3.5 Electric Permittivity 😃

In this section, we will basically go over some important units and constants. Two main constants used in this unit that might be very new to you are k and \epsilonϵ. 🧠

kk is known as the Coulomb's Law constant. You have not yet learned about Coulomb's law yet, but you will in the next section. Here, we are just going to familiarize you with some constant you'll see before we tackle the most important equation in electricity. Coulomb's Constant shows up not only in Coulomb's Law, but in many equations that you will encounter in this unit. This constant has some cool history behind it so let's do a short segment of story time.

This is what is so special about the Coulomb's constant. It itself is not super important in the derivation or theory but exists to make the math and work cleaner. So it's more of a shorthand notation constant. 👀 Because the constant itself is not necessary, you will see some books still use the full notation with the 1/4\piπ\epsilon_0ϵ0​. This constant works best when we have symmetrical charges and objects which is most of the time. That's why it's not an SI unit, but more of a formality. That's why it's also called a rationalized unit.

There are 2 very important characteristics of space and objects: permittivity and permeability. Permittivity is a measure of the obstruction due to an electric field. If something has relatively high permittivity, then the electric forces and fields will be weaker. Permeability is a measure of the flow of a magnetic field. High permeability means more flow of magnetic field/force. 👌🏻

As you saw above, permittivity is more about electricity and polarization, and permeability is more about magnetism. So, we will learn more about permeability later, and for now, we will focus on permittivity. 🔬

Keep note that we mentioned both these constant together, even though one of them comes up a couple of units later because they are connected. Light 💡 (EM waves) is a topic that brings them together. You'll learn about electricity now, magnetism soon, and finish the course with light, optics and modern physics. The speed of light, arguably the most fundamental number in physics, is derived from these 2 constants. That is the beauty of physics!

The constant ϵ is called permittivity. We measure permittivity relative to the permittivity of free space or a vacuum. We notate the permittivity of free space as ϵ0​.

We can measure permittivities if other objects relative to a vacuum. Relative permittivity (ϵr​), also known as the dielectric constant (κ), is simply calculated by ϵ/ϵ0​. We will come back to the dielectric constant later when we learn about conductors and circuits!

There isn't much to know more about this constant until you get into modern physics. The nuances of this constant are beyond the scope of this course and even many college courses!

As you can see from the table above, the relative permittivity of air to vacuum is extremely close to 1. So for the sake of our calculations, we can pretend we are in a vacuum and make calculations because the difference would be insignificant.

1. Body tissue has a relative permittivity of 8. Find the permittivity of body tissue.