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Chapter 10

Magnetism

Class 9 - Concise Physics Selina Solutions


Exercise 10(A)

Question 1

What is a lodestone?

Answer

Lodestone is the first known pieces of magnets, an ore of iron oxide (Fe3O4) found in large quantities in Magnesia, in Asia Minor. This ore was found to possess two properties —

  1. It attracts small pieces of iron, and
  2. It sets itself along a definite direction when it is suspended freely.

Question 2

What is a natural magnet? State two limitations of a natural magnet?

Answer

The pieces of lodestone found in nature were called natural magnets.

The limitations of a natural magnet are —

  1. They are found in quite irregular and odd shapes.
  2. They are not magnetically strong enough for use.

Question 3

What is an artificial magnet? State two reasons why do we need artificial magnets?

Answer

Artificial magnets are prepared from iron in different convenient shapes and sizes.

Examples of artificial magnets are bar magnet, horse shoe magnet, magnetic needle and magnetic compass.

We need artificial magnets because —

  1. Natural magnets are of irregular and odd shapes.
  2. Natural magnets are not magnetically strong enough for use.

Question 4

How will you test whether a given rod is made of iron or copper?

[Hint: Iron rod gets magnetized when placed near a bar magnet by magnetic induction, while copper rod does not get magnetized]

Answer

To test whether a given rod is made of iron or copper, we should bring it near (or in contact with) a magnet, if the rod is of iron, then it will become a magnet i.e., it will acquire the property of attracting iron fillings when they are brought near it's ends.

This happens because iron rod gets magnetized when placed near a bar magnet by magnetic induction, while copper rod does not get magnetized. This way, we can detect which rod is iron and which is copper.

Question 5

You are provided with two similar bars, one is a magnet and the other is a soft iron bar. How will you distinguish between them without the use of any other magnet or bar?

[Hint: A magnet when suspended freely will rest only in north-south direction, but soft iron bar will rest in any direction]

Answer

If we suspend the given bars with a silk thread such that it is free to rotate in a horizontal plane, then we will observe that one bar always sets itself in geographical north south direction as shown in the picture given below, while the other bar sets itself in any direction.

This happens because a magnet when suspended freely will rest only in north-south direction, but soft iron bar will rest in any direction. Hence, by suspending the two given bars freely and observing the position in which they rest we can distinguish between a magnet and a soft iron bar.

You are provided with two similar bars, one is a magnet and the other is a soft iron bar. How will you distinguish between them without the use of any other magnet or bar? Magnetism, Concise Physics Solutions ICSE Class 9.

Question 6

Fill in the blanks to complete the sentences:

(a) Two ends of a magnet are called ___________.

(b) Unlike poles of a magnet _____________ each other.

(c) Like poles of a magnet _____________ each other.

(d) A freely suspended magnet rests in the geographic ___________ direction.

Answer

(a) Two ends of a magnet are called poles.

(b) Unlike poles of a magnet attract each other.

(c) Like poles of a magnet repel each other.

(d) A freely suspended magnet rests in the geographic north-south direction.

Question 7

A small magnet is suspended by a silk thread from a rigid support such that the magnet can freely swing. How will it rest? Draw a diagram to show it.

Answer

When a magnet is suspended with a silk thread such that it is free to rotate in a horizontal plane, it will rest in the geographical north-south direction with north pole towards the geographic north, making some angle with the horizontal.

A small magnet is suspended by a silk thread from a rigid support such that the magnet can freely swing. How will it rest? Draw a diagram to show it. Magnetism, Concise Physics Solutions ICSE Class 9.

Question 8

Explain the term induced magnetism.

Answer

The temporary magnetism acquired by a magnetic material when it is placed near (or in contact with) a magnet, is called induced magnetism.

When an unmagnetised bar AB of a magnetic material such as soft iron or steel, is placed near (or in contact with) a magnet as shown below, the bar AB becomes a magnet i.e., it acquires the property of attracting iron fillings when they are brought near it's ends. If the magnet is now removed, it is seen that nearly all the iron fillings which have clung to it, fall down i.e., the bar AB looses it's magnetism as shown below.

