CHAPTER-16
Electro Magnetic Induction
Text Book Question and Answers
I. Fill in the blanks.
1. Device that converts mechanical energy into electrical energy is called Dynamo
2. The type of electric current that changes its direction twice during one cycle of the dynamo is called Alternating Current
3. Device that converts electrical energy into mechanical energy is Motor
4. In Fleming’s rule the middle finger indicates the direction of Current or e.m.f.
5. The device that detects the induced emf is called Galvanometer
II. Four alternatives are given to each of the following incomplete statement/question. Choose the right answer.
1. Electricity and magnetism are mutually
(a)complementary (b)opposite (c)equal and opposite (d)strongest forces
Answer: (a)Complementary
2. The SI unit of potential difference is
(a)ohm (b)volt (c)coulomb (d)ampere
Answer: (b)Volt
3. Carbon brushes are not necessary in a dynamo, if the coil remains stationary and the magnet moves, because
(a)current is drawn from a stationary source
(b)current is drawn from a moving source
(c)direct current is produced
(d)it is not necessary to reverse the direction of the current
Answer: (d)it is not necessary to reverse the direction of the current
4. AC is not suitable for electrolysis because
(a)it has large current (b)it has high voltage
(c)polarity of the current changes (d)polarity of the current remains the same
Answer:(c) ploarity of the current changes
5. Which of the following graph indicate direct current?
Answer:(d)
6. When the plane of the rectangular coil is parallel to the direction of the magnetic field in a dynamo, then
(a)induced current will be zero (b)induced current is maximum
(c)AC is produced (d)DC is produced
Answer:(a)induced current will be zero
7. A small AC dynamo is connected to a galvanometer and gently rotated, then the needle of the galvanometer,
(a)moves toward left (b)moves towards right (c)remains in the same position (d)moves towards both right and left
Answer:(d)moves towards both right and left
8. A dynamo makes 100 cycles per second. The frequency of the AC is
(a)100Hz (b)200Hz (c)50Hz (d)400Hz
Answer:(a)100Hz
III. Answer the following
1. What is electromagnetic induction?
When the magnetic field linked with a circuit changes an electro motive force (emf) will be induced in the circuit. This phenomenon is known as electromagnetic induction.
2. State faraday’s laws of electromagnetic induction.
1st law: whenever a magnetic field linked with a conductor changes, an induced e.m.f. is generated in the conductor.
2nd law: the magnitude of induced e.m.f.is directly proportional to the rate of change of magnetic field linked with the conductor.
3. State felming’s right hand rule. For what purpose is it used?
Arrange the first three fingers of your right hand mutually perpendicular to each other. If the fore finger indicates the direction of the magnetic field, the thumb the direction of motion of the conductor, then the middle finger will indicate the direction of the induced e.m.f. or current.
This rule is useful to decide the direction of the current since other two parameters are known.
4. Draw a neat diagram of the following and label the parts
1)AC dynamo 2)DC dynamo 3)DC motor
1)AC dynamo
i. An armature which is a rectangular coil of insulated wire ABCD wound over a soft iron. The core is not shown in the figure
ii. Slip rings R1, R2
iii. Brushes B1, B2
iv. Permanent magnet N,S.
v. Lamp L
2. DC Dynamo
i. ABCD coil on armature
ii. Split rings R1, R2
iii. Brushes B1, B2
iv. Permanent magnet pole N,S.
v. Lamp L
3. DC Motor
i. ABCD coil on armature
ii. Split rings R1, R2
iii. Brushes B1, B2
iv. Permanent magnet pole N,S.
v. DC source
5. Mention any two uses of DC and two uses of AC
Uses of DC:
(a)it is used in electrolysis (b)used in torches
Uses of AC:
(a)used in domestic appliances like heaters, fans, ovens etc.
(b)used to produce required frequency of radio waves
(c)used to produce ultrasonic.
IV. Give reasons for the following
1. In the experiment conducted by Faraday the induced electric current was not DC.
In faraday’s experiment induced electric current was not DC as the magnetic field changed direction, the direction of the current also changed.
