conductors and dielectrics
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Conductors and Dielectrics. Conductors Current, current density, drift velocity, continuity Energy bands in materials Mobility, micro/macro Ohm’s Law Boundary conditions on conductors Methods of Images Dielectrics Polarization, displacement, electric field

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conductors and dielectrics
Conductors and Dielectrics
  • Conductors
    • Current, current density, drift velocity, continuity
    • Energy bands in materials
    • Mobility, micro/macro Ohm’s Law
    • Boundary conditions on conductors
    • Methods of Images
  • Dielectrics
    • Polarization, displacement, electric field
    • Permittivity, susceptibility, relative permittivity
    • Dielectrics research
    • Boundary conditions on dielectrics
conductors and dielectrics1
Conductors and Dielectrics
  • Polarization
    • Static alignment of charge in material
    • Charge aligns when voltage applied, moves no further
    • Charge proportional to voltage
  • Conduction
    • Continuous motion of charge through material
    • Enters one side, exits another
    • Current proportional to voltage
  • Real-world materials
    • Plastics, ceramics, glasses -> dielectrics (maybe some conductivity)
    • Metals -> conductors, semiconductors, superconductors
    • Cement, Biosystems -> Both (water high dielectric, salt conductivity)
current and current density
nCurrent and current density
  • Basic definition of current C/s = Amps
  • Basic current density (J perp. surface)
  • Vector current density
current density and charge velocity
Current density and charge velocity
  • Basic definition of current
  • Combining with earlier expression
  • Gives current density
charge and current continuity
Charge and current continuity
  • Current leaving any closed surface is time rate of change of charge within that surface
  • Using divergence theorem on left
  • Taking time derivative inside integral
  • Equating integrands


example charge and current continuity
Example – charge and current continuity
  • Given spherically symmetric current density
  • Current increasing from r = 5m to r= 6m at t=1s
  • Current density from continuity equation
  • Charge density ρ integral w.r.t. time
  • Drift velocity is thus

^^ Why is current increasing ?


energy band structure in three material types
Energy Band Structure in Three Material Types
  • Discrete quantum states broaden into energy bands in condensed materials with overlapping potentials
  • Valence band – outermost filled band
  • Conduction band – higher energy unfilled band
  • Band structure determines type of material
  • Insulators show large energy gaps, requiring large amounts of energy to lift electrons into the conduction band. When this occurs, the dielectric breaks down.
  • Conductors exhibit no energy gap between valence and conduction bands so electrons move freely
  • Semiconductors have a relatively small energy gap, so modest amounts of energy (applied through heat, light,oran electric field) may lift electrons from valence to conduction bands.
ohm s law microscopic form
Free electrons are accelerated by an electric field. The applied force on an electron of charge Q = -e is

But in reality the electrons are constantly bumping into things (like a terminal velocity) so they attain an equilibrium or drift velocity:

where eis the electron mobility, expressed in units of m2/V-s. The drift velocity is used in the current density through:

So Ohm’s Law in point form (material property)

With the conductivity given as:

S/m (electrons/holes)

Ohm’s Law (microscopic form)

S/m (electrons)

ohm s law macroscopic form
Ohm’s Law (macroscopic form)
  • For constant electric field
  • Ohm’s Law becomes
  • Rearranging gives
  • Or
  • Variation with geometry
  • Conductance vs. Resistance
ohm s law example 1
Ohm’s Law example 1
  • Checking ohms law microscopic form
  • Mobility of copper is 0.0032 m2/V-s
  • Charge density
boundary conditions for conductors
Boundary conditions for conductors
  • No electric field in interior
    • Otherwise charges repel to the surface
  • No tangential electric field at surface
    • Otherwise charges redistribute along surface
  • Normal electric field at surface
    • Displacement Normal equals Charge Density (Gauss’s Law)
boundary condition for tangential electric field e

Over the rectangular integration path, we use

To find:



These become negligible as h approaches zero.



