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TUTORIAL 5
1.
What is the potential of the metal sphere at breakdown in Example 10 in Lectures?
2.
A parallel plate capacitor of plate dimensions 10 cm
×
15 cm and separation of 20 microns
is charged to 20 V. The dielectric constant of the filler is 3.2. Find the stored electrostatic
energy using
W
E
=
1
2
D
"
E
dv
#
V
$
.
3.
Why does
"#
J
=
0
for steady currents? Use
"#
J
=
0
to derive Kirchoff’s Current Law.
4.
The current density in a certain region is given in spherical coordinates by
J
=
a
2
r
a
r
.
(i) Find
"#
J
.
(ii) Find
dQ
dt
within a sphere of radius
R
.
5.
Use the definitions of resistance and capacitance to prove the formulae for series/parallel
addition of two resistors/capacitors.
6.
(SADIKU p. 219, problem 5.9)
A composite cylindrical conductor 10 m long consists of an inner core of steel of radius 1.5
cm and an outer sheath of copper whose thickness is 0.5 cm.
(a) Determine the resistance of the conductor.
(b) If the total current in the conductor is 60 A, what current flows in each metal?
(c) Find the resistance of a solid copper conductor of the same length and crosssectional
area as the sheath. Take the resistivities of copper and steel as
1.77
"
10
#
8
and
11.8
"
10
#
8
$
m
, respectively.
7.
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 Spring '07
 
 Electromagnet

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