Dielectrics are insulating media that undergo polarization when subjected to large electric field strengths.
- Block the flow of electric current
- Accumulate electrical charges due to their polarization
- Prevent air dielectric breakdown in high voltage wires
- Isolate electrical components
How polarization of dielectric materials occurs
When dielectric materials are placed in a region of intense electric field , little or no electric current is able to flow through them. This is because these materials are poorly conductive . When propagating inside the dielectrics, the electric field causes the polarization of its molecules, that is, the charge carriers of the material are slightly displaced from their original position, causing an electric field opposite to the external electric field to appear, canceling it.
The figure below shows how the polarization of the dielectric occurs. In white, we have a representation of the molecules of the dielectric, the external electric field “pulls” the negative charges towards it, thus producing a separation of charges that gives rise to an electric field of polarization, opposite to the external electric field:
The external electric field polarizes the dielectric medium.
Breakage of dielectric strength
The breakdown of dielectric strength occurs when the external electric field applied to a dielectric is large enough for the material to stop being an electrical insulator and become a conductor of electricity. When this occurs, the electrons of this material, which were previously strongly bound to atomic nuclei , are now conducted through its crystal lattice . This process is generally violent , produces a large amount of heat, and can cause irreversible damage to the dielectric.
Lightning is formed when air conducts an electric current.
The breakdown of dielectric strength can also occur when the material is heated , the increase in temperature can provide more energy to the electrons, which, when excited, are more easily conducted.
In general, all dielectrics require large amounts of energy to undergo breakdowns of dielectric strength, on the order of at least 3 eV (3 electron-Volts) — about 4.8 x 10 -19 J for each excited electron.
A simple example that depicts the breakdown of dielectric strength is the formation of lightning : when the electric field between clouds is high enough, atmospheric air, which is a good dielectric, polarizes the moment the electric field exceeds a maximum value of 3.10 6 V/m . In this way, the air becomes a conductor. The passage of electrons through the air produces a huge increase in temperature and emits a bang , due to the high thermal expansion of the air.
The corona effect appears in high voltage wires, due to the large electric field around it.
See also: Fun facts about lightning
Examples of dielectric materials
- Distilled water
- Capacitors: These are electrical devices formed by two conductive plates “filled” by a dielectric medium. The electric field formed between the plates polarizes this medium, and this greatly increases the ability of these devices to store electrical charges.
- Transformers : The varnishes used on transformer coil wires are able to insulate them from each other, as well as the oil that is used to cool them has dielectric properties.
- Various sensors : There is a wide range of pressure , temperature and light sensors that use dielectrics to indicate minimum variations in these quantities.
- Touchscreen Screens : Smartphone screens are made of dielectric materials. When the finger touches the screen, a circuit similar to a capacitor is closed. Small load variations are detected indicating the occurrence of touch.
Dielectric constant is the physical constant that influences the build-up of charges in a capacitor . The greater the dielectric constant of a medium, the greater its ability to accumulate charges for a given potential difference . The dielectric constant is defined as a multiple of the vacuum dielectric constant. The vacuum dielectric constant has a value of ε 0 = 8.85418782.10 -12 C 2 N -1 m -2 .
Any material other than a vacuum has its dielectric constant defined based on the following relationship:
ε — dielectric constant of the medium
ε 0 — vacuum dielectric constant
K — relative dielectric constant
Check the values of dielectric constants of some known media:
|Material||Relative dielectric constant (k – C 2 N -1 m -2 )|
|Aluminum||8.1 to 9.5|
|Paper||4 to 6|
|Mica||5.4 to 8.7|
When a material has a constant k = 5 , for example, this means that its capacitance will be five times greater than that of a vacuum. If a capacitor receives this dielectric between its plates, it will be able to store five times as much charge.
Capacitor formula with dielectric
The formula we use for parallel plate capacitors is shown below, please note:
C — capacitance
A — area
d — distance between the capacitor plates
For cases where the capacitor has a dielectric inserted between its plates, we must take into account the constant k, check:
The dielectric inserted between the plates of a capacitor greatly increases its ability to store charge.