ans.
Ferrite magnets
Alloy Magnets
(1) They are made from a
(1) They are made of mixture
powder of iron oxide and
of aluminium, nickel, cobalt
barium oxide
copper and iron.
(2) They are brittle and have
(2) They are very strong and
the properties of ceramics.
do not lose magnetism with
normal treatment.
(3) It is known under the trade (3) It is known under the trade
name magnadur.
names alnico and alcomax.
2.
Ans.
3.
Ans.
4.
Ans.
5.
Ans.
6.
Ans.
Iron and wood.
Iron is a magnetic material but wood is a non-magnetic material.
Magnetic induction and single-touch method.
Magnetic induction is the phenomenon by which an ordinary
piece of iron acquires magnetic properties temporarily due to
the presence of another magnet near it. In single touch method,
the magnetic substance is magnetised by rubbing with a magnet.
Soft iron magnet and alnico magnet.
Soft iron magnet are temporary magnets which can retain their
magnetism only for a short duration. They lose their magnetism
after the magnetising force is removed.
Alnico magnets are permanent magnets i.e. they are able to keep
their magnetism for a long time even when the magnetising force
is removed.
Length of a magnet and effective length of a magnet.
Length of a magnet is the distance between the centre of the
magnet and one of the magnetic poles.
Effective length of a magnet is the distance between the
magnetic north pole and magnetic south pole.
Natural and artificial magnet.
Natural magnets are naturally occurring substances like
lodestone which possesses the property of attracting magnetic
substances.
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Artificial magnets are magnets made by humans using artificial
methods.
7. Permanent and temporary magnet.
Ans. Permanent magnets are the magnets which are able to keep their
magnetism for a long time even when the magnetising force is
removed.
Temporary magnets are magnets which lose their magnetism
after the magnetising force is removed.
8. Steel and plastic.
Ans. Steel is a magnetic material but plastic is a non-magnetic material.
9. Single touch method and double touch method.
Ans. Single touch method is a method of magnetising a magnetic
substance by rubbing with a magnet.
Double touch method is a method of magnetising a magnetic
substance by rubbing with a pair of magnets.
XII. Answer these questions:
1. (a) What is a natural magnet?
(b) How is artificial magnet prepared from a natural magnet?
Ans. (a) A material which occurs naturally and has magnetic
properties is called natural magnet.
(b) Artificial magnets are prepared by imparting the properties
of natural magnet to the magnetic substances by rubbing
with magnet or by process of induction.
2. (a) What are artificial magnets?
(b) Draw diagrams of four artificial magnets.
Ans. (a) Artificial magnets are the magnets made of iron, steel,
cobalt or nickel.
(b) (i) Bar Magnet
(ii) Cylindrical magnet
(iii) Dumb-bell magnet.
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(iv) U-shaped magnet.
3. Why are artificial magnets preferred to natural magnets?
Ans. Artificial magnets are preferred to natural magnets because
(i) they can be made very powerful for using in different
devices.
(ii) they can be made in any desired shape and size.
4. What are (a) magnetic substances (b) non-magnetic substances? Give four examples of each kind.
Ans. (a) Magnetic substances are the substances which are strongly
attracted by magnet or can be converted into magnet
artificially. For examples, iron, cobalt, nickel and steel.
(b) Non-magnetic substances are the substances which are
neither attracted by magnet nor can be converted into
artificial magnets. For examples, stones, glass, wood,
aluminium etc.
5. (a) You have a steel nail 10 cm long. How will you magnetise
it, using a bar magnet?
(b) How will you test the poles of magnet?
Ans. (a) We can magnetise a steel nail of 10 cm long by rubbing
a permanent bar magnet over it.
(b) The poles of magnet can be tested by bringing ends of
magnet near the north pole of freely suspended magnetic
needle. If the north pole of freely suspended magnetic
needle gets repelled, then that end of the magnet is the
north pole of the magnet and if it is attracted then the end
of the magnet is south pole.
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6. How will you convert an iron nail about 8 cm long into a magnet
with the help of electric current?
