Dielectrics

The Physics Hypertextbook
© 1998-2004 by Glenn Elert
All Rights Reserved -- Fair Use Encouraged

prev | up | next


Discussion

overview

Dielectrics are insulators, plain and simple. (The reason why we have two words to describe exactly the same class of materials will be discussed later in this section.) Let's compare and contrast the behavior of charges in conductors (metals in particular) and insulators during a process known as polarization.

When a metal is placed in an electric field the free electrons flow against the field until the run out of conducting material to flow through. In no time at all we have a pile of excess electrons on one side and a deficit on the other. One side of the conductor has become negatively charged and the other positively charged. Release the field and the electrons on the negatively charged side now find themselves too close for comfort. Like charges repel and the electrons run away from each other as fast as they can until they're distributed uniformly throughout; one electron for every proton on average in the space surrounding every atom. Life for a conducting electron in a metal is an interesting one. They are free to roam around as much as they want and get to take long trips spanning the entire length, width, and depth of the metal.

Life is much more restictive for an electron in an insulator. By definition, charges in an insulator are not free to move. This is not the same thing as saying they can't move. An electron in an insulator is like a prisoner -- free to move around, but only as far as the cell walls allow. When an insultor is placed in an electric field, the electrons move against the field as far as they can, each within the confines of its own atomic prison cell. One would hardly think that such tiny motions would have any effect, but small changes multiplied numerous times can result in large scale changes when the actions are directed toward a common goal -- intentional or unintention al. (Electrons don't have a will after all.) This is where we have to depart from our prison analogy and look at this situation from a different perspective.

The electrons in an insulator may not be free to move, but given the impetus they will move as much as they can (an atomically small distance). The effect is much like pushing on a locked door. The door doesn't go anywhere, but that's not to say nothing's happening. The door might not be opening, but it certainly is under some kind of stress. If the door was made thin enough it might even be possible to see it bend slightly under the stress applied. Release the stress and the door pops back into shape. This is what happens when an insulator is placed in an electric field. The field can't make the charges move macroscopically, but it can stretch and distort them microscopically. It can push them slightly into uncomfortable positions and when released allow them to fall back into a relaxed state. The thing that makes the charges in an insulator different from an elastic body (which is essentially what our hypothetical door is) is that eliminating the stress doesn't necessarily release the strain. Some insulators will remain in their stressed state for seconds, minutes, hours, years, and even centuries (although no one has actually left one lying around undisturbed long enough to verify this first hand). Because of this ability to store electrostatic energy for an extended period dielectrics are frequently used in capacitors.

The thing to keep in mind is that the charges "stored" in the dielectric layer of a capacitor aren't available as a pool of free charges like they are in a metal.

messy, detailed atomic descriptions

The charges in a dielectric material won't experience bulk motion in the presence of an external electric field, but they will rearrange themselve in a more subtle manner. This process is known as polarization and a dielectric material so stressed is said to be polarized. There are two principal methods by which a dielectric can be polarized: stretching and rotation.

When an atom is placed in an external electric field, the nucleus is pushed with the field and the electron clouds are pulled against it. This creates an induced dipole.

Polarization by stretching [animate]. This effect occurs in all atoms and molecules.

polar molecules

Polarization by rotation [animate]. This effect occurs only in polar molecules.

numbers, numbers, numbers

About the first discoveries of the Leyden jar. Removing the rod lowers the capacitance. (Air has a lower dielectric constant than rotation.) Voltage and capacitance are inversely proportional when charge is constant. Reducing the capacitance raises the voltage.

The quantity κ [kappa] is unitless.

Dielectric Constant for Selected Materials (~300 K except where indicated)
material κ   material κ
air 1.005364   quartz, crystalline (∥) 4.60
acetic acid 6.2   quartz, crystalline (⊥) 4.51
alcohol, ethyl (grain) 24.55   quartz, fused 3.8
alcohol, methyl (wood) 32.70   rubber, butyl 2.4
amber 2.8   rubber, neoprene 6.6
asbestos 4.0   rubber, silicone 3.2
asphalt 2.6   rubber, vulcanized 2.9
bakelite 4.8   salt 5.9
calcite 8.0   selenium 6.0
calcium carbonate 8.7   silicon 11.8
cellulose 3.7 - 7.5   silicon carbide (αSiC) 10.2
cement ~2   silicon dioxide 4.5
cocaine 3.1   silicone oil 2.7 - 2.8
cotton 1.3   soil 10 - 20
diamond, type I 5.87   strontium titanate, +025 °C 0332
diamond, type IIa 5.66   strontium titanate, –195 °C 2080
ebonite 2.7   sulfur 3.7
epoxy 3.6   tantalum pentoxide 27
flour 3 - 5   teflon 2.1
freon 12, -150 °C (liquid) 3.5   tin antimonide 147
freon 12, +020 °C (vapor) 2.4   tin telluride 1770
germanium 16   titanium dioxide (rutile) 114
glass 4 - 7   tobacco 1.6 - 1.7
glass, pyrex 7740 5.0   uranium dioxide 24
gutta percha 2.6   vacuum 1 (exactly)
jet fuel (jet a) 1.7   water, ice, –30 °C 99
lead oxide 25.9   water, liquid, 000 °C 87.9
lead magnesium niobate 10,000   water, liquid, 020 °C 80.2
lead sulfide (galena) 200   water, liquid, 040 °C 73.2
lead titanate 200   water, liquid, 060 °C 66.7
lithium deuteride 14.0   water, liquid, 080 °C 60.9
lucite 2.8   water, liquid, 100 °C 55.5
mica, muscovite 5.4   wax, beeswax 2.7 - 3.0
mica, canadian 6.9   wax, carnuba 2.9
nylon 3.5   wax, paraffin 2.1 - 2.5
oil, linseed 3.4   waxed paper 3.7
oil, mineral 2.1      
oil, olive 3.1   human tissues κ
oil, petroleum 2.0 - 2.2   bone, cancellous 26
oil, silicone 2.5   bone, cortical 14.5
oil, sperm 3.2   brain, gray matter 56
oil, transformer 2.2   brain, white matter 43
paper 3.3, 3.5   brain, meninges 58
plexiglas 3.1   cartilage, general 22
polyester 3.2 - 4.3   cartilage, ear 47
polyethylene 2.26   eye, aqueous humor 67
polypropylene 2.2 - 2.3   eye, cornea 61
polystyrene 2.55   eye, sclera 67
polyvinyl chloride (pvc) 4.5 fat 16
porcelain 6 - 8 muscle, smooth 56
potassium niobate 700 muscle, striated 58
potassium tantalate niobate, 00 °C 34,000 skin 33 - 44
potassium tantalate niobate, 20 °C 6,000 tongue 38

