Magnetism
EXERCISES
BE PROMPT
A. Fill in the blanks.
1. Unlike poles of a magnet attract and like poles repel each other.
2. A magnet always has two poles.
3. As the distance from the magnet increases the magnetic field decreases as the magnetic force exerted by it decreases.
4. The magnetism of electromagnet is temporary and they can be easily magnetised or demagnetised.
5. The process of demagnetisation is opposite of magnetisation.
6. The magnetic north pole of the earth lies near the geographic south pole.
B. State whether the following statements are true (T) or false (F). Correct the false statements.
1. A bar magnet has only one pole. [F]
= A bar magnet has two poles.
2. Temporary magnets hold their magnetism for longer duration even if the magnetising force is removed. [F]
= Temporary magnets lose their magnetism as soon as the magnetising force is removed.
3. Attraction is a sure test for magnetism. [F]
= Repulsion is a sure test for magnetism.
4. The power of a magnet is the strongest at its poles. [T]
5. Electromagnets are made by striking a magnet on a piece of iron several times. [F]
= Electromagnets are made by coiling an electric wire on a piece of iron several times.
6. An electromagnet gets demagnetised when it is hammered. [F]
= An electromagnet gets demagnetised when the electric flow is disconnected.
C. Choose the correct option.
1. Magnetic substances are made of __________ .
(a) only iron. [ ]
(b) any metal. [ ]
(c) iron, nickel and cobalt. [✓]
(d) plastic. [ ]
2. The north pole of a magnet will attract
(a) the north pole of another magnet. [ ]
(b) the south pole of another magnet and pieces of iron. [✓]
(c) both the pole of another magnet and any magnetic materials. [ ]
(d) none of these. [ ]
3. Repulsion between a magnet and another object indicates that
(a) the latter is made of a metal that is non-magnetic. [ ]
(b) the north pole of the former and the south pole of the latter are in contact with each other. [ ]
(c) the other object is also a magnet. [✓]
(d) the other object is made up of magnetic substance. [ ]
4. To demagnetize a magnet, one can
(a) dip it in water. [ ]
(b) heat it for some time. [✓]
(c) strike it with a magnet. [ ]
(d) break it into two pieces. [ ]
5. If pole A of a magnet repels pole B of a second magnet, and pole B attracts pole C of a third magnet, what can be said about pole A and pole C?
(a) Both A and C are north poles. [ ]
(b) A and C are like poles. [ ]
(c) A and C are unlike poles. [✓]
(d) Both A and C are south poles. [ ]
6. Who suggested the electrical method of magnetisation?
(a) Sir Isaac Newton. [ ]
(b) James P. Joule. [ ]
(c) James Six. [ ]
(d) Hans C. Oersted. [✓]
C. Match the columns.
=
|
Column A |
Column B |
|---|---|
|
1. Magnetisation |
(a) Hammering. [3] |
|
2. Magnetic material |
(b) Earth’s north pole. [4] |
|
3. Demagnetisation |
(c) Cranes. [5] |
|
4. Magnetic compass |
(d) Induction. [1] |
|
5. Electromagnet |
(e) Nickel. [2] |
SHORT AND PRECISE
A. Complete the given crossword puzzle with the help of the clues.
Down:
1. Sure test of magnetism.
2. The process of removing magnetism from a magnet.
3. A non-magnetic substance.
Across:
4. A train which uses electromagnets.
5. A magnetic substance.
6. A natural magnet.
7. The property due to which a freely suspended magnet comes to rest in a north-south direction.
8. A soft iron bar that gets magnetised due to passage of electric current.
B. Give reasons for the following statements.
1. Glass cannot be magnetised.
= Only magnetic substances can be magnetised. Glass is not a magnetic substance. So, it can not be magnetised.
2. A freely suspended bar magnet always comes to rest in north-south direction.
= A freely suspended bar magnet always comes to rest in north-south direction due to its directive property. The north pole of the earth's magnet is towards the geographical south, so as to attract the south pole of the freely suspended magnet. Similarly, the south pole of the earth's magnet is towards the geographical north, so as to attract the north pole of the magnet.