Ans. Wind the iron nail with the insulated copper wire over it. Bare the
ends of copper wire from insulation, then fixed each end on the
terminals of dry cell. Then, the iron nail will change into magnet when
current flows around it.
7. State six important properties of a magnet.
Ans. Six important properties of magnet are:
(a) it attracts magnetic substances.
(b) it points north-south direction when suspended freely.
(c) repulsion is the surest test of magnetism.
(d) poles of magnets are always in pair.
(e) similar poles of magnets repel each other.
(f) magnetic force can easily pass through non-magnetic
substances.
8. A box contains nails of brass and iron. How will you separate them
by using a magnet?
Ans. When we bring a magnet near a box containing nails of brass and
iron, the iron nails will cling at the magnet. Then we can separate
them easily.
9. What are magnetic keepers? How Magnetic keepers are used to
store magnets.
Ans. Magnetic keepers are soft iron bars placed at the either end of the
pair of magnets. It is used to avoid self-demagnetisation of magnets.
The magnets are arranged in pairs, with opposite poles facing each
other. Two soft iron bars are placed at the either end of the pair of
magnets.
10. How do you show the directive properties of a bar magnet?
Ans. A string is tied at the centre of the bar magnet. When, we suspend
it so that it can hangs freely. Then, the magnet will always point in
the north-south direction, when it comes to rest. This show the
directive property of a bar magnet.
11. Classify magnets on the basis of retention of magnetism.
Ans. On the basis of retention of magnetism, we can classify magnets into
two categories.
(i) temporary magnet, and (ii) permanent magnet.
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12. What are the important uses of magnets.
Ans. Uses of magnets:
(i) it is used in magnetic compass,
(ii) it is used in pencil boxes, soap stands, refrigerator etc.
(iii) ceramic magnets are used in large computers,
(iv) it is used in bicycle dynamos,
(v) it is also used in toys to give a magical effect.
13. How do you prove that earth itself is a magnet?
Ans. A freely suspended magnet always points in the north-south
direction. This shows that our earth itself is like a huge bar
magnet with its magnetic poles near the geographical poles. The
north poles of earth’s magnet is near the geographical south
pole and the south pole of the earth’s magnet is near the
geographical north pole.
14. How will you show that the magnet shows attractive property?
Ans. To show the attractive property of a magnet, spread iron filings,
over a sheet of paper. Then, roll the magnet on the filings so
that all parts touch the filings. Iron filings will cling to the
magnet when magnet is picked up.
15. Define magnetic induction. How is an unmagnetised piece of
iron magnetised by this phenomenon?
Ans. The phenomenon by which an ordinary piece of iron (or any
magnetic substance) acquires magnetic properties temporarily
due to the presence of another magnet near it is called magnetic
induction.
An unmagnetised piece of iron can be magnetised by bringing
a magnet near the piece of iron.
16. What happens when a magnet is rubbed on a needle with a
magnet in only one direction (from one end of the needle to the
other)?
Ans. When a magnet is rubbed on a needle with a magnet in only
one direction, then the needle will magnetised temporarily.
17. Explain the ways by which the magnetic properties of a magnet
get destroyed.
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Ans. The magnetic properties of a magnet get destroyed
(i) by rough handling.
(ii) by hammering it violently several times.
(iii) by dropping it on the floor many times,
(iv) by heating it to a very high temperature.
18. Explain how a pair of bar magnets can be stored. Draw a
labelled diagram showing the arrangement of the magnets and
the keepers.
Ans. A pair of bar magnet can
be stored by arranging
such that their opposite
poles lie close together and
a keeper is placed on the
two
different
poles
belonging to two different magnets which attract it.
19. A horseshoe magnet is to be stored. Draw a labelled diagram
showing the arrangement of the magnet and keepers.
Ans.
This shows the arrangement of horseshoe magnet and the
keeper.
20. Describe an experiment to show that like poles repel each other.
Ans. Experiment to show that like poles repel each other:
Suspend a magnet freely from a wooden stand with a thread.
