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1. Electrical Dipole2. Dielectrics in Static Electric Field3. Electric Flux Density and Dielectric Constant 4. Boundary Conditions for Electrostatic Fields
1. A Dipole
A combination of two electric charges with equal magnitude and opposite sign is called a dipole.
+ -+q -q
d
The charge on this dipole is q (not zero, not +q, not –q, not 2q). The distance between the charges is d. Dipoles are “everywhere” in nature.
This is an electric dipole. Later in the course we’ll study magnetic dipoles.
Dipole
i2
i i
qˆE=k r
r
Example: calculate the electric field at point P, which lies on the perpendicular bisector a distance L from a dipole of charge q.
+ -+q -q
d
P
Lto be worked at the blackboard
3o
qdE
4 r
+ -+q -q
d
P
L
3o
qdE
4 r
Caution! The above equation for E applies only to points along the perpendicular bisector of the dipole.It is not a starting equation.
Electric Dipole in anExternal Electric Field
An electric dipole consists of two charges +q and -q, equal in magnitude but opposite in sign, separated by a fixed distance d. q is the “charge on the dipole.”
Earlier, I calculated the electric field along the perpendicular bisector of a dipole (this equation gives the magnitude only).
3o
qdE .
4 r
The electric field depends on the product qd.
Caution! This is not the general expression for the electric field of a dipole!
q and d are parameters that specify the dipole; we define the "dipole moment" of a dipole to be the vector
p qd,
where the direction of p is from negative to positive (NOT away from +).
+q -q
p
caution: this p is not momentum!
To help you remember the direction of p, this is on the equation sheet:
p q d, from to plus
E
A dipole in a uniform electric field experiences no net force, but probably experiences a torque…
+q
-q
pF+
F-
F F F qE qE 0.
There is no net force on the dipole:
E+q
-q
pF+
F-
½ d sin
If we choose the midpoint of the dipole as the origin for calculating the torque, we find
dsin dsinqE qE qdEsin ,
2 2
and in this case the direction is into the plane of the figure. Expressed as a vector,
p E.
½ d sin
Recall that the unit of torque is N·m, which is not a joule!
E+q
-q
pF+
F-
½ d sin½ d sin
The torque’s magnitude is p E sin and the direction is given by the right-hand rule.
What is the maximum torque?
Energy of an Electric Dipole in anExternal Electric Field
If the dipole is free to rotate, the electric field does work* to rotate the dipole.
E+q
-q
pF+
F-
initial finalW pE(cos cos ).
The work depends only on the initial and final coordinates, and not on how you go from initial to final.
*Calculated using , which you learned in Physics 23.zW d
If a force is conservative, you can define a potential energy associated with it.
Because the electric force is conservative, we can define a potential energy for a dipole. The equation for work
initial finalW pE(cos cos )
suggests we should define
dipoleU pE cos .
E+q
-q
pF+
F-
dipoleU pE cos
With the definition on the previous slide, U is zero when =/2.U is maximum when cos=-1, or = (a point of unstable equilibrium).
U is minimum when cos=+1, or =0 (stable equilibrium).
It is “better” to express the dipole potential energy asdipoleU p E.
Recall that the unit of energy is the joule, which is a N·m, but is not the same as the N·m of torque!
dipole moment ( - + )
Potential energy
Torque felt
Ep
EpV
In discussing dielectric materials
P
PN
Definition
dqp
+
- E
Eq
Eq
+-qq d
Polarization = # of dipole moment / unit volume
=
In dielectrics, there are no free charges, but bound charges, i.e. electric dipoles are present.
