1. dielectric polarization 2. method of images 3...
TRANSCRIPT
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Lecture 7 Electrostatics (4)
1. Dielectric Polarization
2. Method of Images
3. Electrostatic Energy
4. Boundary Conditions
5. Coding Example
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Dielectric polarization- Negative/positive charges oriented toward the positive/negative electrode- Charges do not move freely in the material but reoritented.
1. Dielectric Materials and Permittivity
Fig. Polarization in materials [www.pcbdirectory.com]
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Polarization mechanisms in dielectric materials- Electron displacements- Ion displacements- Dipole orientation- Space (interfacial) charge displacements
Fig. Polarization mechanisms [Knowles]
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Polarization charge density- The effect of polarization is modelled using charge densities.- Polarization charges cannot move around. → Bound charges
20
20
0 0
polarization dipole moment per unit volume
ˆ1 (single dipole)4
ˆ1 (distributed dipoles)4
1 1 ( )4 4
ˆ (polarized bound surface charge density)
V
S V
b
VR
V dVR
dV dVR R
P
p R
P R
P S P
P n
(polarized bound volume charge density)b P
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Electric flux density in dielectric materials
0
0
0
: bound charge density due to polarization
: freely moving charge (fee charge) density
(Gauss's law)
( )
(electric flux density)
b f
b
f
b f f
f
f
E P
E P
D E P
D
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Permittivity of dielectric materials
0 0 0 0
0
0
(1 )
: electric susceptibility
(1 ) (permittivity)
1 (dielectric constant or relative permittivity)
e e
e
e
r e
D E P E E E E
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Example: dielectric in a parallel-plate capacitor
0
0| | | | Electric flux density is increased in the dielectric.
E E
D D
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Example: dielectric in an external field
0
00
0
(continuity of electric flux; Gauss's law)
| | | |
D D
E E
E E
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2. Method of Images
1
2
1 1 2 2
1 2
1 23 31 2
( , , ) : field point
Original charge: at (0,0, )
Image charge: at (0,0, )
,
1 1( )4 4
ˆ ˆ1 14 4
q q
x y z
q d
q d
q qV V VR R
q qVR R
r
r
r
R r r R r r
r
R RE
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Example: a point source in a right-angle conductor
1
2
3
4
1 1 2 2 3 3 4
( , , ) : field point
at ( , ,0) : the original point charge
at ( , ,0) : an image charge
at ( , ,0) : an image charge
at ( , ,0) : an image charge
, , ,
x y z
q a b
q a b
q a b
q a b
r
r
r
r
r
R r r R r r R r r R r 4
1 2 3 4
31 2 43 3 3 31 2 3 4
1 1 1 1( )4
ˆˆ ˆ ˆ
4
qV rR R R R
qVR R R R
r
RR R RE
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Example: problems requiring an infinite number of images
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Example: a point charge outside a conducting sphere
1 1
2
aQ qL
aL
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3. Electrostatic Energy
1 1 1
11 2 2 2 2 2
2 1
1 1 21, 2 3 2 3 2 3
2 1 3 1 3 2
1 2 3, 2 3
1
1
only: 0 since no field exists before
, : 4 | |
, :4 | | 4 | | 4 | |
, , ..., : ...
14 | |
N N
ij
ii jj
q W q V
qq q W W q V q
q q qq q q W W W q q
q q q q W W W W
r r
r r r r r r
r r1
1 1
1 1,2 4 | |
N
i
N Nj
i i ii ji j
j i
qq V V
r r
Energy stored in N point charges
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3. Electrostatic Energy
Energy stored in distributed charges
1
12
( )
1 1 12 2 2
N
i ii
i L s v
i
L s vL S V
W q V
q dL dS dV
V
W dL dS dV
r
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Electrostatic energy in terms of electric field
2
1 1( ) ( )2 2
( ) ( ) ( ) ( ) ( ) ( ) ( )
1 1 1 1( ) ( ) ( )2 2 2 2
1 (surface density of electric energy, J/m )2
1 2
v vV V
V V S V
s
v
W VdV VdV
V V V V V V V
W V dV dV V d dV
w V
w
D D
D D D D D D D D E
D D E D S D E
D n
D E
3(volume density of electric energy, J/m )
1 0 as 2
1( )2
S
V
V d V
W dV
D S
D E
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Example: a charged sphere
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Boundary conditions on a dielectric interface
4. Boundary Conditions
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1 2
1 2
1 2
1 2
1 21 2
1 2
( )
0 ,
n n s
n n s
n ns
n ns n n
D dS D D S Q S
D D
E E
E ED D
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Boundary conditions on a dielectric-conductor interface
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Example: infinite charged plate
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Example: angles of electric field lines on a dielectric interface
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5. Coding Example
Example: angles of electric field lines on a dielectric interfaceWrite down a Python code for the stored energy for N point charges.Input data:N : number of point chargesqi (C), (xi, yi, zi) m : i-th point charge and positionOutput data:W (J): stored energy
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# Lecture 7 - Python coding example: Stored energy in n point charges# Input data:# n = number of point charges (maximum 100)# qi = charge (C) of the i-th charge# (xi,yi,zi) = position (m) of the i-th charge# Output data:# w = stored energy (J)
from math import *e0=8.853e-12;pi=3.141593x=[0]*100;y=[0]*100;z=[0]*100;q=[0]*100while True:
n=int(input('Number of point charges='))for i in range(n):print('For charge no.',i)q[i]=float(input('Charge: q(C)='))x[i],y[i],z[i]=map(float,input('Position: x,y,z(m)=').split())
w=0for i in range(n):sum=0for j in range(n):
if(j == i):continue
else:sum=sum+q[j]/sqrt((x[i]-x[j])**2+(y[i]-y[j])**2+(z[i]-z[j])**2)
w=w+q[i]*sumw=w/(4*pi*e0)/2print('Stored: W(J)=',w)
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'''Number of point charges=3For charge no. 0Charge: q(C)=1e-10Position: x,y,z(m)=1 1 3For charge no. 1Charge: q(C)=2e-4Position: x,y,z(m)=3 -1 1For charge no. 2Charge: q(C)=4e-10Position: x,y,z(m)=4 1 -4Stored: W(J)= 0.00018318585243Number of point charges='''
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