When it comes to rest in north-south direction, bring the north
pole of another magnet near the north pole of the suspended
magnet. Then the north pole of freely suspended magnet will
repelled. Repeat the same process with south poles, then also
south pole will repel. This shows that like poles repel each
other.
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21. Describe an experiment to detect whether a given iron piece is
a magnet or not.
Ans. To check whether a given iron piece is a magnet or not.
First keep the given iron piece on a wooden table. Then bring
one pole say N-pole of a magnet close to the ends of the iron
piece one by one. If one end is attracted and other repelled then
the iron piece is a magnet. If both ends are attracted then the
iron piece is not a magnet.
22. Describe an experiment to show that magnetic poles always
occur in pairs.
Ans. If a bar magnet is broken into two pieces each piece, when
freely suspended will point in the north-south direction. This
shows that if a bar magnet is broken into two or more smaller
pieces, then even the smallest piece is a complete magnet with
two poles. This shows that magnetic poles always occur in pairs.
23. Explain the molecular theory of magnetism.
Ans. Molecular theory of magnetism states that,
(i) each molecule of a magnetic substance is an independent
magnet.
(ii) in an unmagnetised substance, the molecular magnets are
randomly oriented forming a closed chain.
(iii) in a magnetised substance the molecular magnets are
arranged such that the north pole of each molecular magnet
points in one direction and the south pole in the opposite
direction and the bar has no magnetic property in its centre.
(iv) when a piece of iron is magnetised the molecular magnets
start setting themselves along straight chains.
(v) when a magnet is strongly heated or hammered the magnet
gets demagnetised. It is because the kinetic energy of the
molecules increases and straight chains get disrupted.
24. On the basis of the molecular theory of magnetism, explain the
following.
(a) Magnetisation of iron
(b) Demagnetisation of a magnetic material
Ans. (a) When a piece of iron is magnetised the molecular magnets
start setting themselves along straight chain. As a result the
magnetism of the piece increases.
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25.
Ans.
26.
Ans.
27.
Ans.
28.
(n) When a magnet is strongly heated or hammered the kinetic
energy of the molecules increases and therefore straight
chains get disrupted. As a result the molecules go back to
the closed chain arrangement. Thus, the magnet gets
demagnetised.
Every iron atom is a tiny magnet, why all iron bars are not
magnets?
In an iron bar, iron atoms are randomly oriented. They form a
closed chain so that they neutralise the effect of each other.
So, all iron bars are not magnet even though every iron atom is
a tiny magnet.
What are the characteristic features of the magnetic lines of force?
Characteristic features of the magnetic lines of force are,
(i) each line is a closed and continuous curve,
(ii) normally they originate at the N-pole and terminate at the
S-pole of a magnet,
(iii) the lines are crowded near the poles where the magnetic
field is strong and are far apart where the field is weak.
(iv) they do not intersect each other.
Describe an experiment to prove that maximum magnetic force
acts at the poles of a magnet.
Take a bar magnet and place a steel pin at some distance.
Nothing happens. Then, bring the steel pin near the pole of the
bar magnet. It will stick to the magnet. This experiment shows
that maximum magnetic force acts at the poles of a magnet.
With the help of a labelled diagram show the difference between
the magnetic lines of force in a bar magnet and a horseshoe magnet.
Ans.
(a) Magnetic lines of force in a bar magnet.
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(b) Magnetic lines of force in a (U-shaped magnet) horseshoe
magnet.
29. With the help of a labelled diagram show the magnetic lines of
force formed when the unlike poles of two bar magnets face
each other.
Ans. Magnetic lines of force when the unlike poles of two bar
magnets face each other.
XIII. Numerical Problems:
1. A long magnet is broken into 10 pieces. How many north poles
will be there in
(a) 10 pieces?
(b) one piece?
Ans. (a) 10 north poles in 10 pieces.
(b) 1 north pole for 1 piece.
2. If the length of a magnet is 2 cm, what is its effective length?
Ans. Length of magnet = Length of one of the magnetic pole from
the centre
= 2 cm
.. . Effective length = Distance between two poles
= 2 cm + 2 cm = 4 cm.
****
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