Explain the term induced magnetism. Magnetism, Concise Physics Solutions ICSE Class 9.

Thus, the bar of a magnetic material behaves like a magnet so long it is kept near or in contact with a magnet.

The magnetism so produced is called induced magnetism.

Question 9

Explain what do you understand by magnetic induction. What role does it play in attraction of a piece of iron by a magnet?

Answer

The process in which a piece of magnetic material acquires the magnetic properties temporarily in presence of another magnet near it is called magnetic induction.

Induction precedes attraction — When a piece of iron is brought near one end of a magnet (or one end of a magnet is brought near the piece of iron), the nearer end of the piece acquires an opposite polarity by magnetic induction.

Since unlike poles attract each other, therefore the iron piece is attracted towards the end of the magnet. Thus, the piece of iron first becomes a magnet by induction and then it is attracted. In other words, induction precedes attraction.

Question 10

Explain the mechanism of attraction of iron nails by a magnet when brought near them.

Answer

If one pole of a bar magnet is brought near small iron nails, they form a chain of nails as shown below, because the bar magnet by induction magnetizes an iron nail which gets attracted to the magnet and clings to it. This magnetized nail magnetizes the other nail near it by magnetic induction and attracts it.

This process continues till force of attraction of magnet on first nail is sufficient to balance the total weight of all the nails in the chain below it.

Explain the mechanism of attraction of iron nails by a magnet when brought near them. Magnetism, Concise Physics Solutions ICSE Class 9.

Now holding the uppermost nail in position by fingers, and if the magnet is removed, we find that all nails fall down. The reason is that on removing the magnet, the uppermost nail loses it's magnetism, so all other nails also lose their magnetism, they get separated from each other and they all fall down due to the force of gravity.

This shows that the magnetism acquired by induction is purely temporary. It lasts so long as the magnet causing induction remains in it's vicinity.

Question 11(a)

Explain the following:

When two pins are hung by their heads from the same pole of a magnet, their pointed ends move apart.

Answer

When two pins are hung by their heads from the same pole of a magnet, their pointed ends move apart because the iron nails by magnetic induction gets magnetized and develop same poles. As we know that like poles repel each other hence, the two pointed ends of the nails repel each other.

Question 11(b)

Explain the following:

Several soft iron pins can cling, one below the other, from the pole of a magnet.

Answer

Several soft iron pins can cling, one below the other, from the pole of a magnet because the bar magnet by induction magnetizes an iron nail which gets attracted to the magnet and clings to it. This magnetized nail magnetizes the other nail near it by magnetic induction and attracts it.

This process continues till force of attraction of magnet on first nail is sufficient to balance the total weight of all the nails in the chain below it.

Now holding the uppermost nail in position by fingers, and if the magnet is removed, we find that all nails fall down. The reason is that on removing the magnet, the uppermost nail loses it's magnetism, so all other nails also lose their magnetism, they get separated from each other and they all fall down due to the force of gravity.

This shows that the magnetism acquired by induction is purely temporary. It lasts so long as the magnet causing induction remains in it's vicinity.

Question 11(c)

Explain the following:

The north end of a freely suspended magnetic needle gets attracted towards a piece of soft iron placed a little distance away from the needle.

Answer

When the north end of a freely suspended magnetic needle is placed near a piece of soft iron, the nearer end of the piece acquires an opposite polarity by magnetic induction.

Since unlike poles attract each other, therefore the iron piece is attracted towards the end of the magnet. Thus, the piece of iron first becomes a magnet by induction and then it is attracted.

Question 12

A small iron bar is kept near the north pole of a bar magnet. How does the iron bar acquire magnetism? Draw a diagram to show the polarity on the iron bar. What will happen if the magnet is removed?

Answer

When a small iron bar is kept near the north pole of a bar magnet as shown below, the bar becomes a magnet due to magnetic induction i.e., it acquires the property of attracting iron fillings when they are brought near it's ends.

A small iron bar is kept near the north pole of a bar magnet. How does the iron bar acquire magnetism? Draw a diagram to show the polarity on the iron bar. What will happen if the magnet is removed? Magnetism, Concise Physics Solutions ICSE Class 9.

