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Class 9 SELINA Solutions Physics Chapter 10 - Magnetism

Magnetism Exercise Ex 10(A)

Solution A.1

(b) lodestone

The first known magnets were pieces of lodestone.

Solution A.2

(c) geographic north-south direction

Solution A.3

(a) temporary, induced

The temporary magnetism acquired by a magnetic material when it is kept near a magnet is called induced magnetism.

Solution A.4

(a) Repel each other 

Solution A.5

(c) opposite, similar

A magnetic pole induces opposite polarity on the near end and a similar polarity on the farther end of the iron bar.

Solution A.6

(c) both (i) and (ii)

Solution A.7

(c) north-south

Solution A.8

(d) neutral points

Solution A.9

(b) Parallel equidistant straight lines 

Solution A.10

(a) either converging or diverging

Solution A.11

(b) normal, parallel

Solution A.12

(a) a tangent at that point

The direction of a magnetic field at a point on a magnetic field line is given by a tangent at that point.

Solution A.13

(d) in any direction

Solution A.14

(a)

  

Solution A.15

(c) nearly vertical with the south pole in a downward direction

Solution A.16

(c) The position of neutral points does not depend on the direction of the magnet

Solution B.1

(a) Poles, (b) Attract (c) repel (d) north-south 

Solution B.2

(a) Uniform, (b) Zero  and (c) On either side of the magnet in east and west. 

Solution B.3

The magnetic south pole of the earth is in Canada at a distance of nearly 2240 km from the geographic north pole.

Solution B.4

Uniform magnetic field is represented by parallel and equidistant lines.

Solution B.5

The angle between the magnetic axis of the earth and the axis of rotation of the earth is 17°.

Solution C.1

Lodestone is an ore of iron oxide (Fe3O4). This ore attracts small pieces of iron and it sets itself along a definite direction when it is suspended freely. It is a natural magnet which was used for the navigation by the mariners.

Solution C.2

The pieces of lodestone found in nature are called the natural magnets. Limitations of a natural magnet are as listed below:

(i) They are irregular and odd shaped.

(ii) They are not magnetically very strong.

Solution C.3

An artificial magnet is a magnetized piece of iron (or other magnetic material). Artificial magnets are required because natural magnets have odd and irregular shape and they are not magnetically very strong. Artificial magnets can be given desired shape and made very strong.

Solution C.4

Iron rod is magnetized when placed near a bar magnet by magnetic induction, while copper rod is not magnetized.

 

Solution C.5

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

Solution C.6

(a) When two pins are hung by their heads from the same pole of a magnet, they acquire same polarity. Because like poles repel each other, their pointed ends move apart.

(b) Several soft iron pins can cling one below the other from the pole of a magnet because the magnet induces magnetism in an iron nail which gets attracted by the magnet and clings to it. This magnetized nail magnetizes the other nail near it by magnetic induction and attracts it. This process continues until force of attraction on first nail is sufficient to balance the total weight of all nails in chain.

(c) When a piece of soft iron is placed a little distance away from the needle, the needle induces magnetism to the piece of soft iron. Thus, soft iron piece starts behaving like a magnet and it attracts the magnetic needle towards it.

 

Solution C.7

Induced magnetism is temporary as it lasts as long as the magnet causing induction remains in it vicinity.

Solution C.8

When a piece of magnetic material is brought near a magnet, it first becomes a magnet by induction and then it is attraction. Thus, we say that induction precedes attraction.

Solution C.9

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

Solution C.10

Properties of magnetic field lines:

1. They are closed and continuous curves.

2. They are directed from the North Pole towards the South Pole outside the magnet.

3. The tangent at any point on a field line gives the direction of magnetic field at that point.

4. Two magnetic lines never intersect each other.

 

Solution C.11

The iron filings take up a definite pattern (curved lines). This happens because each piece of iron filing becomes a magnet to the magnetic induction of the magnet. It thus experiences a force in the direction of magnetic field of the bar magnet at that point and aligns itself along curved lines.

Solution C.12

No two magnetic field lines can intersect each other. If they do, there would be two directions of the field at that point which is not possible.

Solution C.13

In (a) The North Pole of two magnets is facing each other. So, the field lines will be

 

 

In (b) The North Pole of one magnet is facing the South Pole of the other. So, the field lines will be

 

Solution C.14

Two evidences of existence of earth's magnetic field:

(i) A freely suspended magnetic needle always rests in geographic north-south direction.

(ii) Neutral points are obtained on plotting the field lines of a magnet.

Solution C.15

Solution C.16

It can be concluded that magnetic field at that point is zero. This is because the earth's magnetic field at that point is neutralized by the magnetic field of some other magnetized material.

Solution C.17

Neutral points are the points where the magnetic field of the magnet is equal in magnitude to the earth's horizontal magnetic field, but it is in opposite direction. Thus the net magnetic field at the neutral points is zero.

Since the net magnetic field is zero at neutral points, the compass needle remains unaffected (i.e. it comes to rest pointing in any direction) at these points and hence, they can be detected.

Solution C.18

(i) Neutral points will be in east-west direction.

(ii) Neutral points will be north-south direction.

Solution D.1

a.

 

 

b. The magnetic field lines are non-uniform in nature.

 

Solution D.2

(a)

 

 (b)

(c) Magnitude of magnetic field at neutral points is zero. It is so because at these points, the magnetic field of the magnet is equal in magnitude to the earth's horizontal magnetic field, but it is in opposite direction. Hence, they cancel each other.