Every insulator can be forced to conduct electricity. This phenomena is known as dielectric breakdown.

Dielectric Breakdown in Selected Materials
material dielectric strength
(MV/m)
air 3
amber 90
bakelite 12, 24
diamond, type IIa 10
glass, pyrex 7740 13, 14
mica, muscovite 160
nylon 14
oil, silicone 15
oil, transformer 12, 27
paper 14, 16
polyethylene 50, 500-700, 18
polystyrene 24, 25, 400-600
polyvinyl chloride (PVC) 40
porcelain 4, 12
quartz, fused 8
rubber, neoprene 12, 12
strontium titanate 8
teflon 60
titanium dioxide (rutile) 6
water ??

piezoelectric effect

Say all the vowels. Piezoelectricity is an effect by which energy is converted between mechanical and electrical forms.

  • Piezo is the Greek word for pressure (πιεζω).
  • Discovered in the 1880s by the Curie brothers.
  • Cheap piezoelectric microphones. When a polarized crystal is stressed, the stress produces a potential difference. This potential difference is proportional to the stress, which is proportional to the acoustic pressure.
  • A backward piezoelectric microphone is a peizoelectric speaker: alarm clock buzzer, writstwatch chime, all sorts of electronic beepers. When an electrical potential is applied to a polarized crystal, the crystal undergoes a mechanical deformation which can in turn create an acoustical pressure.
  • Collagen is piezoelectric. "When a force is applied to [bone] collagen, a small dc electric potential is generated. The collagen conducts current mainly by negative charges. Mineral crystals of the bone (apatite) close to the collagen conduct current by positive charges. At a junction of these two types of semiconductors, current flows easily in one direction but not in the other direction .... It is thought that the forces on bones producew potentials by the piezoelectric effect and that the junctions of collagen-apatite, currents are produced that induce and control bone growth. The currents are proportional to stress (force per unit area), so increased mechanical bone stress results in increased growth." Physics of the Body (255).
Microphones and How They Work
type sounds produce
changes in ...
which cause
changes in ...
which result in
changes in ...
carbon granule density resistance voltage
condenser plate separation capacitance voltage
dynamic coil location flux voltage
piezoelectric compressional stress polarization voltage

Summary

  • bullet

Problems

practice

  1. Since capacitance is directly proportional to plate area, a lot of metal foil is needed to make a big capacitor. Since capacitance is also inversely proportional to plate separation, this metal foil should be separated by a very thin dielectric film. A thinner film means less metal foil is needed, but dielectric films can only be made so thin. Thus, big capacitances require big capacitors (capacitors with a large volume).

    That is, if one uses conventional materials and conventional designs. Enter the ultracapacitor (also known as the supercapacitor or electrochemical capacitor). Instead of two metal plates separated by a dielectric, an ultracapacitor uses an activated carbon electrode in contact with an electrolytic paste. This effectively compresses a football field worth of surface area into a teaspoon of volume and shrinks plate separation down to the atomic scale.

    According to one manufacturer ...
    An ultracapacitor gets its area from a porous carbon-based electrode material. The porous structure of this material allows its surface area to approach 2000 square meters per gram, much greater than can be accomplished using flat or textured films and plates. An ultracapacitor's charge separation distance is determined by the size of the ions in the electrolyte, which are attracted to the charged electrode. This charge separation (less than 10 angstroms) is much smaller than can be accomplished using conventional dielectric materials. Source: Maxwell Technologies.
    Determine the total ...
    1. surface area,
    2. mass, and
    3. volume
    of the electrodes in a one farad ultracapacitor. (One angstrom is 10-10 m and the density of activated carbon is 0.50 g/cm3.)

    Solution ...
    1. Answer it.
    2. Answer it.
    3. Answer it.
    Ultracapacitors can be used ...
    • in place of rechargeable batteries for long periods in low current devices (like computer back up memory) and for shorter periods in high current devices (like power tools).
    • to provide a bridge current when power is switched from one source to another (when subway cars switch tracks, for example, or while waiting for a backup generator to come online during a blackout).
    • for load levelling high voltage, low energy devices connected to low voltage, high energy sources (remote radio transmitters connected to solar arrays, for example).
  2. Write Something
    • Answer it.
  3. Write something.
    • Answer it.

numerical

  1. problems

Resources

  • no resources for this topic

prev | up | next

Another quality webpage by

Glenn Elert
eglobe logo home | contact

bent | chaos | eworld | facts | physics