Polarity developed at the end A of the bar AB is north (opposite to the polarity of the magnet near end A) and the polarity at end B is south (i.e. similar to the polarity at the end of the magnet near end A).

If the magnet is now removed, the bar will lose it's magnetism.

Thus, the bar of a magnetic material behaves like a magnet so long it is kept near or in contact with a magnet.

The magnetism so produced is called induced magnetism.

Question 13

'Induced magnetism is temporary'. Comment on this statement.

Answer

If one pole of a bar magnet is brought near small iron nails, they form a chain of nails as shown in the fig below. The reason is that the bar magnet by induction magnetizes an iron nail which gets attracted to the magnet and clings to it. This magnetized nail, magnetizes the other nail near it by magnetic induction and attracts it. This process continues till force of attraction of magnet on first nail is sufficient to balance the total weight of all the nails in the chain below it.

Now holding the uppermost nail in position by fingers, and if the magnet is removed, we find that all nails fall down. The reason is that on removing the magnet, the uppermost nail loses it's magnetism, they get separated from each other and they all fall down due to the force of gravity. This shows that the magnetism acquired by induction is purely temporary. It lasts so long as the magnet causing induction remains in it's vicinity.

Question 14

'Induction precedes attraction'. Explain the statement.

Answer

When a piece of iron is brought near one end of a magnet (or one end of a magnet is brought near the piece of iron), the nearer end of the piece acquires an opposite polarity by magnetic induction.

Since unlike poles attract each other, therefore the iron piece is attracted towards the end of the magnet. Thus, the piece of iron first becomes a magnet by induction and then it is attracted. In other words, induction precedes attraction.

Question 15

What do you understand by the term magnetic field lines?

Answer

A magnetic field line is a continuous curve in magnetic field such that tangent at any point of the curve gives the direction of the magnetic field at that point.

Question 16

State four properties of magnetic field lines.

Answer

The magnetic field lines have the following properties —

  1. They are closed and continuous curves.
  2. Outside the magnet, they are directed from the north pole towards the south pole of the magnet.
  3. The tangent at any point on a field line gives the direction of magnetic field at that point.
  4. They are crowded near the poles of the magnet where the magnetic field is strong and far away near the middle of the magnet and far from the magnet where the magnetic field is weak.

Question 17

Explain why iron fillings which are sprinkled on a sheet of cardboard placed over a bar magnet, take up a definite pattern when cardboard is slightly tapped.

Answer

The iron fillings which are sprinkled on a sheet of cardboard placed over a bar magnet, take up a definite pattern when cardboard is slightly tapped because each piece of iron fillings gets magnetized by magnetic induction and experiences a force due to the magnet. Therefore, they arrange themselves along curved lines and these curved lines are called magnetic field lines.

Question 18

Explain the method of plotting magnetic field lines by using a small compass needle.

Answer

Earth's magnetic field is uniform in a limited space. Experimentally we can plot uniform magnetic field lines of the earth as follows —

Experiment — Fix a sheet of paper on a drawing board (or a table top) by means of brass pins. Place a small compass needle at position 1 (as shown in fig.) and looking from top of the needle, mark two pencil dots exactly in front of the two ends of the needle. Then move the compass needle to position 2 in such a way that one end of the needle coincides with the second pencil dot.

Mark the position of the other end of the needle with a dot. Repeat the process of moving the compass needle to position 3,4,.. to obtain several dots. On joining the different dots, you will get a straight line. Thus, one magnetic field line of the earth is traced.

Explain the method of plotting magnetic field lines by using a small compass needle. Magnetism, Concise Physics Solutions ICSE Class 9.

Repeat the process starting from a different point and trace out another magnetic field line. In this manner, draw several magnetic field lines starting from different points. Label each line with an arrow from the south pole of needle towards the north pole to indicate the direction of the magnetic field. Fig. above shows several magnetic field lines so obtained.

It is noticed that these lines do not intersect each other. They are parallel and equidistant. They are directed from geographic south to geographic north (i.e., the direction in which a magnetic needle, suspended freely in a horizontal plane rests).

Question 19

Can two magnetic field lines intersect each other? Give reason to your answer.