Solution D.3

A magnet when suspended freely will rest only in north-south direction, but the soft iron bar will rest in any direction.

Solution D.4

If a small magnet is suspended by a silk thread such that it can swing freely then it rests itself in the geographic north-south direction.

 

 

Solution D.5

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

When a piece of iron is placed near or in contact with a magnet, the piece of iron becomes a magnet i.e., it acquires the property of attracting iron filings when they are brought near its ends. Thus, a piece of iron behaves as a magnet as long as it is kept near (or in contact with) a magnet.

 

Solution D.6

When iron nails are brought near one end of a magnet, the nearer end of piece acquires an opposite polarity by magnetic induction. Since unlike poles attract each other, therefore, iron nails are attracted towards the end of the magnet. Thus, the iron nail first becomes a magnet by induction and then it is attracted.

Solution D.7

The iron bar acquires magnetism due to magnetic induction.

If the magnet is removed, the iron bar loses its magnetism.

Solution D.8

Method of plotting the magnetic field lines using a compass needle:

Fix a sheet of paper on a drawing board by means of board pins. Place a small compass needle at position 1 as shown in fig (a) and looking from the top of the needle, mark two pencil dots exactly at two ends of the needle. Then move the compass needle to position 2 in such a way that one end of needle coincides with the second pencil dot. Repeat the process of moving the compass needle to positions 3, 4,… to obtain several dots. On joining the different dots, you will get a straight line. Thus one line of magnetic field of earth is traced.

This process is repeated starting from a different point and tracing out another line of magnetic field. In this manner, several lines of magnetic field can be drawn. Each line should be labeled with an arrow from the south pole of the needle towards the north pole to indicate the direction of the magnetic field. Fig (b) shows several magnetic lines so obtained.

 

Magnetism Exercise Ex 10(B)

Solution A.1

(d) soft iron

An electromagnet is a temporary strong magnet made from soft iron by flowing current in the coil wound around it.

Solution A.2

(i) (a) south pole

(ii) (b) north pole

(iii) (b) current is switched on, current is switched off

Solution A.3

(d) increasing the number of turns, increasing the current.

Solution A.4

(c) (ii) and (iii)

An electromagnet can produce strong magnetic field and the polarity of electromagnet can be reversed by reversing the direction of current in the solenoid.

Solution A.5

(c)  increasing the current in the coil

The strength of an electromagnet can be increased by

 i. increasing the number of turns of coil, and

 ii. increasing the current through the coil

Hence, the correct answer is option c.

 

Solution A.6

(c) It becomes magnetised.

Solution A.7

(a) Magnetic compass

Solution A.8

(d) the direction of current at the ends of the magnet

Solution B.1

No. An electromagnet is a temporary strong magnet made from a piece of soft iron by flowing current in the coil wound around it.

Solution B.2

The material used for preparing an electromagnet is soft iron.

Solution B.3

The device formed is an electromagnet.

 

Use:

For separating the magnetic substances such as iron from other debris.

Solution B.4

The electromagnet is used in an electric relay.

Solution B.5

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

Solution C.1

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.

Solution C.2

The polarity at X is North and at Y is South.

 

By increasing the number of turns of winding in the solenoid, the strength of the electromagnet can be increased.

Solution C.3

The strength of an electromagnet can be increased by following ways:

 i. Increasing the number of turns of winding in the solenoid.

 ii. Increasing the current through the solenoid.

 

Solution C.4

  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 in its solenoid.

Solution C.5

If an a.c. source is used in place of a battery, the core of the electromagnet will get magnetized, but the polarity at its ends will change. Since attraction of armature does not depend on the polarity of the electromagnet, the bell will still ring on pressing the switch.

Solution C.6

The material used for making the armature of an electric bell is soft iron which can induce magnetism rapidly.

Solution D.1

An electromagnet is made by winding an insulated copper wire around a soft iron core either in the shape of a solenoid or U-shape.

 

The strength of magnetic field of an electromagnet depends on:

1. The number of turns of wire wound around the coil, and

2. The amount of current flowing through the wire.

Solution D.2

Solution D.3

 

Electromagnet 

Permanent magnet 

It is made up of soft iron 

It is made up of steel 

The magnetic field strength can be changed 

The magnetic field strength cannot be changed 

Electromagnets of very strong field can be made. 

Permanent magnets are not so strong. 

 

Solution D.4

Solution D.5

Construction:

The electric bell and its main parts are shown:

  • The armature A is connected to the spring strip SS, which makes contact with the adjusting screw S’ when the switch is not pressed.
  • The hammer H is attached to the upper end of the armature.
  • The coil CC is wound on the two arms of the electromagnet in opposite directions.
  • One end of the coil is connected to the terminal T1 through strip SS and screw S’, while the other end is connected to the terminal T2.
  • A battery is connected in series with the switch K across the terminals T1 and T2.

Working:

  • An electric bell operates based on magnetism. Pressing switch K allows current to flow through coil CC, magnetising the electromagnet's core. This attracts armature A and causes hammer H to strike gong G, creating a ringing sound.
  • When armature A moves towards the electromagnet, loss of connection between strip SS and screw S' stops the current flow, demagnetising the electromagnet, which causes armature A to return to its original position.
  • When armature A touches screw S' again, current flows through the circuit, causing the electromagnet to regain magnetism, and the process repeats as long as switch K is pressed, creating a continuous ringing sound.
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