6.1 Magnetization 6.2 The Field of a Magnetized Object 6.3 The …perera/EM/EM_lec06.pdf · 6.2 The...
Transcript of 6.1 Magnetization 6.2 The Field of a Magnetized Object 6.3 The …perera/EM/EM_lec06.pdf · 6.2 The...
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6.1 Magnetization
6.2 The Field of a Magnetized Object
6.3 The Auxiliary Field
6.4 Linear and Nonlinear Media
6 Magnetization
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Diamagnets, Paramagnets, Ferromagnets
• Magnetism in a material is a result of tiny dipole moments in atomic scale.
• Usually they each other due to their random orientation.• In an external magnetic field those dipoles align and the medium
becomes polarized (or magnetized)
• Paramagnets: magnetization is parallel to the applied magnetic field
• Diamagnets: magnetization is opposite to the applied magnetic field
• Ferromagnets: retain a substantial magnetization indefinitely after the
external field has been removed.
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Torques and Forces on Magnetic Dipoles
A magnetic dipole experiences a torque in a magnetic field :
Torque on a rectangular current loop in a uniform magnetic field
ˆsin
ˆ ( )sin
ˆ sin
N aF i
a bIB i
mB i
θ
θ
θ
=
=
=
�
magnetic dipole moment
m abI=
N m B= � ��
F�
F�
B�
1s
•This torque lines the magnetic dipole up
parallel to �: paramagnetism.
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In a uniform magnetic field, the net force is zero.
( ) ( ) 0F I dl B I dl B= × = × =∫ ∫� �� � �
� �
But in a nonuniform magnetic field, there is a net force
For an infinitesimal loop with dipole moment m in a field B
{ }
ˆ ˆˆ ˆ( ( )) (0, ,0) (0, , ) (0, , ) (0,0, )
ˆ ˆ = (0, ,0) (0, , ) (0, , ) (0,0, )
F I dl B r I dyy B y dzz B z dyy B y dzz B z
I dyy B y B y dzz B z B z
ε ε
ε ε
= × = × + × − × − ×
× − + × −
∫ ∫
∫
� � � ��� �
� � � ��
(0, ,0) (0,0, )
2
ˆ ˆ = -
ˆ ˆ ˆ ˆˆ ˆ =
ˆ ˆ
y z
yx xz
yx
B BI dyy dzz
z y
BB BB B BI z y m y x x z
y z y y z z
BBm y x
y y
ε ε
ε
∂ ∂ × + × ∂ ∂
∂ ∂ ∂∂ ∂ ∂− = − − −
∂ ∂ ∂ ∂ ∂ ∂
∂∂= −
∂ ∂
∫� �
� �
( )
ˆ ˆ ˆˆ ˆ
ˆ ˆ ˆ . 0
ˆ ˆ ˆ ( . ) ( . ) (
yx x xz z
x x x
x
BB B BB Bx z m y x z
z z y y z z
B B Bm x y z B
x y z
m B m x B mx B m
∂ ∂ ∂ ∂∂ ∂− + = − + +
∂ ∂ ∂ ∂ ∂ ∂
∂ ∂ ∂= + + ∇ = ∂ ∂ ∂
= ∇ = ∇ = ∇ = ∇
�∵
� �. )B�
( )F m B= ∇ ⋅� ��
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Effect of a Magnetic Field on Atomic Orbits
Assume the orbit of an electron a circle of radius r
e+ r�
r�
ẑ
2 2since its period 1
c
rT
v w
π π= =
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In a magnetic field
( assume r = constant)
B�
22
20
1 ''
4e
e vev B m
rrπε+ =
22' ( ' )
( ' )( ' )
e
e
mev B v v
r
mv v v v
r
= −
= + −
( 2 )v v v v v′ ′∆
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Bound Currents
for a single dipole
0
2ˆ
4mA
µπ
×=��r
r
m�
', m Md r rτ ′= = −��
r
0
0
0
0 0
2
'
'
ˆtotal ( ) '4
1 ( ' ) '
4
1 1 [ ( ' ) ' ' ( ) ]
4
1 1 ( ' ) ' ( )
4 4
MA r d
M d
M d M d
M d M da
µτ
π
µτ
πµ
τ τπ
µ µτ
π π
×=
= ×∇∫
= ∇ × − ∇ ×∫ ∫
= ∇ × + ×∫ ∫
∫�� �
�
� �
� � ��
r
r
r
r r
r r
2 2 21 1 ˆ1 ( ' ) '( ' ' ' )
' '
r r r r
r r r r
∧ ∧ − − ∇ = ∇ = ∇ = − = =
− −
∵ � � � �r
r r r r
1 1 1' ( ) ( ' ) ( ' )M M M
∇ × = ∇ × − × ∇
� � �∵
r r r
6.2 The Field of a Magnetized Object
magnetization M magnetic dipole moment per unit volume≡�
r
r
r ′
O
( ) problem 1.61b Ad A daτ∇ × = ×∫ ∫� � �
∵ �
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Potential (and field) of a magnetized object is the same as would be produced by a
bound volume current �� �� = �′ × (��) and a surface current � �
� = (��) × ��
0 0
0 0
'1 1( ) ( ' ) ' ( )4 4
( ) ( )( ) ' '
4 4b b
V S
A r M d M da
J r K rA r d da
µ µτ
π π
µ µτ
π π
