Download Question bank of physics - Kendriya Vidyalaya SAC, Vastrapur

KENDRIYA VIDYALAYA SANGATHAN AHMEDABAD REGION
ONE DAY WORKSHOP IN PHYSICS
23 JULY 2016
QUESTTION BANK
ELECTRO MAGNETIC INDUCTION
1 MARK QUESTIONS
Q1
Q2
Define the term self-inductance of a coil. Give its SI unit.
Ans. The self-inductance L is defined as the emf induced in the coil, when the rate of change of current in
that coil is 1 A/s.
If the rate of change of current is 2 ampere/second in a solenoid induces an emf of 40mV in the solenoid,
what is the self-inductance of this solenoid?
Ans. L= Q3
=
=
20X 10-3 H = 20mH
Predict the directions of induced currents in metal rings 1 and 2 lying in the same plane where current I in
the wire is increasing steadily.
Ans Above the wire in the ring clockwise
Below the wire in the ring anticlockwise
Q4
Q5
State Lenz’s law in electromagnetic induction.
Ans. The direction of induced current in a closed circuit is always such that it opposes the cause that
produces it.
Write the expression of electromagnetic energy stored in an inductor of inductance L when steady current is
passed through it.
Ans. U = LI2
2 MARKS QUESTIONS
Q1
Two circular loops are placed with their centres at fixed distance apart. How would you orient the loops to
have (i) maximum (ii) minimum Mutual inductance?
Ans. (i)For maximum mutual induction the orientation should be parallel
(ii) For minimum the orientation should be perpendicular.
Q2
Define eddy current. Give one application of eddy current
The induced currents in the solid conductor due to the motion of the conductor in the magnetic field
Electromagnetic Damping, Magnetic breaks
Q3
What is magnetic flux? Write its expression and SI unit.
Ans. The total no of magnetic field lines through a closed surface is called magnetic flux.
Ф = B.dA
SI unit of magnetic flux is weber.
Q4
Write the expression for the induced emf in a conducting rod of length moving with a velocity v in a
magnetic field B.
e = d Ф/dt
Q5
=
vBl
Two coils P and S are arranged as shown in the figure.
(i) What will be the direction of induced current in S when the switch is closed?
(ii) What will be the direction of induced current in S when the switch is opened?
P
S
Ans: (i) anticlockwise (ii) clockwise
3 MARKS QUESTIONS
Q1
The magnetic flux linked with a coil passing perpendicular to the plane of the coil changes with time
Φ = 4t2 + 2t + 3, where “t” is the time in seconds. What is magnitude of emf induced at t = 1 second?
d
Ans: (e = dΦ/dt =
4t 2  2t  3 ,e = 8t +2 If t = 1s e= 10V)
dt


Q2
Define coefficient of mutual inductance. On what factors does it depend?
Ans. The coefficient of mutual inductance of two coils is defined as the emf induced in the secondary
coil when the rate of change of current in primary coil is 1 A/s.
It is (i) directly proportional to no of turns in both the coils (ii) inversely proportional to the distance
between the coils (iii) orientation between the coils (iv) proportional to area of cross section of the
coils.
Q3
A cylindrical bar magnet is kept along the axis of a circular coil and near it as shown in the fig. Will
there be any induced current at the terminals of the coil when the magnet is rotated a) about its own
axis b) about an axis perpendicular to the length of the magnet?
N
S
Fig (i)
Fig(ii)
Ans.
Fig. (i) No emf will be induced, as these is no change in flux.
Fig (ii) Yes, Φ changes continuously. So emf is induced in the coil.
Q4
Define the term 'mutual inductance' between the two coils.
Obtain the expression for mutual inductance of a pair of long coaxial solenoids each of
length l and radii r1 and r2 (r2 >> r1). Total number of turns in the two solenoids are N 1 and
N2, respectively.
Ans
The ratio of magnetic flux passing through one coil to the current passing through the other is
known as mutual inductance between the two coils.
Suppose a current I is passed through the inner solenoid S1.
A magnetic field B=μ0n1I is produced inside S1 , whereas the field outside it is zero.