Answer

Two magnetic field lines never intersect each other.

If they would intersect, this would mean that there are two directions of the field at the specific point which is not possible.

Question 20

In figure below, draw at least two magnetic field lines between the two magnets.

In the figure, draw at least two magnetic field lines between the two magnets. Magnetism, Concise Physics Solutions ICSE Class 9.

Answer

(a) As the north pole of both the magnets are facing each other so they will repel. The magnetic field lines between the two magnets are shown below:

In the figure, draw at least two magnetic field lines between the two magnets. Magnetism, Concise Physics Solutions ICSE Class 9.

(b) As opposite poles of both the magnets are facing each other so they will attract. The magnetic field lines between the two magnets are shown below:

In the figure, draw at least two magnetic field lines between the two magnets. Magnetism, Concise Physics Solutions ICSE Class 9.

Question 21

State two evidences of the existence of earth's magnetic field.

Answer

The evidences of the existence of earth's magnetic field are —

  1. A freely suspended magnetic needle always rests in geographic north-south direction.
  2. An iron rod buried inside the earth along north-south direction becomes a magnet.

Question 22

Sketch four magnetic field lines as obtained in a limited space on a horizontal plane in the earth's magnetic field alone.

Answer

Magnetic field lines as obtained in a limited space on a horizontal plane in the earth's magnetic field alone are shown below:

Sketch four magnetic field lines as obtained in a limited space on a horizontal plane in the earth's magnetic field alone. Magnetism, Concise Physics Solutions ICSE Class 9.

Question 23

(a) Draw the pattern of magnetic field lines near a bar magnet placed with it's north pole pointing towards the geographic north. Indicate the position of neutral points by marking x.

(b) State whether the magnetic field lines in part (a) represent a uniform magnetic field or non-uniform magnetic field?

Answer

(a) Below diagram shows the magnetic field lines near a bar magnet placed with it's north pole pointing towards the geographic north. The position of neutral points is marked with x:

Draw the pattern of magnetic field lines near a bar magnet placed with it's north pole pointing towards the geographic north. Indicate the position of neutral points by marking x. State whether the magnetic field lines in part a represent a uniform magnetic field or non-uniform magnetic field?  Magnetism, Concise Physics Solutions ICSE Class 9.

(b) The magnetic field lines as shown in part (a) are non-uniform.

Question 24

Figure given below shows a bar magnet placed on the table top with it's north pole pointing towards south. The arrow shows the north-south direction. There are no other magnets or magnetic material nearby.

Figure given below shows a bar magnet placed on the table top with its north pole pointing towards south. The arrow shows the north-south direction. There are no other magnets or magnetic material nearby. Insert two magnetic field lines on either side of the magnet using arrow head to show the direction of each field line. Indicate by crosses, the likely positions of the neutral points. What is the magnitude of the magnetic field at each neutral point? Give a reason for your answer. Magnetism, Concise Physics Solutions ICSE Class 9.

(a) Insert two magnetic field lines on either side of the magnet using arrow head to show the direction of each field line.

(b) Indicate by crosses, the likely positions of the neutral points.

(c) What is the magnitude of the magnetic field at each neutral point? Give a reason for your answer.

Answer

(a) The magnetic field lines for the magnet are as shown below:

Figure given below shows a bar magnet placed on the table top with its north pole pointing towards south. The arrow shows the north-south direction. There are no other magnets or magnetic material nearby. Insert two magnetic field lines on either side of the magnet using arrow head to show the direction of each field line. Indicate by crosses, the likely positions of the neutral points. What is the magnitude of the magnetic field at each neutral point? Give a reason for your answer. Magnetism, Concise Physics Solutions ICSE Class 9.

(b) The likely position of the neutral points is indicated by crosses in the diagram below:

Figure given below shows a bar magnet placed on the table top with its north pole pointing towards south. The arrow shows the north-south direction. There are no other magnets or magnetic material nearby. Insert two magnetic field lines on either side of the magnet using arrow head to show the direction of each field line. Indicate by crosses, the likely positions of the neutral points. What is the magnitude of the magnetic field at each neutral point? Give a reason for your answer. Magnetism, Concise Physics Solutions ICSE Class 9.

