Samacheer Kalvi 9th Science Solutions Chapter 5 Pdf
Tamilnadu Board Samacheer Kalvi 9th Science Solutions Chapter 5: Tamilnadu State Board Solution Class 9 Science Chapter 5 – MAGNETISM.
Samacheer Kalvi 9th Science Solutions Chapter 5: Overview
| Board | Samacheer Kalvi |
| Class | 9 |
| Subject | Science |
| Chapter | 5 |
| Chapter Name | MAGNETISM |
Samacheer Kalvi 9th Science Solutions Chapter 5 Pdf
Chapter 5
MAGNETISM AND ELECTROMAGNETISM
Exercise:
I. Choose the correct answer.
1) Which of the following convert’s electrical energy into mechanical energy?
a) Motor
b) Battery
c) Generator
d) Switch
Ans: a) Motor
Because, motor converts electrical energy into mechanical energy.
2) Transformer works on
a) AC only
b) DC only
c) Both AC and DC
Ans: a) AC only
Because, DC cannot produces varying magnetic field and only constant magnetic field is produced across primary coil and hence no voltage is produced across the secondary of a coil.
3) 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
Ans: d) Brushes
Because, the part of the AC generator that passes the current from the armature coil to the external circuit is brushes.
4) The unit of magnetic flux density is
a) Weber
b) weber/meter
c) weber/meter2
d) Weber. meter2
Ans: c) weber/meter2
II) Fill in the blanks.
1) The SI Unit of magnetic field induction is—–
Ans: The SI Unit of magnetic field induction is Tesla.
2) Devices which is used to convert high alternating current to low alternating current is—
Ans: Devices which is used to convert high alternating current to low alternating current is step down transformer.
3) An electric motor converts—
Ans:An electric motor convertselectrical energy into mechanical energy.
4) A device for producing electric current is—–.
Ans:A device for producing electric current is generator.
III. Match the following.
Ans:
1. Magnetic material: Iron
2. Non-magnetic material: wood
3. Current and magnetism: Oersted
4. Electromagnetic induction: Faraday
5. Electric generator: induction
IV. State whether true or false. If false, correct the statement.
1) A generator converts mechanical energy into electrical energy.
Ans: True
Because, generator can be used for converting mechanical energy into electrical energy.
2) Magnetic field lines always repel each other and do not intersect.
Ans: True
Because, magnetic field lines are not intersecting to each other at any point and repelling to each other.
3) Fleming’s Left hand rule is also known as Dynamo rule.
Ans: true
Because, the dynamo works on the basis of Fleming’s left hand rule.
4) The speed of rotation of an electric motor can be increased by decreasing the area of the coil.
Ans: False
Because, the speed of rotation of an electric motor can be increased by increasing the area of the coil.
5) A transformer can step up direct current.
Ans: False
Because, a transformer can step up alternating current only.
5) In a step down transformer the number of turns in primary coil is greater than that of the number of turns in the secondary coil.
Ans: True
Because, it converts high alternating current into low alternating current.
V) Answer in brief.
1) State Fleming’s Left Hand Rule.
Ans:
Fleming’s left hand rule gives the direction of force acting on the conductor.
According to Fleming’s left hand rule, if the thumb, first finger and middle finger of the left hand are stretched in such way that they are mutually perpendicular to each other.
If first finger gives the direction of magnetic field and middle finger gives the direction of current through the conductor then the thumb gives the direction of force acting on the conductor.
The force acting is given by F = Ilb
Fig. Fleming’s left hand rule
2) Define magnetic flux density.
Ans:
The magnetic flux is the no. of magnetic field lines passing through the given area.
And the magnetic flux density is the number of magnetic field lines crossing unit area which is placed normal to the direction of the magnetic field lines.
The SI unit of magnetic flux density is Wb/m2.
If the no. of lines passing are more then magnetic flux density is also large.
And if no. of lines passing is less then magnetic flux density is also small as shown in fig. bellow.
3) List the main parts of an electric motor.
Ans:
Electric motor is used to convert electrical energy into mechanical energy.
Following are the main parts of an electric motor:
Primary and secondary windings
Armature coil
Stator
Shaft
Commutator
Brushes
4) Draw and label the diagram of an AC generator.
Ans:
5) State the advantages of ac over dc.
Ans:
The direction of AC is changing periodically and hence its magnitude is varying with time. Transformer can be used to vary the voltage of AC.
The power factor for AC is between 0 and 1 and it has low energy losses.
Because of this reason, the AC are used for long distance transmission also.
In India the frequency of AC is 50Hz.
