Dielectric

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A dielectric is a nonconducting substance, i.e. an insulator. The term was coined by William WhewellJ. Daintith, "Biographical Encyclopedia of Scientists" CRC Press, 1994, ISBN 0750302879, page 943 in response to a request from Faraday. Whewell considered "dia-electric", from the Greek "dia" meaning "through", since an electric field passes through the material but felt that "dielectric" was easier to pronounce^{[citation needed]}. Although "dielectric" and "insulator" are generally considered synonymous, the term "dielectric" is more often used when considering the effect of alternating electric fields on the substance while "insulator" is more often used when the material is being used to withstand a high electric field^{[citation needed]}. Von Hippel, in his seminal book A. R. Von Hippel (ed), "Dielectric Materials and Applications", published jointly by the Technology Press of MIT and John Wiley, NY, 1954 takes this definition further. He states,

"Dielectrics ... are not a narrow class of so-called insulators, but the broad expanse of nonmetals considered from the standpoint of their interaction with electric, magnetic, of electromagnetic fields. Thus we are concerned with gases as well as with liquids and solids, and with the storage of electric and magnetic energy as well as its dissipation."

Dielectrics is the study of dielectric materials and involves physical models to describe how an electric field behaves inside a material. It is characterised by how an electric field interacts with an atom and is therefore possible to approach from either a classical interpretation or a quantum one.

Many phenomena in electronics, solid state and optical physics can be described using the underlying assumptions of the dielectric model. This can mean that the same mathematical objects can go by many different names.

Contents 1 Definitions 1.1 Classical 1.2 Behavior 2 Dielectric model applied to vacuum 3 Applications 4 Some practical dielectrics 5 See also 6 References 7 External links

Definitions

Electric field interaction with an atom under the classical dielectric model.

Classical

In the classical approach to the dielectric model, a material is made up of atoms. Each atom consists of a cloud of negative charge bound to and surrounding a positive point charge at its centre. Because of the comparatively huge distance between them, none of the atoms in the dielectric material interact with one another^{[citation needed]}. Note: Remember the model is not attempting to say anything about the structure of matter. It is only trying to describe the interaction between an electric field and matter.^{[citation needed]}

In the presence of an electric field the charge cloud is distorted, as shown the top right of the figure.

This can be reduced to a simple dipole using the superposition principle. A dipole is characterized by its dipole moment, a vector quantity shown in the figure as the blue arrow labeled M. It is the relationship between the electric field and the dipole moment that gives rise to the behavior of the dielectric. Note: The dipole moment is shown to be pointing in the same direction as the electric field. This isn\'t always correct, but it is a major simplification, and it is suitable for many materials.^{[citation needed]}

When the electric field is removed the atom returns to its original state.

Behavior

This is the essence of the model. The behavior of the dielectric now depends on the situation. The more complicated the situation the more rich the model has to be in order to accurately describe the behavior. Important questions are:

• Is the electric field constant or does it vary with time?
  • If the electric field does vary, does it vary quickly or slowly?
• What are the characteristics of the material?
  • Is the direction of the field important (isotropy)?
  • Is the material the same all the way through (homogeneous)?
  • Are there any boundaries/interfaces that have to be taken into account?
• Is the system linear or do nonlinearities have to be taken into account?

The relationship between the electric field E and the dipole moment M gives rise to the behavior of the dielectric, which, for a given material, can be characterized by the function F defined by the equation: $\backslash mathbf\{M\}\; =\; \backslash mathbf\{F\}(\backslash mathbf\{E\})$.

When both the type of electric field and the type of material have been defined, one then chooses the simplest function F that correctly predicts the phenomena of interest. Examples of possible phenomena:

• Refractive index
• Group velocity dispersion
• Birefringence
• Self-focusing
• Harmonic generation

May be modeled by choosing a suitable function F.

Dielectric model applied to vacuum

From the definition it might seem strange to apply the dielectric model to a vacuum, however, it is both the simplest and the most accurate example of a dielectric.

Recall that the property which defines how a dieletric behaves is the relationship between the applied electric field and the induced dipole moment. For a vacuum the relationship is a real constant number. This constant is called the permitivity of free space, ε₀.

Applications

The use of a dielectric in a capacitor presents several advantages. The simplest of these is that the conducting plates can be placed very close to one another without risk of contact. Also, if subjected to a very high electric field, any substance will ionize and become a conductor. Dielectrics are more resistant to ionization than dry air, so a capacitor containing a dielectric can be subjected to a higher operating voltage. Layers of dielectric are commonly incorporated in manufactured capacitors to provide higher capacitance in a smaller space than capacitors using only air or a vacuum between their plates, and the term dielectric refers to this application as well as the insulation used in power and RF cables.

Some practical dielectrics

Dielectric materials can be solids, liquids, or gases. In addition, a high vacuum can also be a useful, lossless dielectric even though its relative dielectric constant is only unity.

Solid dielectrics are perhaps the most commonly used dielectrics in electrical engineering, and many solids are very good insulators. Some examples include porcelain, glass, and most plastics. Air, nitrogen and sulfur hexafluoride are the three most commonly used gaseous dielectrics.

• Industrial coatings such as parylene provide a dielectric barrier between the substrate and its environment.
• Mineral oil is used extensively inside electrical transformers as a fluid dielectric and to assist in cooling. Dielectric fluids with higher dielectric constants, such as electrical grade castor oil, are often used in high voltage capacitors to help prevent corona discharge and increase capacitance.
• Because dielectrics resist the flow of electricity, the surface of a dielectric may retain stranded excess electrical charges. This may occur accidentally when the dielectric is rubbed (the triboelectric effect). This can be useful, as in a Van de Graaff generator or electrophorus, or it can be potentially destructive as in the case of electrostatic discharge.
• Specially processed dielectrics, called electrets(also known as ferroelectrics), may retain excess internal charge or "frozen in" polarization. Electrets have a semipermanent external electric field, and are the electrostatic equivalent to magnets. Electrets have numerous practical applications in the home and industry.
• Some dielectrics can generate a potential difference when subjected to mechanical stress, or change physical shape if an external voltage is applied across the material. This property is called piezoelectricity. Piezoelectric materials are another class of very useful dielectrics.
• Some ionic crystals and polymer dielectrics exhibit a spontaneous dipole moment which can be reversed by an externally applied electric field. This behavior is called the ferroelectric effect. These materials are analogous to the way ferromagnetic materials behave within an externally applied magnetic field. Ferroelectric materials often have very high dielectric constants, making them quite useful for capacitors.

See also

• capacitor
• dielectric strength
• dielectric constant
• dielectric resonator
• dielectric spectroscopy
• electric susceptibility
• electrorotation
• field cage
• low-k
• permittivity
• dielectric constant of vacuum ($\backslash varepsilon\_0$)
• high-k
• leakage
• electret
• piezoelectricity
• ferroelectric

References

External links

• Electromagnetism - a chapter from an online textbook
• Dielectric Sphere in an Electric Field

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