dielectric

 

(dī'ĭlĕk'trĭk) , material that does not conduct electricity readily, i.e., an insulator (see insulation). A good dielectric should also have other properties: It must resist breakdown under high voltages; it should not itself draw appreciable power from the circuit; it must have reasonable physical stability; and none of its characteristics should vary much over a fairly wide temperature range. One important application of dielectrics is as the material separating the plates of a capacitor. A capacitor with plates of a given area will vary in its ability to store electric charge depending on the material separating the plates. On the basis of this variation each insulating material can be assigned a dielectric constant. Generally, the dielectric constant of air is defined as 1 and other dielectric constants are determined with reference to it. Other properties of interest in a dielectric are dielectric strength, a measure of the maximum voltage it can sustain without significant conduction, and the degree to which it is free from power losses.

A dielectric is a physical model commonly used to describe how an electric field behaves inside a material. It is characterised by how an electric field interacts with an atom. It is possible to approach dielectrics from either a classical interpretation or a quantum one. However, the classical is much more intuitive.

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.

Definition

In the classical approach to the dielectric model a material is made up of atoms. The atoms consist of a positive point charge at the centre surrounded by a cloud of negative charge. The cloud of negative charge is bound to positive point charge. The atoms are separated by enough distance such that they do not interact with one another. This is represented by the top left of the figure aside. 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.

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 characterised by its dipole moment. This is a vector quantity and is shown as the blue arrow labeled M. It is the relationship between the electric field and the dipole moment that gives rise to the behaviour 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.

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

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This is the essence of the model. The behaviour 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 behaviour. 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?

Remember it is the relationship between the electric field, E, and the dipole moment, M, that gives rise to the behaviour of the dielectric. So for a given material we want to find the function F defined by the equation:

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 phenoma:

• 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 vaccum 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, 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 ()
• high-k
• leakage
• electret
• piezoelectricity
• ferroelectric

External links

• Dielectric Sphere in an Electric Field

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