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Lecture 13: Advanced
Examples
• Selected
examples taken from Problem Set 4
• Magnetic Potential (as an example of working
with non-conservative fields)
• Plus Maxwell’s Equations (as a concise
example of the power of vector calculus)
Vector Areas (Problem 4.1)
B
z
z
C
OA
y
A
B=(1,1,1)
B
x
O
y
C=(0,2,0)
C
O
x
z
A=(1,0,0)
C
Vector Area of Triangle OAB
B
y
O
A
Vector Area of Triangle OBC
x
Maximum Projected Area
Area Projected in direction of this unit vector
Solid Angle (Problem 4.2)
• Angle (in radians) is dS/r where S
is a circle of radius r centred
on the origin
z
• Solid Angle (in steradians) is
dA/r2 where dA is a sphere of
radius r centred on the origin
y
x
• Applying divergence theorem to red volume
since on sides of closed volume,
field is everywhere parallel to the
sides so surface integrals are zero
•
•Solution to this “paradox”: divergence is non-zero at the origin: applying definition
of divergence to small sphere around origin
and only a fraction
of these field lines emerge into the cone; therefore
divergence theorem works, if origin (source of field lines) is properly treated
Divergence (Problem 4.3)
• Heat up body so it expands by linear factor
• Note fractional increase in volume is
• Divergence is a measure of local expansion/contraction
Centre of mass of body:
doesn’t move as body
expands
Curl (Problem 4.6)
• Consider first, Stokes Theorem applied
to a simple circular loop in the
z=0 plane
z
y
Surface S has cylindrical sides + a circular
cap (taken to be at +ve infinity)
along sides of cylinder
since
is parallel to sides
on cap
x
• Note that double
integral result is the
same however the
loop is wrapped
around the cylinder,
so result is always
Magnetic Potential
• In the curl-free region can define a magnetic potential
such that
where is the cylindrical
polar angle.
as required by
Ampere’s
Theorem
Since In 2D polars
recalling that
• This scalar potential must now be a multi-valued function of
position
• (i) If loop doesn’t enclose wire
• (ii) if loop goes round wire once
• (iii) if loop goes round wire n times
wire carrying current
out of the paper
Maxwell’s Equations
“No magnetic monopoles”
“Gauss’s Theorem”
“Faraday’ s Law” since using
Stoke’s Theorem
emf induced is rate of change of magnetic flux
through the circuit
But “Ampere’s Law”
because
Recalling “continuity equation”
of Lecture 8
if this were true
capacitors couldn’t
charge up!
Maxwell’s Displacement
Current
surface
• Maxwell added
Displacement Current
(4)
cutting space
between
capacitor
plates
I
• Now take
• which is a continuity equation allowing
charge to flow
surface cutting
wire
... and, applying Stoke’s Theorem to get
, and hence , we know that
we now get the same answer for
for a surface passing through
the plates of the capacitor (with zero ) as for a surface, with the
same rim, cutting through the current-carrying wire (with non-zero )
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