# Samacheer Kalvi 9th Science Solutions Chapter 5 Magnetism and Electromagnetism

## Tamilnadu Samacheer Kalvi 9th Science Solutions Chapter 5 Magnetism and Electromagnetism

### Samacheer Kalvi 9th Science Magnetism and Electromagnetism Textbook Exercises

Question 1.
Which of the following converts electrical energy into mechanical energy?
(a) motor
(b) battery
(c) generator
(d) switch
(a) motor

Question 2.
The part of the AC generator that passes the current from the armature coil to the external circuit is
(a) field magnet
(b) split rings
(c) slip rings
(d) brushes
(d) brushes

Question 3.
Transformer works on
(a) AC only
(b) DC only
(c) both AC and DC
(d) AC nor effectively than DC
(a) AC only

Question 4.
The unit of magnetic flux density is
(a) Weber
(b) weber/meter
(c) weber/meter2
(d) weber.meter2
weber/meter2

II. Fill in the blanks.

1. The SI Unit of magnetic field induction is ……………………..
2. Devices which is used to convert high alternating current to low alternating current is …………………..
3. An electric motor converts …………………..
4. A device for producing electric current is ……………………..

1. Tesla
2. Step down transformer
3. Electrical energy into mechanical energy
4. Generator

III. Match the following.

 Column – I Column – II 1. Magnetic material (a) Oersted 2. Non-magnetic material (b) iron 3. Current and magnetism (c) induction 4. Electromagnetic induction (d) wood 5. Electric generator (e) Faraday

1. (b) iron
2. (d) wood
3. (a) Oersted
5. (c) induction

IV. True or False.

1. A generator converts mechanical energy into electrical energy – True

2. Magnetic field lines always repel each other and do not intersect – True

3. Fleming’s Left-hand rule is also known as Dynamo rule – True

4. The speed of rotation of an electric motor can be increased by decreasing the area of the coil – False
Correct Statement: The speed of rotation of the coil can be increased by increasing the area of the coil.

5. A transformer can step up direct current – False
Correct Statement: A transformer can step up the alternating current.

6. In a step-down transformer, the number of turns in the primary coil is greater than that of the number of turns in the secondary coil – True

Question 1.
State Fleming’s Left Hand Rule.
The law states that while stretching the three fingers of the left hand in a perpendicular manner with each other if the direction of the current is denoted by the middle finger of the left hand and the second finger is for the direction of the magnetic field then the thumb of the left hand denotes the direction of the force or movement of the conductor.

Question 2.
Define magnetic flux density.
The number of magnetic field lines crossing unit area kept normal to the direction of field lines is called magnetic flux density. Its unit is Wb/m2

Question 3.
List the main parts of an electric motor.
Main parts of an electric motor

• Field magnet.
• Armature (Rectangular coil)
• Split ring (Commutator)
• Brushes
• Battery

Question 4.
Draw and label the diagram of an AC generator.

Question 5.
State the advantages of AC over DC.

• The cost of generation of AC is less than the cost of generation of DC.
• AC can be easily converted into D.C.
• Only alternating voltage can be stepped up or stepped down by using a transformer.
• AC can be transmitted to distant places without much loss of electric power than DC.

Question 6.
Differentiate step up and step down transformer.

 Step-up transformer Step down transformer The transformer used to change a low alternative voltage to a high alternating voltage is called a step-up transformer, i.e. (Vs>Vp). The transformer used to change a high alternating voltage to a low alternating voltage is called a step-down transformer (Vs Np) In a step-down transformer, the number of turns in the secondary coils are less than the number of turns in the primary coil (Ns < Np)

Question 7.
A portable radio has a built-in transformer so that it can work from the mains instead of batteries. Is this a step up or step down transformer?
It is a step-down transformer. So that rectified DC voltage is equal to battery voltage, hence it can work on mains as well as on battery.

Question 8.
State Faraday’s laws of electromagnetic induction.
Whenever there is a change in the magnetic flux linked with a closed-circuit an emf is produced and the amount of emf induced varies directly as the rate at which the flux changes. This emf is known as induced emf and the phenomenon of producing an induced emf due to a change in the magnetic flux linked with a closed circuit is known as electromagnetic induction.

Question 1.
Explain the principle, construction, and working of a DC motor.
A motor is an electrical machine which converts electrical energy into mechanical energy. The principle of working of a DC motor according to Faraday’s laws of electromagnetic induction is that “whenever a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force”. The various parts of a DC motor are; Permanent magnets on both sides of a coil which consists of carbon brush and commutator as shown in

Working of electric motor is primarily dependent upon the interaction between magnetic field and current. The direction of this force is given by Fleming’s left hand rule and it’s magnitude is given by F = BIL. Where, B = magnetic flux density, I = current and L = length of the conductor within the magnetic field.

Question 2.
Explain two types of transformer.
Transformer is a device used for converting low voltage into high voltage and high voltage into low voltage. It works on the principle of electromagnetic induction. It consists of primary and secondary coil insulated from each other. Depending upon the number of turns in the primary and secondary coils, the two types of transformers are; step-up or step-down transformers.

Step-up transformer:
The transformer used to change a low alternative voltage to a high alternating voltage is called a step-up transformer, ie (Vs > Vp). In a step-up transformer, the number of turns in the secondary coil is more than the number of turns in the primary coil (Ns > Np).

