Chapter 14: ELECTROMAGNETIC INDUCTION (Short Questions/Answers)

CHAPTER # 14

ELECTROMAGNETIC INDUCTION

 

14.1: make list of similarities and differences between the motor effect and electromagnetic induction

in a moving wire (the dynamo effect)

Ans:  

Motor Effect	Electromagnetic Effect
1. It is used to convert the electrical energy  into mechanical energy.  2. Its input is electric source.  3. Torque is used to rotate the load.  4. Working principle is turning effect of coil.	1. It is used to convert the mechanical   energy into electrical energy.  2. Its input source is mechanical.  3. Load is operated at the output by means  of electrical energy.  4. Working principle is faraday’s law of  electromagnetic induction

 

Similarities:  

1. In both phenomenon energy is converted from one form to other. 

2. Both are required magnetic field for their working. 

 

14.2: for a simple motor, why must the back e.m.f. always be smallaer tha n the applied potential

difference?

Ans: Back emf is always smaller then applied potential difference V. because back emf depends upon

the speed of motor. When motor is just started, back emf is almost zero but it increases with the

increase in speed coil. As the back emf opposes the applied voltage so current can be expressed as:

I =(V-Ɛ)/R

Therefore Ɛ is always smaller than applied potential.

 

14.3: what factors limit the size of the back of e.m.f.?

Ans: As the back emf is due to rotation of a coil in magnetic field i.e

Ɛ = NAωBSinθ

Here N= no of turns of coil, B = Magnetic field

A= Area of the coil and ω= Angular speed

A, N and B are constant so back emf depends only upon the angular speed.

 

14.4: why does back e.m.f. tend to decrease as the rate of doing work increases?

Ans: when load is connected to motor then rotational speed of the coil decreases which results decrease

in rate of change of flux and hence back emf of the coil decreases. In this case the value of current

flowing through the coΔil increases.

 

14.5: explain from Ԑ = -Δφ/Δt. Why it possible to say that Ԑ∆I/∆t

Ans: According to Faraday’s law of electromagnetic induction

Ɛ= -NΔφ/Δt or Ɛ = Δ(Nφ)/Δt

 

14.6: show that the relationship Ԑ = -ΔØ/Δt

is dimensionally correct?

Ans: L.H.S = Ɛ = volt = J/c =kgm²s^-2(As)^-1 = kgm²s^-3A^-1

[Ɛ] = [ML²T^-3A^-1]

R.H.S = wb/s =Nm/As = kgm²s^-3 A^-1

R.H.S = [ML²T^-3A^-1]

R.H.S = L.H.S

 

14.7: give the formula for the flux linkage in terms of angular orientation.

Ans: As we know that φ = B.A = BACosθ

For N-turns φ = NBACosθ

But θ=ωt so φ = NBACos ωt

This relation shows the linkage of flux in terms of angular orientation.

 

14.8: explain the eddy current in terms of Lenz’s law. Also by drawing a suitable diagram show the

direction of eddy current and the polarity produced in the sheet as a result of magnetic field.

Ans: According to Len’s law, the induced current will always opposes its cause. Consider a metallic

cylinder which is placed in a magnetic field. A force opposing the rotation will cause to produced heating

effect. This effect can be reduced by replacing metallic cylinder into stack of coins with insulation b/w

them. This avoids the eddy current.

 

14.9: How electromagnetic brake works? Explain.

Ans: In braking system, a magnetic flux is flowing perpendicular to the rotating direction of wheel. So

the eddy current is flowing in the wheel in such a way that it opposes the rotation of the wheel. Hence

the rotation of the wheel will slow down may finally be stopped. Such frictionless braking system help us

to increase the life span as well as reliability.

 

14.10: a bar magnet is dropped inside a long verticle tube. If the tube is made of metal, the magnet

quickly approaches a terminal speed , but if the tube is made of card board, the magnet falls with

constant acceleration. Explain why the magnet falls differently in the metal tube that it does in the

cardboard tube.

Ans: As a bar magnet is dropped into hollow metallic tube, due to rate of change of magnetic flux eddy

current is induced that opposes the change in flux. Such opposing force soon becomes equal to

weight of the magnet and hence it falls with terminal velocity. But when bar magnet is dropped

into the hollow cardboard then no emf is induced and no eddy current is produced and hence

no opposing force acts on it. Therefore it falls with constant acceleration.

 

14.11: the transformer suffers from eddy current loss (a) Explain how eddy current arise. (b) state the

features of transformer designed to minimize eddy current.

Ans: As we know that transformer coil is made up of iron for linkage of flux of primary to secondary coil

by mutual induction. Due to change in flux eddy current is induced in iron core. Due to high

resistance to eddy current energy is lost in form of heat. To minimize the energy loss, iron core

is made up of laminated iron plates instead of one lamina.

 

14.12: Analyse information to explain how induction is used in cook tops in electric ranges?

Ans: As we know that coil is used under the cook top to produce a changing magnetic flux due to

changing current that link with metal pot placed over the top. Hence emf is induced due to

which eddy current is induced that produces extra heat in the metal pot to heat the food placed

in it.

 

14.13: (a) Explain what is meant by the term back e.m.f. in any electric motor operation.

(b) Explain why it is an advantage for the armature to rotate in a radial magnetic field rather

than a uniform one?

Ans: a) when the frequency of an armature increases, then rate of change of flux will also increases.

According to faraday’s law emf is induced in it which is called back emf because it opposes the

applied voltage. The current flowing through the coil decreases because of back emf.

b) In radial magnetic field, the plane of the coil is always parallel to magnetic field during its rotation. so

maximum torque will produced in the coil and hence it rotate smoothly and with fast speed.

 

14.14: if the armatures rotating frely then explain, in terms of electromagnetic principals (a) why the

current in armatures progressively decreases as the angular velocity of the armature

increases. (b) why a maximum angular velocity is eventually reached?

Ans: a) As I = V-Ɛ/R

This relation shows that when rotation of the coil increases then change in flux becomes maximum

hence back emf induces maximum therefore current in the coil decreases.

b) When motor is just started then back emf is zero but after some back emf becomes maximum

because of maximum rotational speed of the coil.

 

14.15 : Transformer cores can be made from a variety of materials . what are the main features that

would require of material to make a god transformer core? Suggest how well each of the

following materials would perform: iron , solid soft iron , laminated soft iron, aluminum.

Ans: The transformer core is preferably made up of laminated soft iron core. It is because power losses

in transformer can be reduced such as eddy current and hysteresis loss.

 

14.6: current is increasing in magnitude from A to B as shown in fig: what is the direction of induced

current, if any , in the loop.

Ans: When current increases in the straight conductor from A to B, then magnetic field due to current

also increases. Such field links with the loop and changes by increasing current. According to

Faraday’s law emf are induced that induces current in the loop. According to Lenz’s Law

direction of induced emf is clockwise so that it opposes its cause.