What is a DIELECTRIC?

Question

In context- Capacitors

Answer 1

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

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

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.


[edit] 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.


[edit] 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.


[edit] 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.

Answer 2

A dielectric is different from an insulator. The crucial idea is that a dielectric polarizes (as many already pointed out).

When you put a dielectric material in an existing electric field, electric charges within the dielectric get displaced -- the negative charges one way and the positive charges the other. This gives rise to layers of what's called "bound" charge on opposite surfaces of the dielectric, and a lessened net electric field within the dielectric. That in turn leads to increased capacitance when the dielectric material is used inside a capacitor.

Answer 3

Most dielectric materials are solid. Examples include porcelain (ceramic), mica, glass, plastics, and the oxides of various metals. Some liquids and gases can serve as good dielectric materials. Dry air is an excellent dielectric, and is used in variable capacitors and some types of transmission lines. Distilled water is a fair dielectric. A vacuum is an exceptionally efficient dielectric.

You may notice that these materials are insulators. They don't permit charges to flow through them.

The center of mass of electrons and center of mass of nucleus coincide in these materials when they are not placed inside an electric field.



But when a is placed in an electric field, the positive charges are pulled along the direction of electric field and negative charges are pulled opposite to the direction of electric field . Thus the center of mass of electrons and the center of mass of nucleus are separated by some distance which depends upon the material used.
Thus in the presence of an electric field, the charge cloud is distorted, A positive and negative cahrge separated by a distance is called dipole.A dipole is characterised by its dipole moment. It is the relationship between the electric field and the dipole moment that gives rise to the behaviour of the dielectric.

Answer 4

Dielectric is a material that does not allow electricity to pass through it but when placed in an electric field the material gets polarised or better to say charge separation takes place.In capacitors generally dielectrics are used between the metal plates to store energy

Answer 5

A dielectric is a material that doesn't allow the electricity to pass through it. You put the dielectric between the capacitator's plates to increase the capacity. The dielectric gets polarized when you apply an electric current, the charge is accumulated on the surface of each plate and you are able to store energy.

Answer 6

A dielectric material is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields. If the flow of current between opposite electric charge poles is kept to a minimum while the electrostatic lines of flux are not impeded or interrupted, an electrostatic field can store energy. This property is useful in capacitors, especially at radio frequencies. Dielectric materials are also used in the construction of radio-frequency transmission lines.













In practice, most dielectric materials are solid. Examples include porcelain (ceramic), mica, glass, plastics, and the oxides of various metals. Some liquids and gases can serve as good dielectric materials. Dry air is an excellent dielectric, and is used in variable capacitors and some types of transmission lines. Distilled water is a fair dielectric. A vacuum is an exceptionally efficient dielectric.

An important property of a dielectric is its ability to support an electrostatic field while dissipating minimal energy in the form of heat. The lower the dielectric loss (the proportion of energy lost as heat), the more effective is a dielectric material. Another consideration is the dielectric constant, the extent to which a substance concentrates the electrostatic lines of flux. Substances with a low dielectric constant include a perfect vacuum, dry air, and most pure, dry gases such as helium and nitrogen. Materials with moderate dielectric constants include ceramics, distilled water, paper, mica, polyethylene, and glass. Metal oxides, in general, have high dielectric constants.

The prime asset of high-dielectric-constant substances, such as aluminum oxide, is the fact that they make possible the manufacture of high-value capacitors with small physical volume. But these materials are generally not able to withstand electrostatic fields as intense as low-dielectric-constant substances such as air. If the voltage across a dielectric material becomes too great -- that is, if the electrostatic field becomes too intense -- the material will suddenly begin to conduct current. This phenomenon is called dielectric breakdown. In components that use gases or liquids as the dielectric medium, this condition reverses itself if the voltage decreases below the critical point. But in components containing solid dielectrics, dielectric breakdown usually results in permanent damage.

Answer 7

Dielectric is how an electric current behave in a materials..

high dielectric materials mean that it is a poor conductor of electricity but effecient supporter of electrostatic field.

Answer 8

1) Any electrical insulating medium between two conductors.
2) The medium used to provide electrical isolation or separation.

Answer 9

ITS A SUBSTANCE WHICH GETS POLARISED IN PRESENCE OF AN EXTERNAL ELECTRIC FIELD..........

Answer 10

I it is basically a semiconductor material..