12 - Standard Physics 2, 3, 5 marks
4. Electromagnetic Induction And Alternate Current
4. ELECTROMAGNETIC INDUCTION AND ALTERNATE CURRENT
SHORT ANSWER QUESTIONS (BOOK BACK)
1. Define electromagnetic induction.
Whenever the magnetic flux linked with a closed coil changes, an emf is induced and hence an
electric current flows in the circuit.
This emf is called induced emf and the current is called induced current. This phenomenon is called
electromagnetic induction.
2. State Faraday’s laws of electromagnetic induction.
(i) Whenever magnetic flux linked with a closed circuit changes, an emf is induced in the circuit.
(ii) The magnitude of induced emf in a closed circuit is equal to e time rate of change of magnetic
flux linked with the circuit.
3. State Lenz’s law.
Lenz’s law states that the direction of the induced current is such that is always opposes the cause
responsible for its production.
4. State Fleming’s right hand rule.
The thumb, index finger and middle finger of right hand are stretched out in mutually perpendicular
directions. If index finger points the direction of magnetic field and the thumb points the direction of
motion of the conductor, then the middle finger will indicate the direction of the induced current.
Fleming’s right hand rule is also known as generator rule.
5. What are called eddy currents? How are they produced?
When magnetic flux linked with a conductor in the form of a sheet or a plate changes, an emf is
induced. As a result, the induced current flow in concentric circular paths which resembles eddies of
water. Hence these are known as Eddy currents or Foucault currents.
6. What the methods of producing induced emf?
By changing the magnetic field ‘B’
By changing the area ‘A’ of the coil
By changing the relative orientation ‘θ’ of the coil with magnetic field.
7. What is called inductor?
Inductor is a device used to store energy in a magnetic field when an electric current flows through
it. (e.g.) solenoids and toroids
8. What is called self induction?
The phenomenon of inducing an emf in a coil, when the magnetic flux linked with the coil itself
changes is called self induction.
The emf induced is called self-induced emf.
9. What is called mutual induction?
When an electric current passing through a coil changes with time, an emf is induced in the
neighboring coil. This phenomenon is known as mutual induction and the emf is called mutually
induced emf.
10. State the principle of AC generator (alternator)
It work on the principle of electromagnetic induction. (i.e.) The relative motion between a conductor
and a magnetic field changes the magnetic flux linked with the conductor which in turn induces an
emf.
The magnitude of the induced emf is given by Faraday’s law and its direction by Fleming’s right
hand rule.
J.Sisubaladhanasekar M.Sc., M.Phil, B.Ed 102 K.Gomathi M.Sc., B.Ed
Govt. Boys Hr. Sec. School, Vennandur, Rasipuram, Namakkal District
12 - Standard Physics 2, 3, 5 marks
4. Electromagnetic Induction And Alternate Current
11. What are the advantages of stationary armature - rotating field alternator?
Advantages of stationary armature - rotating field alternator :
The current is drawn directly from fixed terminals on the stator without the use of brush contacts.
The insulation of stationary armature winding is easier.
The number of slip rings is reduced. Moreover the sliding contacts are used for low-voltage DC
source.
Armature windings can be constructed more rigidly to prevent deformation due to any mechanical
stress.
12. Distinguish between step up and step down transformer.
Step up transformer Step down transformer
If the transformer converts an alternating If the transformer converts an alternating
current with low voltage in to an alternating current with high voltage in to an alternating
current with high voltage is called step up current with low voltage is called step down
transformer. transformer.
13. Define mean value or average value of AC.
The mean or average value of alternating is defined as the average of all values of current over a
2
positive half cycle or negative half cycle.
= = 0.6371
14. Define RMS value of AC.
The root mean square value of an alternating current is defined as the square root of the mean of the
square of all currents over one cycle.
