M.Sc. (Semester II) (CBCS) Examination Nov/Dec-2018
Physics (Materials Science)
ELECTRODYNAMICS
Time: 2½ Hours Max. Marks: 70
Instructions: Q 1 and Q 2 are compulsory.
Attempt any three questions from Q. 3 to 7.
All questions carry equal marks.
Use of non programmable calculator is allowed.
Q.1 Choose correct alternatives. 06
When a negative charge is placed at the centre of the sphere then the
direction of electric field on the Gaussian surface is
Radially outward
Radially inward
Along the tangent to the surface
None of the above
A monochromatic electromagnetic waves means that
The field strength at a point varies with time according to sine or
cosine functions
The wave always travels in the same direction
Electric filed vector E lies in one direction only
Magnetic field vector B must be ⊥lar to the direction of propagation.
When angle of incidence is greater than Brewster's angle then the
reflected ray suffers a phase change of

0
The displacement current can be represented as
For good conductors, skin depth varies inversely with power of
frequency.
One Two
Half Three
The power radiated by an electric dipole is proportional to the angular
frequency by


State True or False/ Fill in the blanks. 08
The electric flux through any open surface is a measure of the total
charge inside.
The quadrupole potential goes off like
1

For a uniformly polarized sphere, the electric field inside the sphere is
uniform.
Magnetic forces do work.
Page 2 of 2
SLR-VN-382
In a uniform magnetic field B the torque on any localized current
distribution is, N M × B .
The microscopic form of ohm's law is, J bE .
In TEM waves, both EZ 0 and BZ 0.
The total power radiated by electric dipole is independent of the radius of
the sphere.
Q.2 Write a short note on:
Show that the magnetic field of a dipole can be written in coordinate free form
as B dip


1
r3 m . r r − m
05
Obtain the continuity equation and interpret it. 05
Sketch the electric field lines of a "pure" and "physical" dipole. 04
Q.3 Show that the potential of a polarized object is the sum of potential produced
by a volume bound charge density and a surface bound charge density.
08
Obtain the multipole expansion of scalar potential, at a distance due to a
localized charge distribution.
06
Q.4 Derive the expression for magnetic vector potential of a localized current
distribution. Explain the magnetic vector potential due to monopole and dipole
contributions.
08
Find the magnetic field at a distance Z above the center of circular loop of
radius which carries a steady current I. Write the magnetic field at the
center of a circular loop.
06
Q.5 State and prove Poynting's theorem 08
What are the general boundary conditions of electrodynamics? 06
Q.6 Discuss the case of reflection and transmission at normal incidence and show
that the sum of reflection coefficient and transmission coefficient is
unity.
10
Write the Maxwell's equations in matter. 04
Q.7 Derive the formula for total power radiated by an oscillating electric dipole. 10
Write a note on skin depth of conductors. 04