More formally:

Boundary Condition for Tangential Electric Field E
boundary condition for the normal displacement d
Gauss’ Law is applied to the cylindrical surface shown below:


This reduces to:

as h approaches zero





More formally:

Boundary Condition for the Normal Displacement D
Tangential E is zero

At the surface:

Normal D is equal to the surface charge density

example boundary conditions for conductors
Example - Boundary Conditions for Conductors
  • Potential given by
  • Potential at (2,-1,3) is 300 V. Also 300 V along entire surface where
  • Thus we can “insert” conductor in region provided the conductor follow hyperbola
  • The Electric Field is at all times normal to conducting surface
  • Electric field at point 2,-1,3)
    • Ex = -400 V/m, Ey = -200 V/m
    • Down and to left
example streamlines of electric field
Example – Streamlines of Electric Field
  • Slope of line equals electric field ratio
  • Rearranging
  • Evaluate at P(2,-1,3)


method of images
The Theorem of Uniqueness states that if we are given a configuration of charges and

boundary conditions, there will exist only one potential and electric field solution.

In the electric dipole, the surface along the plane of symmetry is an equipotential with V = 0.

The same is true if a grounded conducting plane is located there.

So the boundary conditions and charges are identical in the upper half spaces of both configurations

(not in the lower half).

In effect, the positive point charge images across the conducting plane, allowing the conductor to be

replaced by the image. The field and potential distribution in the upper half space is now found much

more easily!

Method of Images
example of the image method
Want to find surface charge density on conducting plane at the point (2,5,0). A 30-nC line of charge lies parallel to the y axis at x=0, z = 3.

First step is to replace conducting plane with image line of charge -30 nC at z = -3.

Example of the Image Method
example of the image method continued
Vectors from each line charge to observation point:

Electric Fields from each line charge

Add both fields to get: (x component cancels)

Example of the Image Method (continued)


image method using potentials
Image Method using Potentials
  • Conducting plane at x = 4 with vertical wire in front.
  • Potential for wire in front at x = 6, y=3:
  • Boundary condition for wire in front at x = 6, y=3:
  • Boundary condition for image wire in back at x=2, y=3:
image method using potentials cont
Image Method using Potentials (cont)
  • Total potential becomes
  • At point (7,-1,5) gives
  • To get electric field must write V(ρ) as V(x,y) and take gradient
  • Material has random oriented dipoles
  • Applied field aligns dipoles (negative at (+) terminal, positive at (-) terminal
  • Effect is to cancel applied field, lower voltage
  • OR, increase charge to maintain voltage
  • Either increases capacitance C= Q/V
review dipole moment
Review Dipole Moment
  • Define dipole moment
  • Potential for dipole
  • Written in terms of dipole

moment and position

  • Dipole moment determines “strength” of polar molecule

amount of charge (Q) and offset (d) of charge

polarization as sum of dipole moments per volume
Introducing an electric field may increase the charge separation in each dipole, and possibly re-orient dipoles so that

there is some aggregate alignment, as shown here. The effect is small, and is greatly exaggerated here!


The effect is to increase P.

n = charge/volume

p = polarization of individual dipole

P = polarization/volume

Polarization as sum of dipole moments (per volume)
polarization near electrodes
EPolarization near electrodes


  • From diagram
    • Excess positive bound charge near top negative electrode
    • Excess negative bound charge near bottom positive electrode
    • Rest of material neutral
  • Excess charge in bound (red) volumes
  • Writing in terms of polarization
  • Writing similar to Gauss’s law

(Note dot product sign, outward normal leaves opposite charge enclosed)

- - - - - - - - - - - - - - - -



+ + + + + + + + + + + + +

combining total free and bound charge
ECombining total, free, and bound charge


  • Total, free, and bound charge
  • Total
  • Free
  • Bound
  • Combining

- - - - - - - - - - - - - - - -



+ + + + + + + + + + + + +

d p and e in dielectric
D, P, and E in Dielectric
  • D continuous
  • Polarization increases
  • E decreases
  • C/m2
charge densities
Bound Charge:

Total Charge:

Free Charge:

Charge Densities

Taking the previous results and using the divergence theorem, we find the point form expressions:

electric susceptibility and the dielectric constant
A stronger electric field results in a larger polarization in the medium. In a linear medium, the relation

between P and E is linear, and is given by:

where e is the electric susceptibility of the medium.