= ∇ × + ×∫ ∫
′ ′= +∫ ∫
� � �� ��
� �� � �
r r
r r
surface chargecf: in ES volume charge b Pρ = −∇ ⋅ ˆb P nσ = ⋅�
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Example: Magnetic field of a
uniformly magnetized sphere
ẑ
Since is uniform:
ˆˆ0 sinb bJ M K M n M θ ϕ= ∇× = = × =� � � �
0
ˆsin
2 ˆfield produced ( )3
K v R
B zR z inside
σ σω θ ϕ
µ σ ω
= =
=
� �
�
02 ( )3
inside B M uniformµ=� �
34outside field is same as that of a dipole 3
m R Mπ=��
This is similar to the currents on a uniformly charged rotating spherical shell (Ch. 5)
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Physical Interpretation of Bound Currents
For uniform
b b
m Mat
Ia I Mt
K t K M
=
= ⇒ =
= ⇒ =
��
ˆbK M n= ×� �
Internal loop currents cancel
Dipole
currents
Only the current flowing
around the boundary remains
I
Each charge moves in a small loop, but the net effect is a macroscopic
current flowing over the surface.
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For non-uniform internal currents do not cancel.
A nonuniform M in y contributes to a current in x direction.
A nonuniform M in z, also contributes to
current in x direction
[ ( ) ( )]
x z z
z
I M y dy M y dz
Mdydz
y
= + −
∂=
∂
x zx
I MJ
dydz y
∂= =
∂
yx
MJ
z
∂= −
∂
( )yz
b x
MMJ
y z
∂∂∴ = −
∂ ∂
0b bJ M J⇒ = ∇ × ∇ ⋅ =� �
Current density:
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6.3 The Auxiliary Field �
0
0
In Electrostatics
b fb
f
P E
D E P D
ρ ρρ
ε
ε ρ
+= −∇ ⋅ ∇ ⋅ =
= + ∇ ⋅ =
� �
� � � �
In a magnetized medium there are bound currents �� = � ×
So the total current is sum of bound currents and other (free) currents � = �� + ��
So the Ampere law: 0 0 0
1 0
or 0
( ) ( )
( )
1 where
b f f
f
f
B J J J M J
B M J
H J H B M
µ
µ µ µ
µ
∇ × = = + = ∇ × +
⇒∇ × − =
∇ × = = −
� � � � � �
� � �
� � � � �
,fH dI I enc⋅ =∫� �
�In integral form Total free current passing through an Amperian loop
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0
0above below
encB d I
B B K
µ
µ
⋅ =
⇒ − =
∫� �
��ℓ�
Boundary Conditions
Since
,fH dI I enc⋅ =∫� �
�
0 above belowB da B B⊥ ⊥⋅ = ⇒ =∫
� ��
0
1 H B Mµ
= −� � �
( )
above above below below
above below above below
H M H M
H H M M
⊥ ⊥ ⊥ ⊥
⊥ ⊥ ⊥ ⊥
+ = +
− = − −
0above below fH H Kµ− =� �
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linear and Nonlinear Media
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6.4. Magnetic Susceptibility and Permeability
In Electrostatics
Polarization
Similar parameters can be defined for magnatostatics. In paramagnetic and diamagnetic materials magnetization is sustained by the applied magnetic field and proportional to B
As a convention magnetization is written in terms of H,
�� magnetic susceptibility
0 ( )eP E linear dielectricε χ=� �
00
( ) (1 )eE D Eρ
ε ε ε χε
∇⋅ = = = +� � �
��mM = χ H
��>0 paramagnets, ��
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Example: Magnetic field of an infinite length solenoid filled with
linear material of susceptibility ��
Since B depends on bound currents, apply ampere Law for H
I
ẑ
n turns
per unit
length
0
ˆ
ˆ(1 )
enc
m
H dl I
H nIz
B nIkµ χ
⋅ =
=
= +
∫�
�
ˆˆ ˆˆ( )b m mK M n nI z n nIχ χ ϕ= × = × =� �
Bound surface currents
If the medium is paramagnetic (��>0) and field is (slightly) enhanced and if the medium diamagnetic field is reduced.
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6.4.2 Ferromagnetism
Paramagnetic
Ferromagnet
no B
applied remove 0=M�
no with
B�
B�
0M =�
B�
B�
B�
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6.4.2(2)
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Iron is ferromagnetic,
but if T>Tcurie ( 770。 for iron )
curie point
it become a paramagnetic.
This is a phase transition.
6.4.2(3)