(c) The magnitude of magnetic field at each neutral point is zero because at these points the magnetic field of the magnet and the magnitude of the earth's horizontal magnetic field are equal in magnitude but in opposite directions such that the two fields neutralize each other.

Question 25

What conclusion is drawn regarding the magnetic field at a point if a compass needle at that point rests in any direction? Give reason for your answer.

Answer

If a compass needle at a point rests in any direction then we can say that the magnitude of the magnetic field at that particular point is zero.

The reason for this is that the earth's magnetic field at that point is neutralized by the magnetic field of some other magnetized material.

Question 26

What is a neutral point? How is the position of a neutral point located with the use of a compass needle?

Answer

Neutral points are the points at which two magnetic fields are equal in magnitude, but opposite in direction. The net magnetic field at a neutral point is zero.

A compass needle if placed at the neutral point, will rest in any direction. Hence, we can find the position of neutral points with the help of compass needle.

Question 27

State the positions of neutral points when a magnet is placed with it's axis in the magnetic meridian and with it's north pole (i) pointing towards the geographic north, (ii) pointing towards the geographic south.

Answer

(i) When a magnet is placed with it's axis in the magnetic meridian and with it's north pole pointing towards the geographic north then the neutral point will be in east-west direction.

(ii) When a magnet is placed with it's axis in the magnetic meridian and with it's north pole pointing towards the geographic south then the neutral point will be in north-south direction.

Question 28

Complete the following sentences:

(a) If the field lines in a magnetic field are parallel and equidistant, the magnetic field is __________ .

(b) At a neutral point, the resultant magnetic field is _____________ .

(c) The neutral points of a bar magnet kept with it's north pole pointing towards geographic north are located ___________ .

Answer

(a) If the field lines in a magnetic field are parallel and equidistant, the magnetic field is uniform.

(b) At a neutral point, the resultant magnetic field is zero.

(c) The neutral points of a bar magnet kept with it's north pole pointing towards geographic north are located on either side of the magnet in east and west directions.

Multiple Choice Type

Question 1

Two like magnetic poles:

  1. repel each other ✓
  2. attract each other
  3. first attract each other, then repel
  4. neither attract nor repel.

Answer

It is the property of magnets that like poles repel each other while unlike poles attract each other.

Question 2

In a uniform magnetic field, the field lines are:

  1. curved
  2. parallel and equidistant straight lines ✓
  3. parallel, but non-equispaced straight lines
  4. nothing can be said.

Answer

Parallel and equidistant field lines represent a uniform magnetic field. The earth's magnetic field in a limited space is uniform.

Exercise 10(B)

Question 1

What is an electromagnet?

Answer

An electromagnet is a temporary strong magnet made from a piece of soft iron when current flows in the coil wound around it. It is an artificial magnet.

Question 2

Name the material used for preparing an electromagnet.

Answer

The material used for preparing an electromagnet is a piece of soft iron.

Question 3

How is an electromagnet made? Name two factors on which the strength of the magnetic field of an electromagnet depends.

Answer

An electromagnet can be made in the following two shapes:

  1. I-shape or bar magnet.
  2. U-shape or horse-shoe magnet.

The principle behind making both the magnets is the same. An electromagnet can be made by winding an insulated copper wire over a piece of soft iron in U-shape or a solenoid.

The factors on which the strength of the magnetic field of an electromagnet depends are —

  1. The number of turns of wire wound around the coil — By increasing the number of turns of wire wound around the coil, the magnetic field of an electromagnet can be increased.
  2. The amount of current flowing through the wire — By increasing the current through the wire, the magnetic field of an electromagnet can be increased.

Question 4

You are required to make an electromagnet from a soft iron bar by using a cell, an insulated coil of copper wire and a switch. (a) Draw a circuit diagram to represent the process. (b) Label the poles of the electromagnet.

Answer

The labelled circuit diagram for making an electromagnet from a soft iron bar is shown below:

You are required to make an electromagnet from a soft iron bar by using a cell, an insulated coil of copper wire and a switch. Draw a circuit diagram to represent the process. Label the poles of the electromagnet. Magnetism, Concise Physics Solutions ICSE Class 9.