2. Split rings are used instead of slip rings to construct DC dynamo.
Split rings are used so that the current which flows in the external circuit is unidirectional.
3. Stationary coil and stationary magnet can not produce induced elcetric current
Stationary coil and stationary magnet cannot produce induced electric current because the magnetic field linked with the circuit does not change.
V. Draw neat diagrams of the following and label the parts.
1. Step down transformer
2. Setp up transformer
3. Induction coil
Additional Questions and Answers
1. Define magnetic effect of electric current.
A conductor carrying current produces a magnetic field around it. This effect of electric current is known as magnetic effect of electric current.
2. Define e.m.f. of a cell
When a cell is not connected to a load, the potential difference between the poles of the cell is known as its electromotive force (e.m.f.)
3. Define electromagnetic induction
When the magnetic field linked with a circuit changes, an electromotive force (e.m.f) will be induced in the circuit. This phenomenon is known as electromagnetic induction.
4. On what factors does the induced e.m.f. in a coil depend?
i. The number of turns of the coil
ii. The strength of the magnetic field
iii. The speed with which the magnet or the coil moves
iv. Area of the coil.
5. State felming’s left hand rule
Arrange the first three fingers of your left hand mutually perpendicular to each other. If the fore finger indicates the direction of the magnetic field, the thumb the direction of the mechanical force acting on the conductor, then the middle finger will indicate the direction of flow of current
6. What is Alternating Current(AC)?
Alternate current is that which changes its direction to and periodically.
7. Write the applications of electromagnetic induction
i. AC and DC Dynamo.
ii. Transformers.
iii. Induction coil.
8. What is dynamo?
Dynamo is a device which converts mechanical energy into electrical energy by the principle of electromagnetic induction.
9. Define Armature?
The coil along with the rings is called armature
10. How does alternating current differ from direct current?
If the direction of the current keeps reversing every half a cycle, such a current is called Alternating current. The current which flows in the same direction is called Direct current.
11. Write the difference between AC dynamo and DC dynamo.
| AC dynamo | DC dynamo |
| 1. it produces alternating current | 1. it produces direct current |
| 2. Slip rings are used in AC dynamo | 2. Split ring commutator is used in DC dynamo. |
12. What is an electric motor?
A device that converts electrical energy into mechanical energy is known as an electric motor.
13. Explain the principle of an electric motor.
A motor works on the principle that a conductor carrying current in a magnetic field experiences a mechanical force.
14. Represent alternating current and direct current diagramatically.
Alternating Current:
Direct current:
15. Mention andy three contributions of Michael Faraday.
i. He showed that chlorine can be liquefied
ii. He discovered benzene.
iii. He conducted experiments on electromagnetic induction.
16. What is transformer?
The device which is used for stepping up or stepping down the AC voltage it is called transformer.
17. What is mutual induction?
Production of induced e.m.f. in one coil due to change of current in a neighbouring coil is called mutual induction.
18. What is step up transformer?
Transformer which increases the input voltage of an AC source is called step up transformer.
19. What is step down transformer?
Transformer which decreases the input voltage of an AC Source is called step up transformer.
20. What is primary coil?
The winding on one side which is connected to input source is called primary coil.
21. What is secondary coil?
The winding on the other side from which output voltage is obtained is called secondary coil.
22. What is induction coil?
An induction coil is a device for obtaining a very high DC voltage starting from a low DC voltage
23. On what factors does the e.m.f. induced in the secondary coil of transformer depend.
The magnitude of the e.m.f. induced in the secondary coil of a transformer depends on.
i. e.m.f. in the primary
ii. Number of turns in the primary
iii. Number of turns in the secondary
iv. The core material.
24. Write the relationship between current and voltage of primary and secondary coils of a transformer.
The relationship between current and voltage of primary and secondary coils of a transformer can be expressed as,
Ip x Vp = Is x Vs
Primary Current X voltage of primary coil = Secondary current X voltage of secondary coil
Or
The relationship suggests that the primary of a step up transformer should have less number of turns than the secondary and should be thick to sustain larger electric current. The primary of a step down transformer should have more number of turns than the secondary.
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