The flux through each turn S2 is Bπr12=μ0n1 I πr12
The total flux through all the turns in a length l of S2 is
ϕ=(μ0n1 I πr12)n2l= (μ0n1n2πr12l) I
⇒M=μ0n1n2πr12l
Q5
(a) Define self-inductance. Write its S.I. units.
(b) Derive an expression for self-inductance of a long solenoid of length l, cross-sectional area
A having N number of turns.
Ans.
(a) The phenomenon in which emf is induced in a single isolated coil due to change of flux
through the coil by means of varying the current through the same coil is called selfinductance. S.I unit of inductance is Henry.
(b) Magnetic field B inside a solenoid carrying a current i is
.
B=
Let n be the number of turns per unit length.
Where,
N is total number of turns
l is the length of the solenoid
Inductance,
Substituting, we obtain
Substituting the value of B, we obtain
Inductance L of a solenoid is:
5 MARKS QUESTIONS
Q1
(i)State Faraday’s laws of electromagnetic induction.How the Lenz’s law is in accordance with law of
conservation of energy? Explain.
(ii) A 12V battery is connected to a 6Ω; 10 H coil through a switch drives a constant current in the circuit. The
switch is suddenly opened. Assuming that it took 1ms to open the switch calculate the average emf induced
across the coil.
Ans. (i) Correct statement and explanation
(ii initial=2A I final= 0
=-Ldi/dt = 20000V)
Q2
State the working of a.c. generator with the help of a labelled diagram.
The coil of an a.c. generator having N turns, each of area A, is rotated with a constant angular
velocity ω. Deduce the expression for the alternating emf. Generated in the coil.
What is the source of energy generation in this device?
Ans.
AC Generator:
Principle − Based on the phenomenon of electromagnetic induction
Construction:
Main parts of an ac generator:
• Armature − Rectangular coil ABCD
• Filed Magnets − Two pole pieces of a strong electromagnet
• Slip Rings − The ends of coil ABCD are connected to two hollow metallic rings R 1 and R2.
• Brushes − B1 and B2 are two flexible metal plates or carbon rods. They are fixed and are
kept in tight contact with R1 and R2 respectively.
Theory and Working − As the armature coil is rotated in the magnetic field, angle θ between
the field and normal to the coil changes continuously. Therefore, magnetic flux linked with the
coil changes. An emf is induced in the coil. According to Fleming’s right hand rule, current
induced in AB is from A to B and it is from C to D in CD. In the external circuit, current flows
from B2 to B1.
To calculate the magnitude of emf induced:
Suppose
A → Area of each turn of the coil
N → Number of turns in the coil
→ Strength of magnetic field
θ → Angle which normal to the coil makes with at any instant t
∴ Magnetic flux linked with the coil in this position:
= NBA cos θ = NBA cos ωt …(i)
Where, ‘ω’ is angular velocity of the coil
As the coil rotates, angle θ changes. Therefore, magnetic flux Φ linked with the coil changes
and hence, an emf is induced in the coil. At this instant t, if e is the emf induced in the coil,
then
∴e = NAB ω sin ωt
The generator converts the mechanical energy into electrical energy. The mechanical energy
may be obtained from the rotation of the turbine associated with the generator. The turbine in
turn, may be working by the kinetic energy of running water, wind or steam.
HOT QUESTIONS
Q1
A rectangular conductor LMNO is placed in a uniform magnetic field of 0.5 T. The field is
directed perpendicular to the plane of the conductor. When the arm MN of length of 20 cm is
moved towards left with a velocity of 10 ms−1, calculate the emf induced in the arm. Given the
resistance of the arm to be 5 (assuming that other arms are of negligible resistance) find the
value of the current in the arm.
Solution:
Let ON be x at some instant.
The emf induced in the loop = e.
Q2
A wheel with 8 metallic spokes each 50 cm long is rotated with a speed of 120 rev/min in a
plane normal to the horizontal component of the Earth’s magnetic field. The Earth’s magnetic
field at the place is 0.4 G and the angle of dip is 60°. Calculate the emf induced between the
axle and the rim of the wheel. How will the value of emf be affected if the number of spokes
were increased?
Ans.
This component is parallel to the plane of the wheel. Thus, the emf induced is given as,
The value of emf induced is independent of the number of spokes as the emf’s across the
spokes are in parallel. So, the emf will be unaffected with the increase in spokes.