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Observe the figure. If the current in the coil A is changed, will some current be induced in the coil B ? Explain.




1.      If the current in coil A is changed, then some current will be induced in coil B.
2.      As the current in coil A changes, the magnetic field related to it also change.

3.      Due to the changing magnetic field, current is induced in coil B.

Electric generator


An electric device which converts mechanical energy into electric energy is called an electric generator.

Principle:  Electric generator works on the principle of electromagnetic induction. When the coil of electric generator rotates in a magnetic field, the magnetic field induces a current in this coil. This induced current then flows into the circuit connected to the coil.

Types of electric generators:
1.       AC Generator
2.       DC Generator

1.       AC Generator:  A generator which converts mechanical energy into electric energy in the form of alternating current is called AC Generator.

Construction:

The main components of AC generator are as shown in figure.
1. Armature    2. Strong magnets   3. Slip rings and     4. Brushes.
    Armature, strong magnets and brushes are the same as used for electric motors.

Slip rings:

The two ends of the armature coil are connected to two brass slip rings R1 and R2. These rings rotate along with the armature coil.

Working:

a.        When the armature coil ABCD rotates in the magnetic field provided by the strong magnets, it cuts the magnetic lines of forces.
b.       Thus the changing magnetic field produces induced current in the coil. The direction of induced current is determined by the Flemings right hand rule.
c.        The current flows out through the brush B1 in one direction in the first half of the revolution and through the brush B2 in the next half revolution in the reverse direction, this process is repeated.
d.       Therefore, the induced current produced is of alternating nature. Such a current is called as alternating current.

2.       DC Generator:

A generator which converts mechanical energy into electrical energy in the form of direct current is called as DC generator.



Construction:



The main components of D.C. generator are:
1.       Armature coil 2. Strong magnets 3. Split rings or commutator 4. Brushes 5. Bulb.

The components such as armature coil, strong magnet, brushes are the same which are used for AC generators. Slip rings or commutator is the same as used in electric motor.

Working of DC generator:

(a)     When the coil of DC generator rotates in the magnetic field, potential difference is produced in the coil. This gives rise to the flow of current. This is shown by glowing of the bulb.
(b)     In D.C. generator, the flow of current in the circuit is in the same direction as long as the coil rotates in the magnetic field.

(c)     This is because one brush is always in contact with the arm of the armature moving up and other brush is in contact with the arm of the armature moving downward in the magnetic field.

Electric motor.


Ans. A device which converts electrical energy into mechanical energy is called an electric motor.

Principle: - Electric motor works on the principle that a current carrying conductor placed in a magnetic field experiences a force.

Construction:



ELECTRIC MOTOR
i.                     Armature coil: - A large number of turns of insulated copper wire wound on iron core in rectangular shape forms an armature coil ABCD as shown in the above figure.
ii.                   Strong magnet: -The armature coil is placed in between two pole pieces (N and S) of a strong magnet. This provides a strong magnetic field.  
iii.                 Split ring commutator: - It consists of two halves (R1 and R2) of a metallic ring. The ends of the armature coil are connected to these rings. Commutator reverses the direction of current in the armature coil.
iv.                 Brushes: - Two carbon brushes B1 and B2, used to press the commutator.
v.                   Battery: - The battery supplies the current (D.C.) to the armature coil.

Working: -

i.                     When current is passed through the coil ABCD, arms AB and CD experience force. According to Fleming’s left hand rule the force experienced by arm AB is in the downward direction and arm CD in the upward direction.
ii.                   Both these forces are equal and opposite. This force rotates the coil in clockwise direction until the coil is vertical.
iii.                 At this position, the contact between commutator and brushes break. So the supply to the coil is cut off. Thus no force acts on the coil.
iv.                 But the coil does not stop due to inertia. It goes on rotating until the commutator again comes in contact with the brushes B1 and B2.
v.                   Again the current starts passing through coil and the arm AB rotates through 900, 1800, 2700 and 3600. Now the force acting on arm AB is downward and CD is upward. Again this force moves the coil in clockwise direction.
vi.                 Thus, the coil rotates with the help of electrical energy. The coil of DC motor continues to rotate in the same direction.  

Uses of DC motor:

i.                     These motors are used in domestic appliances like mixers, blenders, refrigerators and washing machines.
ii.                    These motors are used in electric fans, hair dryers, record players, tape recorders and blowers.

iii.                  These motors are used in electric cars, rolling mills, electric cranes, electric lifts and electric trains. 

A magnetic crane is used to load and transport scrap iron.


i.                     It is not feasible to create a permanent magnet of a big size required to load and transport scrap iron.
ii.                    It is also not possible to store such a big magnet.
iii.                  Hence, we create a powerful electromagnet by passing a current through a big iron disc.

iv.                  Therefore, a magnetic crane is used to load and transport scrap iron. 

