lesson 7 electrostatics in materialssdyang/courses/em/lesson07... · 2011-04-10 · introduction...
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Lesson 7Electrostatics in Materials
楊尚達 Shang-Da YangInstitute of Photonics TechnologiesDepartment of Electrical EngineeringNational Tsing Hua University, Taiwan
Introduction
Materials consist of atoms, which are made up of nuclei (positive charges) and electrons (negative charges).The “total” E-field must be modified in the presence of materials.Discuss the field behaviors inside some material, and on the interface of different materials.
Classification of materials − Classical view
Conductors: Electrons in the outermost shells of the atoms, very loosely held, can freely migrate due to thermal excitation ( ). These electrons are sharedby all atoms.Dielectrics: Electrons are confined within the inner shells, can hardly migrate.Semiconductors: Electrons are moderately confined, movable when applying an external electric field. Free electron density (conductivity) can be changed by doping or bias voltage.
23kTEk =
Classification of materials − Quantum view (1)
Classification of materials − Quantum view (2)
Electric properties of materials depend on the band structure, and how they are filled by the electrons (Pauli’s exclusion principle).
Outline
Static E-field in the presence of conductorsStatic E-field in the presence of dielectricsGeneral boundary conditions
Sec. 7-1Static E-field in the Presence of Conductors
1. Inside a conductor2. Air-conductor interfaces3. Example: Conducting shell
pushes free charges away from one another, modify by turns. The process continues until steady state is reached.
Inside a conductor
Ev
Ev
0=ρ0=E
v
sec10 19−≈t
0=t
Why no E-field inside a conductor? (1)
0≠Ev
1V
2V
,nonuniform is )(,21
rVVV
v>⇒
If
⇒ work has to be done to move free charges (contradiction!)
Why no E-field inside a conductor? (2)
Correspond to the lowest system energy, state of equilibrium
0=ρ0=E
v
Air-conductor interface-1
Charge is movable, not in equilibrium (contradiction!)
0≠tEIf
nnt EaEE vv==⇒ ,0
Air-conductor interface-2
tE
,0
=⋅∫CldEvv
00
0
=Δ⋅+Δ⋅=
⋅→Δ∫
wwE
ldE
t
habcda
vv
0=tE⇒
Air-conductor interface-3
⇒
, 0
εQsdE
S=⋅∫
vv
0
0
ερ SSE
sdE
sn
hS
Δ=Δ⋅=
⋅→Δ∫
vv
0ερ s
nE =
Example 7-1: Conducting shell (1)
Spherical symmetry:
)(REaE RRvv
=⇒
1. :iRR <
( )0
24)(ε
π QRRER =⋅=
=⋅∫3 S
sdE vv
204
)(R
QRER πε=⇒
:oi RRR <<
Example 7-1: Conducting shell (2)
2. ,0=Ev
0 2
=⋅∫SsdE vv
⇒ -Q induced on the inner surface⇒ +Q induced on the outer surface
Example 7-1: Conducting shell (3)
3. :oRR > =⋅∫1 S
sdE vv
0
2 )(4ε
π QQQRER R+−
==
204
)(R
QRER πε=⇒
Example 7-1: Conducting shell (4)
Conclusion
A “floating” conducting shell cannot isolate the effect of a charge.
Sec. 7-2Static E-field in the Presence of Dielectrics
1. Microscopic and macroscopic dipoles2. Polarization charge densities3. Electric flux density4. Example: Parallel-plate capacitor5. Example: Dielectric shell
Microscopic electric dipoles − Nonpolar molecules
H2 CH4
Non-polar covalent bond (electrons are equally shared by the atoms).
Symmetric arrangement of polar bonds.
Microscopic electric dipoles − Polar molecules
HF
Polar covalent bond
NH32 lone electrons
Asymmetric arrangement of polar bonds
H2O
Macroscopic electric dipoles-1
A dielectric bulk made up of a large number of randomly oriented molecules (polar or non-polar) typically has no macroscopic dipole moment in the absence of external E-field.
Macroscopic electric dipoles-2
In the presence of external E-field:
1. Non-polar molecule is polarized
2. Individual dipole moments are aligned
H2
Ev
dqpvv =
_
+Ev
E-torque Epvv //
Macroscopic electric dipoles-3
Each dipole (though electrically neutral) provides nonzero field, modifying the total E-field
(physics.udel.edu)
Strategy of analysis
It is too tedious to directly superpose the elementary fields:
( )θθπε θ sincos2
4)( 3
0
aaR
prE Rvvvv
+≈
Instead, we define polarization vector as:
vp
P k
v Δ≡ ∑
→Δ
vv
0lim
the kth dipole moment inside a differential volume Δv
⇒ Polarization charge ρp,
⇒ Electric flux density Dv
Polarization surface charge density-1
On the air-dielectric interface, is dis-continuous, net polarization exists.
Pv
Ev
Polarization surface charge density-2
Consider a differential parallelepiped Vbounded by Sb. The net polarization charge is:
( ) ( ) SaPSadnqQ nn Δ⋅=Δ⋅=Δ vvvv
Surface charge density:
⇒ΔΔ
= ,SQ
psρ
)mC( 2nps aP vv
⋅=ρ
Differential volume
Polarization volume charge density-1
In the interior of a dielectric, net polarization charge exists where is inhomogeneous:P
v
Polarization volume charge density-2
Consider two polarization vectors , in a differential volume V bounded by S.
