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Lecture 151
Calculation of Torque
•
Suppose the coil has width
b
(the side
we see) and length
a
(into the screen).
The torque
about the center
is given by:
2 sin
2
b
τ r F F
F IaB
sin
sin
Iab B
IAB
area of loop
• Define
magnetic dipole moment
by
IAn
B
where
n
is normal to the
loop with
RHR
along I
.
E
p
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View Full Document Lecture 152
Potential Energy of Dipole
•
Work must be done to change the
orientation of a dipole (current loop)
in the presence of a magnetic field.
•
Define a potential energy
U
(with
zero at position of max torque)
corresponding to this work.
θ
τdθ
U
90
θ
θdθ
μB
U
sin
Therefore,
θ
θ
μB
U
cos
θ
U
cos
B
x
F
F
.
B
U
Lecture 153
Van Allen belts
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View Full Document Lecture 154
Hall Effect
• A conducting strip in crossed
E
and
B
fields Applied
E
along
the strip leads to a charge
buildup on the sides of the strip
and thus an electric field
E
H
develops perpendicular to both
the applied
E
and
B
.
Determines the sign and
number of carriers.
Measures
B
.
E
H
E
H
Lecture 155
Carrier Sign and Density from Hall Effect
Sign and density of charge carrier
is determined at equilibrium
0
H
d
H
H
H
d
q E
q v
B
E
E d
V
v
B
Bd
Bd
and
/
dd
HH
JI
n
v q
v qA
BId
BI
n
V qA
V q A d
Hall
voltage
E
H
E
H
B
d
A
nq
I
V
H
)
/
(
•
This relation can also be
used to detect an unknown
field strength B.
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This note was uploaded on 04/23/2011 for the course PHYS 241 taught by Professor Wei during the Spring '08 term at Purdue UniversityWest Lafayette.
 Spring '08
 Wei

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