The frequency of DC is zero.
The power factor of DC is 1 and it has high energy losses, due to which DC cannot be used for long distance transmission.
The AC can be easily converted into DC.
6) Differentiate step up and step down transformer.
Ans:
The transformer is a device used to convert high alternating voltage into low alternating voltage and low alternating voltage into high alternating voltage. And it works on the principle of electromagnetic induction.
Depending on the no. of turns in primary coil and secondary coil the transformer is classified into two types as given bellow:
Step up transformer:
It is used to convert low alternating voltage into high alternating voltage.
In this transformer, the no. of turns of secondary coil is more than no. of turns in primary coil.
Step down transformer:
It is used to convert high alternating voltage into low alternating voltage.
In this transformer, the no. of turns of primary coil is greater than the no. of turns in secondary coil.
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? Give reason.
Ans: A portable radio having a built in transformer so that it can work from the mains instead of batteries. Then the transformer used must be step-down transformer, because it converts high alternating voltage into low alternating voltage.
So that the voltage will be reduced and which becomes equal to the voltage of battery.
And hence, it can work on mains instead of batteries.
8) State Faraday’s laws of electromagnetic induction.
Ans:
According to Faraday’s law of electromagnetic induction,
Whenever there is a change in magnetic flux linked with the coil, an emf is produced in the coil. And this emf induced varies directly with the rate at which magnetic flux through the coil changes.
This emf produced is called as induced emf.
And the phenomenon of producing induced emf due to change in the magnetic flux in the coil is called as electromagnetic induction.
VI) Answer in detail.
1) Explain the principle, construction and working of a dc motor.
Ans:
An electric motor is the device which converts electrical energy into mechanical energy.
This motors are used in water pump, fan, washing machines, mixers, grinders etc.
The figure shows the labelled diagram of simple electric motor. It has rectangular coil ABCD insulated by copper wire. The coil is placed between two poles of magnet such that the sides AB and CD are normal to the direction of magnetic field.
Fig. Electric motor/ dc motor
The ends of coil are connected to halves P and Q whose inner sides are insulated and are attached to axle. The edges of P and Q are touching to two conducting stationary brushes X and Y. Current will enter the coil through brush X and flows back to battery through Y.
By applying Fleming’s left hand rule, the force acting on side AB pulls it downwards and force acting on CD pulls it up words, due to which the coil and axle rotates freely in anticlockwise direction.
After half rotation Q meet to brush X and P meets to brush Y. In this way current get reversed and flows along the path DCBA. Commutator is the device which reverse the direction of current through circuit. Here split rings act as a commutator.
When the direction of current is reversed, the force acting on sides AB and CD get also reversed, therefore the next half turn would be in same direction. The direction of current is repeating continuously due to which the coil and axle rotate continuously.
a) The speed of rotation of coil can be increased by
b) Increasing magnitude of current in the coil
c) Increasing the no. of turns of the coil
d) Increasing the area of the coil
e) Increasing the strength of the coil
2) Explain two types of transformer.
Ans:
The transformer is a device used to convert high alternating voltage into low alternating voltage and low alternating voltage into high alternating voltage. And it works on the principle of electromagnetic induction.
Depending on the no. of turns in primary coil and secondary coil the transformer is classified into two types as given bellow:
Step up transformer:
It is used to convert low alternating voltage into high alternating voltage. In this transformer, the no. of turns of secondary coil is more than no. of turns in primary coil.
Step down transformer:
It is used to convert high alternating voltage into low alternating voltage.
In this transformer, the no. of turns of primary coil is greater than the no. of turns in secondary coil.
3) Draw a neat diagram of an AC generator and explain its working.
Ans:
An AC generator is an electronic device which is used to convert mechanical energy into electrical energy. The labelled circuit diagram of electric generator is as shown in figure.
It consists of a rotator coil PQRS which has been placed in between two poles of magnet the end points of coil are connected to two rings R1 and R2. B1 and B2 are the brushes and axel is attached internally to this rings. The outer ends of brushes are directly connected to the galvanometer for measuring current.
When the coil PQRS is rotating in clockwise direction then according to Fleming’s right hand rule, the current is induced in side PQ and RS. The current to the coil depends on number of turns. If number of turns increased current get added up and galvanometer shows large deflection. After half rotation, the direction of current induced changes. Hence the direction of current changes after every half rotation which is called as an AC current and this device is called AC generator.
To produce a uni directional DC current we have to use a split ring type of commutator, due to which the each brush will be in contact with only one arm and hence uni-directional current flows called as DC current like in an electric motor.
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