We may now write:

where the dielectric constant, or relative permittivity is defined as:

Leading to the overall permittivity of the medium:


Electric Susceptibility and the Dielectric Constant
isotropic vs anisotropic media
In an isotropic medium, the dielectric constant is invariant with direction of the applied electric field.

This is not the case in an anisotropic medium (usually a crystal) in which the dielectric constant will vary

as the electric field is rotated in certain directions. In this case, the electric flux density vector components

must be evaluated separately through the dielectric tensor. The relation can be expressed in the form:

Isotropic vs. Anisotropic Media
permittivity of materials
Permittivity of Materials
  • Typical permittivity for various solids and liquids.
    • Teflon – 2
    • Plastics - 3-6
    • Ceramics 8-10
    • Titanates>100
    • Acetone 21
    • Water 78
  • Actual dielectric “constant” varies with:
    • Temperature
    • Direction
    • Field Strength
    • Frequency
    • Real & Imaginary components
variation with frequency
Variation with frequency
  • Charge polarization due to:
    • Ionic (low frequency)
    • Orientation (medium, microwave)
    • Atomic (IR)
    • Electronic (Visible, UV)
  • Dielectric relaxation
    • As frequency is raised, molecule can no longer “track”.
    • Real permittivity decreases and

imaginary permittivity peaks

  • In medium and microwave range
    • Rotation, reorientation, etc >>
  • Modeling:
    • Permittivity & impedance diagrams.
    • Statistical relaxation functions (Debye, Cole Davidson).
application to polymer composites
Application to Polymer Composites
  • Dielectric Permittivity in Epoxy Resin 10Hz -10 MHz
  • Polar-group rotation in epoxy resin.
  • Low-frequency range 10 Hz – 10 MHz.
  • Permittivity-loss transition at 1 MHz, at –4°C.
  • Transition frequency increases with temperature.

dielectric permittivity in epoxy resin 1 mhz 1 ghz
Dielectric Permittivity in Epoxy Resin 1 MHz -1 GHz
  • Aerospace resin Hexcel 8552.
  • High frequency range 1 MHz – 1 GHz.
  • Temperature constant 125°C, transition decreases with cure.
  • TDR measurement method.

application to cement hydration
Application to cement hydration
  • Cement Conductivity - Variation with Cure
  • Imaginary counterpart of real permittivity (’’).
  • Multiply by  to remove power law (o’’).
  • Decrease in ion conductivity, growth of intermediate feature with cure
  • Frequency of intermediate feature does not match permittivity

cement cure dielectric relaxation model
Cement Cure -Dielectric Relaxation Model


  • Provide free-relaxation, two intermediate-frequency relaxations
  • Provide conductivity and electrode polarization

Debye for free & medium. Cole-Davidson for low. (literature, biosystems)


9 variables fit over entire range, real & imaginary, 2-stage fit, f = 8.2 ps

cement cure model fitting
Cement Cure - Model Fitting
  • Fits permittivity – both low and free relaxation.
  • Fits conductivity – both medium and free relaxation.
  • Fits permittivity polarization.
  • Fits conductivity baseline.

other applications
Other applications
  • Other Applications
    • Bio
    • Liquid Crystal
    • Composite polymers
    • Titanates
    • Wireless characterization
    • MRI dyes
    • Ground water monitoring
    • Oil Drilling fluid characterization (GPR)
boundary condition for tangential electric field e1
Since E is conservative, we setup line integral straddling both dielectrics:

Left and right sides cancel, so

Leading to Continuity for tangential E

Boundary Condition for Tangential Electric Field E

And Discontinuity for tangential D

E same, D higher in high permittivity material

boundary condition for normal displacement d

Apply Gauss’ Law to the cylindrical volume straddling both dielectrics


Flux enters and exits only through top and bottom surfaces, zero on sides

Boundary Condition for Normal Displacement D

Leading to Continuity for normal D (for ρS = 0)

And Discontinuity for normal E

D same. E lower in high permittivity material

bending of d at boundary
Bending of D at boundary
  • Boundary conditions
    • DN continuous
  • Trigonometry
  • Eliminating DN



  • Teflon εr = 2.1
  • Displacement and Polarization outside
  • Displacement and Polarization inside
  • At boundary D is continuous, so inside
example continued
Example (continued)
  • Polarization up, E field down, D maintains continuity