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Encyclopedia of Electrical Engineering
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Encyclopedia of Electrical Engineering
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Electric Permittivity

What is Electric Permittivity?

Permittivity is a measure of how easy or difficult it is to form an electric field inside of a medium.
Permittivity is the measure of capacitance that is encountered when forming an electric field in a particular medium.
Permittivity is the ability of a substance to store electrical energy in an electric field.
Permittivity, usually denoted by the Greek letter $\epsilon$, and its SI unit is farad/meter (f/m). The lowest possible permittivity is that of a vacuum. which is denoted by $\epsilon_0$.
The permittivity of free space (a vacuum) is a physical constant equal to approximately $8.85 \times 10^{-12}$ farad per meter (F/m).
Fig.no.1: Electric Permittivity.

How electric permittivity works?

To understand how electric permittivity works let's start by looking at a capacitor. A basic capacitor has two charged plates that are separated by some substance. One of the plates gathers positive charge on it, and the other gathers negative charge. This creates an electric field through the substance between them.
The trick to electric permittivity is what happens to the substance between the capacitor plates when it is placed in an electric field. That substance is made out of atoms, which bind together to form molecules. When these atoms form into molecules they often form dipole moments. This means that positive charge is at one end of the molecule, and negative charge at the other.
These molecules normally align randomly with each other in a substance, but when an external electric field is introduced they align themselves in such a way that the electric field their dipole moments produce resists the external electric field. In other words, the electric field created by the dipole moments acts in the opposite direction of the external electric field. This process can be seen in the image below where the red vectors represent the electric field lines.
The better the molecules align the more they resist the external electric field. This can be seen by the red field lines in the image above.
Electric permittivity is a measure of how well the molecules of substance align, polarize, under an electric field. The higher the electric permittivity the better the molecules polarize, and the more that substance resists the external electric field.