PHY/VI/11 Student's Copy
2 0 1 8
6th Semester
PHYSICS
ELEVENTH PAPER
Electromagnetic Theory
Pre-revised
Full Marks 55
Time 2½ hours
PART A—OBJECTIVE
Marks 20
The figures in the margin indicate full marks for the questions
SECTION—A
Marks 5
Tick the correct answer in the brackets provided 1×5=5
1. In inductor, energy is stored in the form of
electrical energy
magnetic energy
heat energy
No energy is stored in the inductor

2. Energy density of electric field in vacuum is given by (where E0 is
amplitude of electric field)
u E e0 0
2 u 1 E
2 0 0
2 e
u 1 E
4 0 0
2 e u e E B
3. When an electromagnetic wave incidents normally on dielectric surface, the
relation between reflection coefficient and transmission coefficient is
R R T 0
R T RT
4. Lorentz gauge condition is given by
5. If P be the average power radiated from an oscillating dipole and frequency
of the oscillation is doubled keeping all other variables constant, the new
average power radiated from the same dipole is
2P 4P
8P 16P
SECTION—B
Marks 15
Answer the following questions 3×5=15
1. Write a short note on displacement current.
2. What do you mean by Poynting vector? Write down the relation for the
same.
3. What do you mean by Brewster's angle? Hence obtain the relation for
Brewster's law.
4. Show that the magnetic scalar potential satisfies Laplace's equation.
5. Write the differences between Normal dispersion and Anomalous dispersion.
Marks 35
The figures in the margin indicate full marks for the questions
1. Write down the four Maxwell's equations. Derive any two relations.
OR
Show that the normal component of electric field in the boundary of two
different media is discontinuous by s
e0
where s is surface charge
density. 4
Show that displacement current density is given by
2. Obtain the wave equations satisfied by electric and magnetic fields in
vacuum and hence show that the speed of electromagnetic wave in
vacuum is given by c 1
The amplitude of electric field in an electromagnetic wave is
20 V/m. Calculate the amplitude of magnetic field 2
OR
Establish orthogonality and transverse nature of electromagnetic wave. 5
What do you mean by momentum of electromagnetic wave? Write down
the expression for it when the surface is perfect reflector. 2
3. Show that when an electromagnetic wave is incident on conducting
medium, the amplitude decreases exponentially. 7
OR
Consider an electromagnetic wave incident obliquely on a dielectric surface.
Prove the following (kinematic properties) 7
Frequency of the wave remains the same
Angle of incidence is equal to angle of reflection
Snell's law

4. What do you mean by Gauge transformation? Show that electric and
mragnreticr fields are gauge invariants using the transformations
simultaneously.
5
Starting with Maxwell's equation, show that magnetic field can be
written in the form
r r r
B Ñ ´ where
r
A is magnetic vector potential. 2
OR
Show that the total force experienced by a charged particle moving in
the region, where both magnetic and electric fields are present is
4
Show that the momentum of a charged particle in an electromagnetic
field is given by
r r r
p mv qA. 3
5. What do you mean by retarded potential? Discuss it quantitatively.

What do you mean by TE wave? What is cut-off wavelength? 3
OR
Write a note on Lorentz theory of dispersion and hence obtain Cauchy's
dispersion law. 7