Question 5

Figure shows a coil wound around a soft iron bar XY. (a) State the polarity at the end X and Y as the switch is pressed. (b) Suggest one way of increasing the strength of the electromagnet so formed.

Figure shows a coil wound around a soft iron bar XY. State the polarity at the end X and Y as the switch is pressed. Suggest one way of increasing the strength of the electromagnet so formed. Magnetism, Concise Physics Solutions ICSE Class 9.

Answer

(a) The polarity at X — south pole and at Y — north pole.

(b) The strength of the electromagnet so formed can be increased by increasing the current in the coil.

Question 6

A coil of insulated copper wire is wound around a piece of soft iron and current is passed in the coil from a battery. What name is given to the device so obtained? Give one use of the device mentioned by you.

Answer

The device obtained when a coil of insulated copper wire is wound around a piece of soft iron and current is passed in the coil from a battery, is an electromagnet.

Electromagnets are mainly used for lifting and transporting heavy iron scrap, girders, plates, etc. particularly when it is not convenient to take the help of human labour. Electromagnets are used to lift as much as 20,000 kg of iron in a single lift. To upload iron objects at the desired place, the current in the electromagnet is switched off so that the iron objects gets detached.

Question 7

Show with the aid of a diagram how a wire is wound on a U-shaped piece of soft iron in order to make it an electromagnet. Complete the circuit diagram and label the poles of the electromagnet.

Answer

Below labelled circuit diagram shows an electromagnet made by winding a wire on a U-shaped piece of soft iron:

Show with the aid of a diagram how a wire is wound on a U-shaped piece of soft iron in order to make it an electromagnet. Complete the circuit diagram and label the poles of the electromagnet. Magnetism, Concise Physics Solutions ICSE Class 9.

Question 8

State two ways through which the strength of an electromagnet can be increased.

Answer

The strength of a magnet can be increased by —

  1. Increasing the number of turns of windings in the solenoid.
  2. Increasing the current through the solenoid.

Question 9

Name one device that uses an electromagnet.

Answer

An electric bell uses an electromagnet to function.

Question 10

State two advantages of an electromagnet over a permanent magnet.

Answer

Advantages of an electromagnet over a permanent magnet are —

  1. An electromagnet can produce a strong magnetic field.
  2. The strength of the magnetic field of an electromagnet can easily be changed by changing the current (or the number of turns) in it's solenoid.

Question 11

State two differences between an electromagnet and a permanent magnet.

Answer

The differences are as follows —

ElectromagnetPermanent magnet
It is made of soft iron.It is made of steel.
It produces magnetic field so long as current flows in it's coil, i.e., it produces temporary magnetic field.It produces a permanent magnetic field.

Question 12

Why is soft iron used as the core of the electromagnet in an electric bell?

Answer

The soft iron bar acquires the magnetic properties only when an electric current flows through the solenoid and loses the magnetic properties as soon as the current is switched off. That's why soft iron is used as the core of the electromagnet in an electric bell.

Question 13

How is the working of an electric bell affected, if alternating current is used instead of direct current?

Answer

If alternating current is used instead of direct current then the core of electromagnet will get magnetized, but the polarity at it's ends will change. Since attraction of armature does not depend on the polarity of electromagnet, so the bell will still ring on pressing the switch.

Question 14

Name the material used for making the armature of an electric bell. Give a reason for your answer.

Answer

Soft iron is used for making the armature of an electric bell because it acquires magnetic properties only when an electric current flows through the solenoid and loses the magnetic properties as soon as the current is switched off.

Multiple Choice Type

Question 1

Electromagnets are made up of:

  1. steel
  2. copper
  3. soft iron ✓
  4. aluminum

Answer

Electromagnets are made from soft iron.

An electromagnet is a temporary strong magnet made from a piece of soft iron when current flows in the coil wound around it.

Question 2

The strength of an electromagnet can be increased by:

  1. reversing the directions of current
  2. using alternating current of high frequency
  3. increasing the current in the coil ✓
  4. decreasing the number of turns of coil

Answer

The strength of electromagnet can be increased by increasing the current in the coil.

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