Alloys like alnico or nipermag are used in industry.


i.                     Alloys like alnico or nipermag are very hard and are used in the production of permanent magnets.
ii.                    Permanent magnets of these alloys are used in microphones, loudspeaker, ammeters, voltmeters, etc.
iii.                  Alnico is an alloy of iron containing aluminium, nickel and cobalt. Nipermag is an alloy of iron containing nickel, aluminium and titanium.

iv.                  Hence, permanent magnets prepared from these alloys are most suitable for use in industries. 

We should not use many electrical appliances simultaneously.


i.                     Many of the electrical appliances that we use at home have a high power rating. E.g. grinder, A.C, Oven, washing-machine, etc.
ii.                    When we use these electrical appliances simultaneously, it causes overloading, that is flow of large amount of current in the circuit, occurs.
iii.                  This causes fire.
Hence, we should not use many electrical appliances simultaneously

Wires carrying electricity should not be touched bare footed.

1. 
i.                     Our body is a good conductor of electricity.
ii.                    If we touch the wire bare footed, a large current may pass through our body.
iii.                  Due to this, we may receive severe electric shock and sometimes even death.

iv.                  Therefore, wires, carrying electricity should not be touched bare footed.

State the characteristics of magnetic lines of force.


Ans. 

The path along which the unit north pole moves in a magnetic field is called magnetic lines of force or magnetic field lines.
The characteristics of magnetic lines of force are:
1.       Magnetic lines of force are closed continuous curves. They start from North Pole and ends on South Pole.
2.       The tangent at any point on the magnetic lines of force gives the direction of the magnetic field at that point.
3.       No two magnetic lines of force can intersect each other.
Magnetic lines of force are crowded where the magnetic field is strong and far from each other where the field is weak. 

ELECTRIC MOTOR V/S ELECTRIC GENERATOR

Electric motor
Electric generator
1.       Electric motor converts electrical energy into mechanical energy.
2.       It uses electricity.
3.       It is based on the principle that current carrying conductor placed in a magnetic field experiences a force.
1.       Electric generator converts mechanical energy into electrical energy.
2.       It generates electricity.
3.       It is based on the principle of electromagnetic induction.

Right hand thumb rule



Ans.

 Imagine that you are holding a current carrying straight conductor in your right hand such that the thumb points towards the direction current, then the curled fingers around the conductor will give the direction of the magnetic field. This is known as right hand thumb rule.

Fleming’s Left hand rule:


Ans. 

Stretch the forefinger, the central finger and the thumb of you left hand mutually perpendicular to each other. If the forefinger shows the direction of the field and the central finger shows the direction of the current, then the thumb will point towards the direction of the motion of the conductor.


Fleming’s right hand rule


Ans.


i.                     Stretch the thumb, forefinger and middle finger of the right hand so that they are perpendicular to each other.
ii.                    If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of the motion of conductor, then the middle finger will show the direction of induced current.

iii.                  The induced current is found to be the maximum when the direction of motion of the coil is at right angles to the magnetic field. 

Electric motor

 A device which converts electrical energy into mechanical energy is called an electric motor. 

Solenoid

1A coil of many turns of insulated copper wire wrapped in the shape of cylinder is called a solenoid. 


Magnetic field

1 The region around magnet, in which the force of attraction and repulsion exists, is called as ‘magnetic field’. 

Find the odd one out.


1.       Loudspeaker, microphone, electric motor, bar magnet.
Ans. Bar Magnet: - It is a magnet while the rest are devices working on electromagnetism.
2.       Armature coil, brushes, commutator, direct current.
            Ans. Direct current: - The rest are parts of a generator.
3.       Fuse, insulating material, rubber shoes, generator.
Ans. Generator: - The rest are safety measures in using electricity.
4.       Voltmeter, ammeter, galvanometer, thermometer.
Ans. Thermometer: - The rest are dives working on electromagnetism. 

Say true or false. If false, write the correct sentence.


1.       Magnetic poles exist in pairs.
Ans. True
2.       Magnetic field increases as we go away from a magnet.
Ans. False:- Magnetic field decreases as we go away from a magnet.
3.       Magnetic lines of force always cross each other.
Ans. False:- Magnetic lines of force never cross each other.
4.       Electric generator is used to generate current.
Ans. True

Write a letter to your Mother on Mother's Day

8A, Heaven colony,
M.G. Road,
Dharavi,
Mumbai - 400017.
13th February, 2013.


