Electricity and Magnetism (PHYS2239)

BSc, Physics - PHYS

Semester: Second Semester

Level: 200

Year: 2017

Answers (200 )
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PS:).
4)
THE UNIVERSITY OF BAMENDA
FACULTY OF SCIENCE PHYSICS DEPARTMENT
CONTINUOUS ASSESSMENT
COURSE: PHY53225 - PRINCIPLES OF ELECTRICITY AND MAGNETISM
DATE: Saturday,
8
TH
July 2017 DURATION: 2 hours
Permittivity of free space
ɛ
o
= 8.85 x 10
-12
F/m; permeability of free space µ
o
= 4 x 10
-7
N/A
2
;
elementary charge
e =
1.60 x 10
-19
C; mass of electron m
e
= 9.101 x 10
-31
kg; coulomb constant k =

=
8.99 x 10
9
Nm
2
/C
2
1)
a) State
two properties
of electrostatic field lines.
b) Sketch the electric field lines for an electric dipole.
c) The electric potential at a point is given by
V
(x,y) =x
2
+x -3xy
2
+ 7.
Calculate the electric field
at that
point.
d) A capacitor is connected to a battery. What happens to the energy when a dielectric plate is inserted? Explain your
answer.
(8 marks)
2) Shown in figure 1 is the cross-section of a conducting sphere of radius
surrounded by a very thin conducting spherical
shell of radius R. The inner sphere carries a charge +Q
0
uniformly distributed with a charge density
p
and the outer
shell carries a charge —Q
0
.
a) On the figure, indic ate the distribution of charge on the inner sphere.
b) Express
p
as function of
R.
c) Using Gauss's Law, find the strength of the electric field
E(r)
at given a point, as a
function of r, where
r
is the distance of the point from the center of the sphere:
i) for r
2
ii) for r >
R
(7
marks)
Figure 1
3)
Figure 2 is a circuit with four branches. The currents
I
1
and
I
2
,
and the emf ɛ
1
are unknown. The current flowing from
c
to
e
is 2A.
a) What is the current flowing through each of the 12Ω resistors?
b) Wh at i s th e v a lu e of t h e e rn f
ɛ
1
?
c) What are the currents
I
1
and
I
2
, and in what directions do they flow?
d) Is the emf
ɛ
2
providing energy or is it absorbing energy? What is the
power it is providing or absorbing?
(7 marks)
Consider eight of the ions of Cl
-
and Na
+
on a crystal lattice of common
salt as shown in figure 3.
The ions are located at the vertices of a cube of
side measuring 2.82 x 10
- 1 0
m as shown in figure 3. Calculate the
magnitude of the electric force that seven of these ions exert on the
eighth.
(8 marks)