3. The magnetic field around a magnet is the strongest near the magnet.
= The magnetic field around a magnet is the strongest near the magnet because the length of the magnetic field of a magnet decreases as we move away from it.
4. Soft iron is a very good material for making the core of electromagnets.
= Soft iron is a very good material for making the core of electromagnets because it is a good magnetic substance with all the magnetic properties.
5. Maximum number of iron filings always sticks near the two ends of the magnet.
= Maximum number of iron filings always sticks near the two ends of the magnet because magnetic force is the strongest at the poles.
6. The poles of an electromagnet are not fixed.
= The poles of an electromagnet are not fixed because it depends on the direction of the current flow. The end of the electromagnet at which current is flowing in clockwise direction behaves as the south pole and the other end where the current is flowing in an anticlockwise direction behaves as the north pole.
C. Answer in short.
1. In which direction does a freely suspended bar magnet come to rest?
= A freely suspended bar magnet comes to rest in north-south direction.
2. List the methods of magnetising a magnetic substance.
= The methods of magnetising a magnetic substance are -
i. Single touch method.
ii. Double touch method.
iii. Magnetic induction.
iv. Electrical method.
3. Why do we have to take care while storing the magnets?
= We have to take care while storing the magnets because a magnet becomes weak and loses its magnetism over the period of time when its poles are left free.
4. How is an electromagnet different from other magnets?
= The magnetism of an electromagnet is temporary and can be switched on and off unlike that of a permanent magnet. It is a temporary magnet.
The strength of an electromagnet can be changed as per the need.
5. Write any four uses of a magnet.
= Four uses of a magnet are -
i) Permanent magnets are used in televisions, computers, speakers and microphones.
ii) These are used in doors of refrigerators for closing mechanism.
iii) These are used in magnetic compasses.
iv) Magnetic strips are used in credit cards and ATM cards to store information.
6. Explain magnetisation by induction.
= If a magnet is kept close to a magnetic substance, it magnetises the magnetic substance and that gets the property of a magnet for that moment. It is called magnetic induction.
If we clamp a iron nail and keep a magnet to its head, the nail attracts the iron filings kept below. As soon as we remove the magnet from the head of the nail, the iron filings fall down.
7. How does the magnetic field around a magnet depend on the distance from the magnet?
= The strength of the magnetic field of a magnet decreases as we move away form it. Magnetic lines of force are used to represent magnetic field around a magnet. Conventionally, outside the magnet, the direction of magnetic lines of force is from north pole to south pole of a magnet, whereas inside the magnet, the magnetic lines of force move form south pole to north pole. Thus, magnetic lines form closed curves in a magnetic field.
AT LENGTH
A. Explain the following terms.
1. Magnet.
= Magnet is naturally occurring material like lodestone which possess the inherent property to attract iron.
2. Artificial magnet.
= Artificial magnets are the magnets made by human.
3. Electromagnet.
= Electromagnet is a device in which magnetic field is produced by the flow of electric current.
4. Magnetic axis of bar magnet.
= Magnetic axis of bar magnet is the line joining the magnetic north pole of the bar to its magnetic south pole of the same.
5. Magnetisation.
= Magnetisation is the conversion of an ordinary piece of magnetic material like iron, steel etc, into a magnet.
6. Magnetic field.
= Magnetic field is the region around the magnet where its influence can be felt.
7. Demagnetisation.
= Demagnetisation is the process of removing magnetism from a magnet.
8. Magnetic keepers.
= To prevent the magnets from losing their magnetism quickly, we should store the magnets in pairs by placing their opposite poles close to each other and by using soft iron strips across the ends of the bar magnets. These soft iron strips are called the magnetic keepers.