= = 0.707
√2
15. Define phasor and phasor diagram.
A sinusoidal alternating voltage or current can be represented by a vector which rotates about the
origin in anti-clockwise direction at a constant angular velocity ‘ω’. Such a rotating vector is called a
phasor.
The diagram which shows various phasors and phase relations is called phasor diagram.
16. Define resonance.
When the frequency of the applied source is equal to the natural frequency of the RLC circuit, the
current in the circuit reaches it maximum value. Then the circuit is said to be in electrical resonance.
17. Define resonance frequency.
The frequency at which resonance takes place is called resonant frequency.
Hence the condition for resonance is : XL = XC
18. Define Q - factor or quality factor.
Q - factor is defined as the ratio of voltage across L or C to the applied voltage at resonance.
19. Define wattless current.
If the power consumed by an AC circuit is zero, then the current in that circuit is said to be wattless
current. This wattless current happens in a purely inductive or capacitive circuit.
Power factor (cos ∅) of a circuit is defined as the cosine of the angle of lead or lag
20. Define power factor.
Power factor is also defined as the ratio of true power to the apparent power.
21. What are called LC oscillations?
Whenever energy is given to a circuit containing a pure inductor of inductance L and a capacitor of
capacitance C, the energy oscillates back and forth between the magnetic field of the inductor and
the electric field of the capacitor.
Thus the electrical oscillations of definite frequency are generated. These oscillations are called LC
oscillations.
J.Sisubaladhanasekar M.Sc., M.Phil, B.Ed 103 K.Gomathi M.Sc., B.Ed
Govt. Boys Hr. Sec. School, Vennandur, Rasipuram, Namakkal District
12 - Standard Physics 2, 3, 5 marks
5. Electromagnetic Waves
5. ELECTROMAGNETIC WAVES
SHORT ANSWER QUESTIONS (BOOK BACK)
1. Define displacement current.
The displacement current can be defined as the current which comes into play in the region in which
the electric field and the electric flux are changing with time
That is when ever the change in electric field takes place, displacement current is produced.
2. Define electro magnetic waves.
Electromagnetic waves are non-mechanical waves which move with speed equals to the speed of
light (in vacuum)
3. Give the modified form of Ampere’s circuital law.
If IC and ID are the conduction and displacement current, then the modified Ampere’s circuital law is
given by,
=
.
( + )
.
= + .
This is also known as Ampere - Maxwell’s law.
4. Write notes on Gauss' law in magnetism.
The surface integral of magnetic field over a closed surface is zero.
This law can also be called as Gauss’s law for magnetism.
.
= 0
5. Give two uses each of (i) IR radiation, (ii) Microwaves and (iii) UV radiation.
(i) IR radiation
It is used to produce dehydrated fruits,
It is used to in TV remote as a signal carrier,
(ii) Microwaves
It is used in radar system for aircraft navigation,
It is used in microwave oven for cooking
(iii) UV radiation.
It is used in burglar alarm,
It is used in the study of atomic structure.
6. What are Fraunhofer lines? How are they useful in the identification of elements present in the Sun?
When the spectrum obtained from the Sun is examined, it consists of large number of dark lines (line
absorption spectrum).
These dark lines in the solar spectrum are known as Fraunhofer lines.
The absorption spectra for various materials are compared with the Fraunhofer lines in the solar
spectrum, which helps in identifying elements present in the Sun’s atmosphere.
7. Write notes on Ampere-Maxwell law.
It is modified Ampere’s circuital law. This is also known as Ampere – Maxwell law.
This law relates the magnetic field around any closed path to the conduction current and
displacement current through that path.
.
= + .
8. Why are e.m. waves non-mechanical?
Electromagnetic waves do not require any medium for propagation. So electromagnetic wave is a
non-mechanical wave.
J.Sisubaladhanasekar M.Sc., M.Phil, B.Ed 135 K.Gomathi M.Sc., B.Ed
Govt. Boys Hr. Sec. School, Vennandur, Rasipuram, Namakkal District
