Chapter-16
Electromagnetic Induction
Learning Card-1
Choose the correct answer:
1. ‘volt’ is the unit of which of the following quantities?
a. Electric current b. Potential difference c. Electro motive energy c. power
2. The magnet is pushed in all four coils shown below the coil which produces lowest e.m.f is
3. How many times direction of the electric current change when the armature of an A.C. dynamo makes one cylce?
a. 5 b.10 c. 2 d.20
4. A device converting mechancial energy in to electrical energy is
a. Dry cell b. motor c. dynamo d. solar cell
5. A primary coil of a transformer has 10 turns and secondary has 100 turns the transformer is called
a. step up transformer b. step in transformer
c. step out transformer. d. step down transformer
Question and Answers
1. Who discoverd that “The current carrying conductor induces magnetic field around it”
Oersted.
2. Define electro magnetic induction.
Rate of change of the magnetic field linked with the coil induces on e.m.f in the coil. This is called electro magnetic induction.
3. State flemings night hand rule?
Arrange the first three fingers of your right hand mutually perpendicular to each other. If fore-finger indicates magnetic field, thumb indicates motion of conductor, then second finger indicates the direction of induced current flowing through the conductor.
4. What is magnetic effect of electric current?
An electric current passing through the coil produces magnetic field around it. This effect is called magnetic effect of electric current.
5. Name two instruments which work on electromagnetic induction.
1) Dynamo 2) Transformer
6. Define dynamo.
It is a device which converts mechanical energy in to electrical energy.
7. What is a motor.
It is a device which converts electrical energy in to mechanical energy.
8. Name two instruments having A.C. Motor.
Fan, mixer, grinder.
9. What are electric transformers?
It is a device used for stepping up or stepping down the AC voltage.
10. Define mutual inductions.
Production of induced e.m.f in one coil due to change of current in a neighbouring coil is called mutual induction.
11. Name two types of transformers.
a) Step up transformer. b) Step down transformer.
12.
Write the relation b/w primary and secondary current and voltage.
13. State Flemings left hand rule.
The first three fingers of your left hand mutually perpendicular to each other. If for finger indicates the direction of magnetic field. second finger indicates the direction of current passing through the conductor then thumb indicates the direction of mechanical force acting on the conductor.
14. State the factors which influences the induced e.m.f. in a coil.
a) the number of turns of the coil b) the strength of the magnetic field
c) the strength of the magnetic field d) area of the coil
15. State faradays laws of electromagnetic induction.
I Law : Whenever a magnetic field a linked with a conductor changes an induced e.m.f is generated in the conductor.
II Law : The magnitude of induced e.m.f is directly proportional to the rate of change of magnetic field linked with the conductor.
16. Write any two uses of A.C.
1. AC is used in ultrasonics
2. It is used in RADARS
3. Used in Domestic appliances.
17. Write a difference between motor and dynamo
| Motor | Dynamo |
| Converts Electrical energy in to mechanical energy | Converts mechanical energy in to electrical energy |
18. Write any two difference between AC dynamo and DC dynamo.
| AC dynamo | DC dynamo |
| Produces Alternating current slip rings are used | Produces direct current split rings are used. |
Learning Card-2
Choose the correct answer:
1. An AC has a frequency of 50Hz. The number of the armature should be produce it direction changes, the current makes in one second
(a) 25 (b) 50 (c) 100 (d) 200
2. The frequency of OC in India is
(a) 25Hz (b) 50Hz (c) 20Hz (d) 0Hz
3. A transformer has 20 turns in the primary coil and 80 turns in the secondary. If 10v is given in the primary coil, the voltage in the secondary coil would be
(a) 10v (b) 20v (c) 40v (d) 80v
4. The turns ration in a stepdown transformed will be
(a) > 1 (b) < 1 (c) 0 (d) ≥ 0
5. The part of a DC dynamo which may not be present in AC dynamo is
(a) split rings (b) slip rings (c) armature (d) magnet
Question and Answers:
1. What are the factors influencing induced emf in the secondary coil in the transformers?
(a) e.m.f in the primary (b) Number of turns in the primary
(c) Number of turns in the secondary (d) The core material
2. Explain the working of an AC dynamo?
When the armature is rotated the current moves in the direction ABCD during first half of the rotation. During the second half of the rotation the current moves in the direction DCBA. The current in the external circuit therefore, flows in one direction, for one half cycle and in the opposite direction for the second half cycle. It is therefore an alternating current. The dynamo is called alternating current dynamo.
3. Explain the working of a DC dynamo?
When the armature is rotated, during first half of rotation the current flows along ABCD in the coil and along B2 < B, in the external circuit, the split rings changes their contact. S1 comes in contact with B2 S2 with B1. In the second half, the current is along DCBA in the coil but along B2 > B1 in the external circuit. But the magnitude of the current is not constant.
4. Differentiate between stepup transformer and stepdown transformer.
| Step up transformer | Stepdown transformer |
| 1. No of turns in secondary is more than primary 2. Primary coil should have thick wire to sustain larger current | 1. Number of turns in primary is more than secondary 2. Secondary coil should have thick wire to sustain larger current |
Learning Card-3
Choose the correct answer:
1. Given below direction of magnetic field (line A) and the direction of an electron entering the magnetic field (line B) In Which direction the electron deflected?
a) Out of the paper you are reading b) In wards the paper you are reading
c) Towards your right hand d) Towards your left hand
2. Which one the following in not a method of obtaining maximum from your cycle dynamo?
a) Riding the bicycle upwards the hill b) Pedalling the bicycle with more frequency
c) Moving downhill on a bicycle. d) Applying break on the bicycle oftenly
3. A DC dynamo can be easily used as DC motor only change we should make in doing so is
a) Replacing load with battery b) Replacing battery with load
c) By connecting additional pair of commutators d) Any of the above
4. The maximum mechanical energy obtained in a motor is where
a) more turns in armuture is used b) Magnets of more strength are used
c) Using the armature of lesser area and making it rotate faster d) All the above
Question and Ansewers:
1. A nature of a dynamo rotates 20 times per minute what is the frequency of the current obtained.
20Hz
2. Think you are sitting in a chamber with your back to one wall. An electron beam moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side what is the direction of magnetic field?
Movement of from electron beam from back wall to the front wall is equivalent to the flow of electric current from front wall to the back wall. The deflection of the beam means, the force is acting towards our right side according to Flemings left hand rule, the direction of magnetic field is vertically downward. That is, the magnetic field is perpendicular, to the plane of paper and directed inward. Such magnetic field is shown by.
3. Why does a current carrying conductor kept in a magnetic field experience force? What is the direction of force acting on the conductor?
Current carrying conductor contains moving negatively charged particles Each electrons experience a force. The total force experienced by electrons is equal to the force experienced by the conductor. This force acts perpendicular to both the magnetic field and the direction of current in the conductor.
4. What is the frequency of DC?
Zero
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