D. Differentiate between the following.
1. Magnetic materials and non-magnetic materials.
=
|
Magnetic materials |
Non-magnetic materials |
|---|---|
|
1. These are easily attracted by a magnet. |
1. These are not attracted by a magnet. |
|
2. These can be magnetized. |
2. These can not be magnetized. |
|
3. Examples – Iron, steel, nickel and cobalt. |
3. Examples – Wood, aluminium, copper, rubber, stone, sand etc. |
2. Permanent magnets and temporary magnets.
=
|
Permanent magnet |
Temporary magnet |
|---|---|
|
1. It shows magnetic properties at all times. |
1. It shows magnetic properties as long as the magnetizing force is
present around it. |
|
2. Its magnetic strength cannot be changed. |
2. Its magnetic strength can be changed. |
|
3. Its polarity cannot be changed. |
3. Its polarity can be changed by reversing the direction of the magnetizing
force. |
|
4. It is not easily demagnetized. |
4. It is easily demagnetized. |
3. Magnetic induction and electrical method.
=
|
Magnetic induction |
Electric method |
|---|---|
|
1. Magnet is needed. |
1. Electric is needed. |
|
2. Magnetic strength cannot be increased or decreased. |
2. Magnetic strength can be increased or decreased. |
|
3. Poles cannot be changed. |
3. Poles can be changed by changing the electric flow. |
4. Magnetic axis and geographical axis.
=
|
Magnetic axis |
Geographical axis |
|---|---|
|
1. It is the line joining the magnetic north pole of the earth to the
magnetic south pole of the earth. |
1. It is the line joining the geographical north pole to the
geographical south pole of the earth. |
C. Answer in detail.
1. Explain types of magnets.
= Magnets are of two types - Permanent Magnets and Temporary Magnets.
Permanent Magnets :
Magnets which retain their magnetism even when the magnetising force is removed are called the permanent magnets. The permanent magnets are used in a radio speaker, telephone ear piece, a lining on the door of the refrigerator, etc.
Temporary Magnets :
Magnets which retain their magnetism only for a short period of time and lose it as soon as the magnetising force is removed are known as temporary magnets. The magnets made by induction process and electrical methods get demagnetised as soon as the magnetising force is removed. Therefore, the magnetism acquired by these processes is temporary in nature.
Temporary magnets are used in devices such as electric door bell, electric cranes and generators.
2. The poles of a magnet always exist in pairs. Explain.
= Every magnet has north and south poles. Both the poles always exist in pairs. When a magnet is broken into pieces, each piece has a north pole and a south pole. The earth itself acts like a giant magnet. Due to this, a freely suspended magnet always comes to rest in north-south direction because it is attracted by the magnetic field of the earth. This makes all the magnets have two poles.
3. State the properties of a bar magnet.
= The properties of a bar magnet are -
i) A magnet exerts force of attraction on magnetic substances called magnetic force. This property is known as the attractive property of magnets.
ii) When a magnet tied to a massless string is freely suspended horizontally, it always comes to rest in the north-south direction. Even if it is disturbed form its position, after a few seconds the magnet will again come to rest in the north-south direction.
iii) Every magnet has north and south poles. Both the poles always exist in pairs. When a magnet is broken into pieces, each piece has a north pole and a south pole.
iv) Like poles of two magnets repel each other, whereas unlike poles attract each other.
v) The strength of a magnet is maximum at its poles and minimum at its centre.
4. How can you find out whether a metallic bar is a magnet or not?
= To find whether a metallic bar is a magnet or not, we have to take it near a magnet. If both of the poles of the metallic bar is attracted to the magnet, it is not a magnet. If one pole is attracted and the other is repelled by one end of the magnet, then it is a magnet.
5. Write a note on the theory of magnetism. Draw suitable diagrams.
= About 4000 years ago, the ancient Greeks discovered rocks which possessed the property to attract iron objects. They named them magnetite. Later on due to the discovery of the directive property of magnetite, its name was changed to lodestone.
Naturally occurring materials like lodestone which possess the inherent property to attract iron are known as natural magnets. But they are weak magnets. However, the magnets made by human, i.e., artificial magnets are stronger magnets. They are made of iron, cobalt, nickel, etc. as these materials can be magnetised easily. Such magnets are available in different shapes and sizes according to the purpose they serve.. Magnets have a number of applications. They are used in refrigerators, computers, televisions, CDs, generators, doorbells, etc.
