What Are Dielectric Materials?

A dielectric gives a capacitor its’ capacitance.

Why it’s important: Dielectric materials are insulators that have a high degree of polarization when a voltage is applied across them. Another way of saying this is that they have high capacitance, or a high ability to store charge. For this reason, they make great capacitors. Dielectrics are one of the most important types of material used in electronics. If you can understand how a dielectric becomes polarized, you will have a solid understanding of capacitors.

What are Dielectric Materials?

A dielectric material, or dielectric, is an insulator that can be polarized by an applied voltage.

The voltage generates an electric field which then influences the atoms and molecules in the dielectric material.

Instead of conducting electricity, the atoms or molecules in a dielectric react to the electric field. The material becomes polarized; instead of being electrically neutral, one side accumulates a negative charge and the other accumulates a positive charge.

Since a dielectric is an insulator, the charge does not flow through the circuit but instead forms an electric field that stores electrical energy. A capacitor will store energy up to the maximum voltage applied, like the voltage of the battery it is connected to. Then, when you disconnect the battery, reverse the battery, or create a current path, the energy from the capacitor will discharge into the circuit.

Polarization in Dielectrics

The simplest way to think about polarization is that the atoms or molecules in the dielectric rearrange themselves in response to the applied voltage. In a dielectric without a voltage applied, the atoms in the material sit in their normal configuration, with their electrons centered around the nucleus.

When we apply a voltage, an electric field is generated across the dielectric material. It’s electrons move positions slightly because they are attracted to the positive side. They shift so that they are a little closer to the positive side.

The atoms on the side of the dielectric that is connected to the positive terminal, will shift a little so that their negative charges are closer to the positive side. The atoms on the side connected to the negative terminal will shift so that their positive charges are just a bit closer to the negatively charged plate. Like a spring being compressed, the atoms or molecules respond to the voltage. When the voltage is released and the dielectric is allowed to relax, the atoms and molecules spring back to their normal positions, pushing the charges that have accumulated on the plates back through the circuit.

Dielectrics vs. Insulators

The terms ‘dielectric material’ and ‘insulator’ are often used interchangeably, but this clouds the purpose of each. An insulator is a material that is selected because of its’ resistance to current flow. A dielectric is a material that is selected because of it’s high polarizability.

Even though dielectrics are insulators because they don’t allow current to flow, a perfect insulator is not dielectric. This is because using a dielectric in place of an insulator could cause charge buildup where we don’t want it. So the perfect insulator is actually a poor dielectric; whereas a perfect dielectric is both highly polarizable and a perfect resistor.

Where Does Capacitance Come From?

It’s important to note that capacitance is an inherent property of a device. The amount of charge that can be stored per volt is not a result of the applied voltage, nor of the charge itself. It is the result of the type of dielectric used, and the construction of the capacitor. If we increase the size (area) of the two capacitor, the total capacitance will increase. If we increase the distance between the two plates, the total capacitance will decrease. This is expressed by the following formula:

C = \epsilon \frac{A}{d}

Here, ε is the permittivity of the dielectric used. Permittivity is a constant used to describe how easily a dielectric polarizes. It is basically a constant that tells us how good a material is at storing electrical energy. The higher the permittivity of a material, the higher the capacitance will be. This means that more charge can be stored per volt.

Capacitance and Permittivity

Permittivity is actually measured in Farads per meter; how much capacitance the material has per meter.

Even a vacuum has permittivity, which is eloquently called the vacuum permittivity. The vacuum permittivity has its own symbol, and is an important quantity in its own right. The symbol for vacuum permittivity is ε0, and its value is very small:

\epsilon_0 = 8.854×10^{-12} \frac{F}{m}

The permittivity of any material is expressed by comparing it to the vacuum permittivity. This ratio is called the relative permittivity, which is sometimes written as εr and sometimes written as κ:

relative\hspace{1mm}permittivity = \epsilon_r = \kappa = \frac{\epsilon}{\epsilon_0}

The permittivity of materials is usually expressed as relative permittivity. Vacuum has a relative permittivity of 1 by definition, and air is just a bit higher. Remember that the higher the relative permittivity, the higher the capacitance. A material with a high relative permittivity can store a lot more charge per volt than air. Here’s a table with permittivity values for some common materials:

Materialεr
Air1.00058986
Alcohol16-31
Body Tissue8
Glass3.7-10
Mineral Oil2.1
Paper1.4
Vacuum1 (By Definition)
Water80.2
Wood2-6

We’ve also put together a comprehensive table of relative permittivity values for a wide variety of materials, which you can find here.

Lesson 0: Introduction to Module 3

Lesson 1: Introduction to DC Circuits

Lesson 2: Series and Parallel Circuits

Lesson 3: DC Power Sources and Batteries

Lesson 4: Resistors, Capacitors, and Inductors

Lesson 5: Resistors in Series

Lesson 6: Resistors in Parallel

Lesson 7: Voltage Dividers

Lesson 8: Kirchoff’s Current Law

Lesson 9: Kirchoff’s Voltage Law

Lesson 10: Capacitors

Lesson 11: Dielectric Materials

Lesson 12: Capacitors in Parallel

Lesson 13: Capacitors in Series

Lesson 14: Capacitors in Series and Parallel