Dielectric & capacitors

[blazinfury]

The purpose of a dielectric is to aid a capacitor in maximizing the amount of charge that it can store. However I am confused about whether the best material for a dielectric would be a polar or nonpolar? I had always thought that you wanted a vacuum or a near vaccuum between the two plates to minimize charges from flowing between the plates and thus maximizing charge storage on each plate. However it seems that the more polar a material the better the dielectric and thus the higher the capacitance. Would someone be able to clarify why a polar material is better?

[swansont]

A dielectric has an induced polarity, not an intrinsic one. But by being able to induce this polarity, the electric field inside the capacitor is reduced. You can store more charge at a given potential difference.

[x(x-y)]

Indeed - as swansont stated - the dielectric material is an insulator (typically air, glass etc used in between capacitor plates) which will be partially polarised when an electric field is passed through it; a partial dipole is created. If you'd just spent a long time charging up the plates of a capacitor and then someone came along and put an intrinsically polar material (i.e. a conductor) between the plates then, of course, you wouldn't be a happy person.

 

Note that the capacitance C of a capacitor is given by

 

[latex]C = \frac{Q}{V}[/latex]

 

where Q is the charge and V is the potential difference, and one can express the latter as V = Ed, where E is the magnitude of the electric field (strength) and d is the distance between the plates, yielding

 

[latex]C = \frac{Q}{Ed}[/latex]

 

Thus, by decreasing E you will act to increase the capacitance C assuming Q remains constant.

Edited February 7, 2013 by x(x-y)

[Enthalpy]

Dielectrics work by reducing the field hence voltage for a give charge, and increase that way the capacitance.

 

They do so by allowing their charges to move a little bit when the field is applied. These charges can be electronic orbitals which deform, they can also be ions. Sometimes the ions can have two stable positions, in which case the material has an intrinsic polarization which the external field can reverse; these materials are called ferroelectric by analogy with ferromagnetism hysteretic behaviour. These are PVDF and the type II and type III ceramic capacitors, very compact but with high losses, microphonic sensitivity, temperature drift and so on.

 

Such a comparison holds only to maximize the capacitance... One may want to maximize the energy instead, and then the allowed electric field is twice as important as the capacitance increase (=the permittivity), especially if a capacitor can be made of layers as plentiful and thin as desired. Then non-polar plastics or vacuum can be about as good as ceramics; it depends on the capacitor's voltage. In our complicated world, the maximum field depends on the voltage.

 

Most users, especially for electronics design, want other properties than compact capacity from their components: temperature stability, time stability, frequency uniformity, low losses, low "dielectric polarization" (charge give back with a delay), low inductance, linearity, self-healing ability... These subtle properties, which would deserve much more attention, tend to demand dielectrics that don't make compact capacitors.