1Pv
2Pv
Polarization charges on top and bottom surfaces:
( )∫ ⋅=+S npsps dsaPQQ
21vv
( )∫∫ ⋅∇=⋅=VS
dvPsdP
vvv
psρ
divergence theorem
Polarization volume charge density-3
By definition:
⇒
To maintain the electric neutrality:
( ) ( ) ( )∫∫ ⋅∇−=⋅∇−=+−=VVpspsp dvPdvPQQQ 21
vv
∫= V pp dvQ ρ
( ) )mC( 3Pp
v⋅∇−=ρ
Comment-1
can be regarded as a special case of , where( )Pp
v⋅∇−=ρ
nps aP vv⋅=ρ
∞→⋅∇ Pv
∞→⋅∇ Pv
Comment-2
Equivalent charge densities:
can be substituted into the formulas:
to evaluate the influence of polarized dielectrics.
( )Pp
v⋅∇−=ρ,nps aP vv
⋅=ρ
∫ ′′
′′
=V
vR vd
rrRrarE
20 ),(
)(4
1)( vv
vvvv ρ
πε
vdrrRrrV
Vv ′
′′
= ∫ ′ 0 ),(
)(4
1)( vv
vv ρ
πε
Electric flux density-definition
Total E-field is created by free & polarization charges:
0ερ
=⋅∇ Ev
0ερρ pE
+=⋅∇
v( )P
v⋅∇−
( ) ( )PEvv
⋅∇−=⋅∇⇒ ρε 0
ρ=⋅∇ Dr
⇒PEDvvv
+= 0ε (C/m2)
…Gauss’s lawQsdDS
=⋅∫ vv
only free charge
Electric flux density - Application (1)
For linear, homogeneous, and isotropicdielectrics, the polarization vector is proportional to the external electric field:
EP e
vvχε 0=
0EeE vv=
qdep vv =
02EeE vv=
qdep 2vv =
0=Ev
0=pv
Susceptibility, independent of magnitude, position, direction of E
v
Electric flux density-application (2)
( )EPED e
vvvvχεε +=+= 100
Ee
vχε 0
EDvv
ε=⇒
( ) 01 εχε e+= …permittivity of the dielectric
In this way, a single constant replaces the tedious induced dipoles, polarization vector, equivalent polarization charges in determining the total electric field.
ε
Example 7-2: Physical meanings of D, E, P
: free charge: polarization charge
: total charge
Dv
Pv
Ev
0ε
Example 7-3: Dielectric shell (1)
Spherical symmetry:
)(RDaD RRvv
=⇒
For all :0>R
( ) QRRDR =⋅= 24)( π
=⋅∫SsdD
vv
24)(
RQRDR π
=⇒
freecharge
Dv
Example 7-3: Dielectric shell (2)
By ,EDvv
ε=
,4
)( 2RQRER πε
=⇒
⎩⎨⎧ <<
=otherwise ,
for ,
0
00
εεε
εRRRir
Ev
Example 7-3: Dielectric shell (3)
decay slower
Example 7-3: Dielectric shell (4)
By ,
EDvv
ε=
EDPvvv
0ε−=
( )⎪⎩
⎪⎨⎧ <<−
=
−
otherwise ,0
for ,4
1
)(
021 RRR
RQ
RP
ir
R
πε
Pv
Example 7-3: Dielectric shell (5)
By ,Pv
nps aP vv⋅=ρRn aa vv =
Pv
Rn aa vv −=
( )
( )⎪⎪⎩
⎪⎪⎨
⎧
=>−
=<−−
=
−
−
oo
r
ii
r
ps
RRR
Q
RRR
Q
for ),0(4
1
;for ),0(4
1
21
21
πε
πε
ρ
Example 7-3: Dielectric shell (6)
By ,SdsQ psS psps ρρ == ∫
( )( )⎪⎩
⎪⎨⎧
=>−
=<−−
=
−
−
or
ir
ps
RRQ
RRQ
Q
for ),0(1
;for ),0(11
1
ε
ε
_
_ _
_
+
+
+ +
+ +
+ +
_ _
_ _Ev
Polarization charges create a radially inward E-field to reduce the total field.
Sec. 7-3General Boundary Conditions
1. Tangential boundary condition2. Normal boundary condition
General interface-1
tE
,0
=⋅∫CldEvv
( ) ( ),0
21
0
=Δ⋅+Δ−⋅=
⋅→Δ∫
wEwE
ldE
tt
habcda
vv
⇒ tt EE 21 =
General interface-2
⇒
( )( )S
SaDaD
sdD
s
nn
hS
Δ⋅=Δ⋅+⋅=
⋅→Δ∫
ρ1221
0
vvvv
vv
QsdDS
=⋅∫ vv
( ) sn DDa ρ=−⋅ 212
vvv
snn DD ρ=− 21⇒
only free charge
Comment-1
0=tE
0ερ s
nE =
tt EE 21 =
( ) sn DDa ρ=−⋅ 212
vvv{ } 0, 22 =DE
vv
If M2 is a conductor:
Comment-2
Only free surface charge density counts in:
( ) sn DDa ρ=−⋅ 212
vvv
If the two interfacing media are dielectrics:
nns DD 21 ,0 =⇒=ρ
Comment-3
tt EE 21 =
( ) sn DDa ρ=−⋅ 212
vvv
remain valid even the E-fields are time-varying.