6. Explain the method of magnetising an iron piece by single touch method. How is it different from the double touch method?
= Single Touch Method : Take a single iron bar of length AB and place it on the table. Now, take a bar magnet and select one of its pole. Move the north pole of the bar magnet along the iron bar A to B. When you reach the end point B, lift your bar magnet and move it along the same direction again. Repeat this process several times.
You will observe that after sometime the iron bar gets magnetised with one of the end acting as the north pole and another end acting as the south pole. You can also test the iron bar for magnetic properties by bringing its north pole closer to the north pole of the bar magnet. You will observe that the two repel each other. Now bring some iron filings near the magnetised iron bar. You will observe that these will stick to the bar.
The differences between single touch method and the double touch method are -
|
Single Touch Method |
Double Touch Method |
|---|---|
|
1. One bar magnet is needed. |
1. Two bar magnets are needed. |
|
2. The magnet is rubbed from one end to the other end. |
2. The magnets are rubbed from the centre to the two respective end
points of the iron bar. |
7. What is an electromagnet? Give its uses.
= An electromagnet is a device in which magnetic field is produced by the flow of electric current. The magnetic field generated lasts as long as there is current in the circuit.
The uses of an electromagnet are -
❐ An electromagnet is used in electric motors, electric bells, loudspeakers, generators, etc.
❐ It is employed in magnetic separation equipments for separation of iron ores in industries, for picking up and moving heavy iron objects like scrap iron.
❐ It is used by doctors to detect and treat a variety of ailments.
❐ It is used to deflect electronic beam in a television.
❐ It is used for making strong, permanent magnets.
❐ The bottom of maglev trains (the fastest trains in the world) and their tracks both have electromagnets to run the train very fast.
8. What are the advantages of using an electromagnet?
= The advantages of using an electromagnet are -
❐ The poles can be changed by changing the direction of the electric flow.
❐ The strength of an electromagnet can be changed as per the need. The strength of an electromagnet depends on the following factors.
i) The strength of current flowing in the coil.
ii) The number of turns in the coil.
iii) The nature of material of the core.
9. What is demagnetisation? What are the ways in which a magnet can be demagnetised?
= The process of destroying the magnetism of a magnet is called demagnetisation. The magnetic properties of a magnet can be destroyed in the following manner.
❐ By rough handling.
❐ By dropping the magnet on the floor or on a hard surface a number of times.
❐ By hammering the magnet repeatedly.
❐ By heating the magnet to a very high temperature.
❐ By passing alternating electric current around the magnet.
By Heating the Magnet to a Very High Temperature :
If a magnet bar is heated at a very high temperature and then allowed to cool in the east-west direction, it loses magnetism. On heating the magnet, its molecules gain thermal energy and then kinetic energy increases. This breaks the chain of the molecules and results in demagnetisation of the magnet.
By Hammering the Magnet Repeatedly :
When a magnet is hammered repeatedly, disorientation of the molecules of the magnet occurs. This results in the loss of magnetism altogether.
By Passing Alternating Electric Current Around the Magnet :
If the alternating electric current of high frequency is passed through an insulated copper wire would over a magnet, the orientation of the molecules of the magnet changes rapidly with the change in the direction of the current. Due to this, the orderly alignment of the molecules of the magnet is broken leading to demagnetisation of the magnet.
10. Write a note on earth as a magnet.
= The earth itself acts like a giant magnet. Due to this, a freely suspended magnet always comes to rest in the north-south direction because it is attracted by the magnetic field of the earth. A magnetic compass is also based on this principle. This means that the north pole of the earth's magnet is towards the geographical south, so as to attract the south pole of the freely suspended magnet. Similarly, the south pole of the earth's magnet is towards the geographical north, so as to attract the north pole of the magnet. The line joining the geographical north pole to the geographical south pole of the earth is known as the geographical axis. The line joining the magnetic north pole of the earth to its magnetic south pole is known as the magnetic axis. There is a difference of approximately 11 and half degrees between the geographical axis and the magnetic axis of the earth.