Dear Mom,


I love you Mom. I miss you a lot since I came to hostel for my studies. You are the greatest gift ever given to me by God. Though you are not highly educated in the formal sense, but you are well cultured, very kind and sympathetic. I had been inspired by you in every moment of my life.

In this mother day I want to thank you mom, for your deeds. But I knew very well that a merely thanks or any valuable gifts given to you will not equalize your deeds. But I'm sending my heartily kisses to you 'mom'. I think you will like this other than anything else. In this world I had been with everyone in the last 20 years but I had been with you for more than that of 10 months (i.e.) from the time you conceived me. So, our relation is beyond than everyone else.

I will always be thankful to you 'mom' in my life time. I love you crores of times and miss you a lot in this one week.

Your loving son,

Xyz

Formal Letters


Informal Letters

Your younger brother who is staying in a hostel has failed in the Terminal examination and has become nervous. He is thinking to give up studies. Write a letter persuading him to continue the efforts and get success.


                                                                                                                                                                                                                                                          8C, Sonia Apartments,
                                                                                      Mango colony,
                                                                                      M.G. Road,
                                                                                      Dharavi,
                                                                                      Mumbai - 400 016.
                                                                                      13th February, 2013    
                          
Dear brother,

                   I hope this letter of mine finds you in the good health. But what is this I hear about you? Are you really seriously planning to give up your studies, just because you failed in the terminal examination? It would then be a rash decision made without any thinking about your future. A single  failure should not dishearten you. It is not the end of the world for you yet. You must count your blessings. You are full of bouncing energy and very much talented in your activities. If you decide to give up your studies then you will be a big loser. You will be always identified as a school dropout.

                   Moreover, without adequate formal education, you will not be able to achieve ahead in life. The prospects for you anywhere and everywhere are bound to be at risk. Dearest brother, keep on working and revise your weak subjects then you are likely to pass your next examination with flying colours.

                   Try to work harder and this will make your task much easier and simpler. Best of luck for your studies.  
                                                                                                                             
Yours lovingly,
A B C

Write an application letter for the post of accountant.

Write an application letter for the post of accountant.

Formal Letter Writing.

96, Sonia Apartment,

7G, Rainbow colony,

M.G. Road,

Dharavi,

Mumbai - 400017.

13th February, 2021.

                         

To,

The Manager,

The General company,

Dadar (W),

Mumbai – 400 021.


Reference  : Your advertisement in “The Times of India” dated 13th, February, 2021.


Subject : Application letter for the post of Accountant.


Respected sir,


With reference to your advertisement for the post of accountant in “The Times of India” dated 13th, February, 2013. I wish to apply for the same.


I am a very energetic person with excellent communication skill and ready to face any target set by your esteemed company. I am willing to work hard and being young I am also eager to add knowledge with experience. I hold an M.Com degree from “University of Mumbai” and a Diploma in Cost Account from “Indira Gandhi Open University”, New Delhi. I have also done computer courses like Basic, Tally – 9 as well as programming languages like C, C++ from NIIT, Matunga. I have 2 years experience as an assistant accountant in local company, Mahim.


My resume is sent with this letter, for your reference. I shall be much thankful to you, if you grant me an interview call at your convenient time. I am looking forward to join your esteemed company as early as possible.


Thanking You,


Yours Faithfully,


 Xyz.


Encl: - Resume and Xerox copy of educational certificates.


Formal Letters


Informal Letters

Write an application letter for the post of librarian for a public library.

Write an application letter for the post of librarian for a public library.

Formal Letter Writing.

96, Sonia Apartment,

7G, Rainbow colony,

M.G. Road,

Dharavi,

Mumbai - 400017.

13th February, 2021. 

                           

To,

The Manager,

The General Public Library,

Dadar (W),

Mumbai – 400 021.


Reference: Your advertisement in “The Times of India” dated 13th, February, 2013.


Subject: Application letter for the post of Librarian.


Respected sir,


With reference to your advertisement for the post of librarian in “The Times of India” dated 13th, February, 2021. I wish to apply for the same.


I am a very energetic person with excellent communication skills and ready to face any target set by your esteemed public library. I am willing to work hard and being young I am also eager to add knowledge with experience. I hold a M.Com degree from “University of Mumbai” and a Diploma in Library Science from “Indira Gandhi Open University”, New Delhi. I have also done computer courses like Basic, Tally – 9 as well as programming languages like C, C++ from NIIT, Matunga. I have 2 years’ experience as an assistant in the local library, Mahim.


My resume is sent with this letter, for your reference. I shall be much thankful to you, if you grant me an interview call at your convenient time. I am looking forward to joining your esteemed Public library as early as possible.


Thanking You,


Yours Faithfully,


Xyz.


Encl: - Resume and Xerox copy of educational certificates.


Formal Letters


Informal Letters

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