Step down transformer:
The transformer used to change a high alternating voltage to a low alternating voltage is called a step-down transformer (Vs < Vp). In a step-down transformer, the number of turns in the secondary coils is less than the number of turns in the primary coil (Ns < Np).
The formulae pertaining to the transformers are given in the following equations.

• The number of primary turns Np / The number of secondary turns Ns = The primary voltage Vp/ The secondary voltage Vs
• The number of secondary turns Ns / The number of primary turns Np – The primary current Ip/ The secondary current Is

Question 3.
Draw a neat diagram of an AC generator and explain its working.
An alternating current (AC) generator, consists of a rotating rectangular coil ABCD called armature placed between the two poles of a permanent magnet. The two ends of this coil are connected to the two slip rings S1 and S2. The inner sides of these rings are insulated. Two conducting stationary brushes B1 and B2 are kept separately on the rings S1 and S2 respectively. The two rings S1 and S2 are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field. Outer ends of the two brushes are connected to the external circuit.

When the coil is rotated, the magnetic flux linked with the coil changes. This change in magnetic flux will lead to the generation of induced current. The direction of the induced current, as given by Fleming’s Right Fland Rule, is along ABCD in the coil, and in the outer circuit, it flows from B2 to B1. During the second half of rotation, the direction of current is along DCBA in the coil, and in the outer circuit it flows from B1 to B7. As the rotation of the coil continues, the induced current in the external circuit is changing its direction for every half a rotation of the coil.

ACTIVITY

Question 1.
Put a magnet on a table and place some paper clips nearby. If you push the magnet slowly towards the paper clips, there will be a point at which the paper clips jump across and stick to the magnet. What do you understand from this?

The invisible magnetic field that surrounds the magnet acts at a particular distance. This magnetic field attracts the paper clip which is made of steel.

### Samacheer Kalvi 9th Science Magnetism and Electromagnetism In-Text Problems

Question 1.
A conductor of length 50 cm carrying a current of 5 A is placed perpendicular to a magnetic field of induction 2 × 10– 3 T. Find the force on the conductor.
Solution:
Force on the conductor = ILB
= 5 × 50 × 10– 2 × 2 × 10– 3
= – 5 × 10– 3 N

Question 2.
A current-carrying conductor of a certain length, kept perpendicular to the magnetic field experiences a force F. What will be the force if the current is increased four times, the length is halved and the magnetic field is tripled?
Solution:
F = ILB = (4I) × ( L / 2) × (3 B) = 6 F
Therefore, the force increases six times.

Question 3.
The primary coil of a transformer has 800 turns and the secondary coil has 8 turns. It is connected to a 220 V ac supply. What will be the output voltage?
Solution:
In a transformer, Es / Ep = Ns / Np
Es = Ns / Np × Ep
= $$\frac { 8 }{ 800 }$$ × 220 = $$\frac { 220 }{ 100 }$$ = 2.2 volt

### Samacheer Kalvi 9th Science Magnetism and Electromagnetism Additional Questions

Question 1.
What are natural magnets?
Natural magnets exist in nature and can be found in rocks and sandy deposits in various parts of the world.

Question 2.
How can the speed of rotation of a coil be increased? Write at least three methods.
The speed of rotation of a coil can be increased by;

• increasing the strength of the Current in the coil
• increasing the area of the coil
• increasing the strength of the magnetic field.

Question 3.
What is the connection between electricity and magnetism?
Electricity and magnetism are closely related to each other. The current flowing through the wire produces a circular magnetic field outside the wire. The direction of this magnetic field depends on the electric current.

Similarly, a changing magnetic field produces an electric current in a wire or conductor. The relationship between electricity arid magnetism is called electromagnetism.

Question 4.
What are the factors that determine the strength of the magnet?
The strength of the magnetic field at a point due to current-carrying wire depends on:

• the current in the wire
• distance of the point from the wire
• the orientation of the point from the wire and
• the magnetic nature of the medium.

Question 5.
Name some equipment that uses electromagnetism for functioning.
Many of the medical equipment such as scanners, x-ray equipment, and other equipment also use the principle of electromagnetism for their functioning.

Question 6.
Explain why the ozone layer is not affected by the solar wind.
Magnetic field can penetrate through all kinds of materials. The Earth produces its own magnetic field, which shields the earht’s ozone layer from the solar wind.

Question 7.
Write the properties of magnetic lines of force.

• Magnetic lines of force are closed continuous curves, extending through the body of the magnet.
• Magnetic lines of force start from the North Pole and end at the South Pole.
• Magnetic lines of force never intersect.
• They will be maximum at the poles than at the equator.
• The tangent drawn at any point on the curved line gives the direction of the magnetic field.

Question 1.
Michael Faraday discovered that a current-carrying conductor also gets deflected when it is placed in a magnetic field. Michael Faraday was a British Scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic induction, diamagnetism, and electrolysis. Although Faraday received little formal education, he was one of the most influential scientists in history. Faraday was an excellent experimentalist who conveyed his ideas in clear and simple language.

The SI unit of capacitance is named in his honour: the farad. Albert Einstein kept a picture of Faraday on his study wall, alongside pictures of Isaac Newton and James Clerk Maxwell. Faraday is one of the greatest scientific discoverers of all time.

Question 2.
Explain in detail the application of electromagnets.