Dielectrics
LV
E 00
Electric field inside the parallel plate :
Now, insert a slab of dielectric modify the field to a new value E
2. Dielectrics in Static Electric Field
3. Electric Flux Density and Dielectric Constant
3m
C D
0 E
)( CQdsDdvDdvv v s
Gauss’s Law: The total outward flux of the dielectric displacement (or simply the outward flux) over any closed surface is equal to the total free charge enclosed in the surface
200
000
m
C E)1( EE
EEPED
re
e
Where- ε is the absolute permittivity (F/m) -εr is the relative permittivity or the dielectric constant of the medium -ε0 is the permittivity of free space -χe is the electric susceptibility (dimensionless)
z
y
x
z
y
x
E
E
E
D
D
D
333231
232221
131211
Homogeneous-εr independent of position
Anisotropic εr is different for different of the electric field
Biaxial, Uniaxial and Isotropic Medium
z
y
x
z
y
x
E
E
E
D
D
D
3
2
1
00
00
00
21
-biaxial
-uniaxial
-isotropic
321
Vacuum 1
Glass 5-10
Mica 3-6
Mylar 3.1
Neoprene 6.70
Plexiglas 3.40
Polyethylene 2.25
Polyvinyl chloride 3.18
Teflon 2.1
Germanium 16
Strontiun titanate 310
Titanium dioxide (rutile) 173 perp
86 para
Water 80.4
Glycerin 42.5
Liquid ammonia(-78°C 25
Benzene 2.284
Air(1 atm) 1.00059
Air(100 atm) 1.0548
Material Dielectric Constants
The maximum electric field intensity that a dielectric material can stand without breakdown
Material Dielectric Strength (V/m)
Air 3e6
Bakelite 24e6
Neoprene rubber 12e6
Nylon 14e6
Paper 16e6
Polystyrene 24e6
Pyrex glass 14e6
Quartz 8e6
Silicone oil 15e6
Strontium titanate 8e6
Teflon 60e6
Dielectric Strength
4. Boundary Conditions for Electrostatic Fields
abcda
ldE 0CONDITION I (tangential components)
Tangential Components of the Electric Field
0 a
d
d
c
c
b
b
a
ldEldEldEldE
0 2/
0
0
2/
02/
0
0
2/0
h
h
w
h
h
w
ldEldE
ldEldEldEldE
02/
0
0
2/
0
2/
0
0
2/0
212
211
h
nnN
h
nnN
w
TTT
h
nnN
h
nnN
w
TTT
adlaEadlaEadlaE
adlaEadlaEadlaE
022
22
21
2211
hE
hE
wEh
Eh
EwE
NN
TNNT
Condition I (tangential components)
21
21
11
TT
TT
DD
EE
CONDITION II (normal components) QsdD
Perpendicular Components of the Displacement Vector
bottomsides top
sdDsdDsdDsdD
sDsD
adsaDadsaDsdD
NN
bottom
nnN
s top
nnN
21
11
)(
s
ssencl sdsQ
Condition II (normal components)
221 m
C )(
21 sNN DDa
The normal component of D field is discontinuous across an interface where a surface charge exists, the amount of discontinuity being equal to the surface charge density
Example: Two dielectric media with permittivity ε1 and respectively ε2 , are separated by a charge free boundary. The electric field intensity in medium 1 at point P1 has a magnitude E1 and makes an angle α1. Determine the magnitude and direction of the electric field intensity at point P2 in medium 2.
α1
α2
P1
P2
0)( since
21
21
11
12
sNN
NN
TT
DDa
DD
EE
222111
2211
coscos
sinsin
EE
EE
2
1
2
1
2
2
1
1
tan
tan
tantan
2
112
12112
2
112
1211
222
222
222
cossin
cossin
cossin21
EEE
EE
EEEEE NT
The magnitude of E2 :
Boundary conditions at a Dielectric/Conductor Interface
-inside a good conductor
E=0 ET=0
D=0 Dn=ρs
2 212 1212 24 x y x y
xy xy VV y xy
x y m
2E a a a a
22
-1.1 2.2 r o x y
nCy xy
m D E a a
The potential field in a material with εr= 10.2 is V = 12 xy2 (V). Find E, P and D.
1 9.2e r
12 22
9.2 8.854 10 -9.8 2.00 e o x y
nCx y xy
m P E E = a a
Practical Problems: Electric Potential
For z ≤ 0, r1 = 9.0 and for z > 0, r2 = 4.0. If E1 makes
a 30 angle with a normal to the surface, what angle does E2 make with a normal to the surface?
1 1 1 2 2 2 1 2sin , sin , and T T T TE E E E E E
More Practical Problems: Boundary Conditions
1 1 1 1 2 2 2 2
1 2
cos , cos ,
and since 0N r o N r o
N N s
D E D E
D D
1 2
1 2
,T T
N N
E E
D D Therefore
and after routine math we find 1 2
2 11
tan tanr
r
Using this formula we obtain for this problem 2 = 14°.
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