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What Is Permittivity? - Definition & Examples

Instructor: Damien Howard

Damien has a master's degree in physics and has taught physics lab to college students.

Discover what the electric permittivity of a substance is, and how it's determined by that substance's reaction to an external electric field. Then go further and learn what the permittivity of free space and the dielectric constant have to do with electric permittivity.

Conduction

Most people are familiar with the concept of electric conductors and insulators. You learn at an early age not to stick a fork into an electrical socket. This is because the metal the fork is made out of is a conductor, and will allow the electric current to pass through it into whoever is holding it. Conversely, electrical wires are wrapped in plastic because it's an insulator. This stops you from getting shocked when touching an electric cord.

The metal electrical cable is wrapped in plastic to protect you from being shocked.
electrical wires

If an object is a conductor it has high electric conduction, and if it's an insulator it has low electric conduction. Electric conduction tells us how easily a substance allows for electric current to pass through it. In your studies you may have learned along with electric current, an electric field is also capable of passing through a substance.

The question would then be: is there something similar to conduction that tells us how easily an electric field can pass through a substance? The answer is yes, and it's called electric permittivity. In this lesson, you're going to learn how electric permittivity works, and see some common examples of it.

Electric Permittivity

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 capacitor creates an electric field shown as red field lines between its metal plates.
capacitor diagram


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 left image shows the molecules in a substance under normal conditions, and the right shows them in an external electric field.
molecules in an electric field


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, a.k.a. 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.

Permittivity Examples

There are three different main ways you see electric permittivity (ε) expressed in a science course. The first is the general form of electric permittivity. You'll most likely see it in a physics course relating electric field (E) to electric flux density (D). Electric flux is the number of electric field lines that pass through a given area.


electric flux density equation


The value for electric permittivity varies depending on the substance in question. The most famous of these values for electric permittivity is our second of our common ways you see this concept expressed in science, the permittivity of free space (ε0). This is the electric permittivity of a vacuum, i.e. the absence of any substance. It's a famous constant that shows up multiple places in physics such as the speed of light in a vacuum (c) and Coulomb's constant (ke).


equations with permittivity of free space


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