Dielectric materials are essential in the technologies we use every day for example they are used for energy storage in capacitors and to enhance devices based on semiconductors they are also present in liquid crystal displays industrial coatings cooling processes and this is not an extensive list in other words modern society wouldn't even exist without dielectric materials but what are dielectric materials what does the word dielectric even mean well get ready to find out let's look at the word itself dielectric the word is constructed from the Greek word dia that means passing through an electric well that refers to electric fields giving you dia electric this term was proposed by a scientist and philosopher from the 19th century William dielectric is difficult to spell so with time naturally it compressed into the word dielectric this etymology suggests that the dielectric material is a material that allows electric fields to pass through it that is what we will discuss we will also see why this property is so useful first we will look at what happens to an electric field when it is applied to a conducting material like a piece of metal then we will look at what happens to that electric field when it is applied to a dielectric material like a piece of ceramic that's where we will discuss the concept of electric susceptibility permittivity and the dielectric constant of a material dielectrics are used mostly in capacitors a capacitor is made of two parallel conducting plates if the plates are connected to a dc source a uniform electric field will establish between the plates that is why we will also review in this video uniform electric fields and their properties everything will come together in the last section of the video where we will place a dielectric material between the two conducting plates of a capacitor by doing so we will realize how that improves the capability of the capacitor to store energy but first let's go back to basics for a minute by defining what an electric field is an electric field is a region of space where charge q placed in that space experiences a force f equals q bye where e is a strength of the field or if you prefer the force experienced by one coulomb of positive charge placed in that field if I place a charge free to move around in an electric field it will feel a force and accelerate its position will change now let's imagine I place a piece of metal in a uniform field of strength e the piece of metal is a conductor, so electrons are free to move within it subjected to an electric fields the electrons will rush towards one end leaving the other end with a deficit of electrons in other words excess of negative charges on one side excess of positive charges on the other minus plus you get an electric dipole the trick lies here this new dipole generates its own electric field e prime that opposes the applied electric field e until an equilibrium is established and that the charges do not move anymore because the charges are totally free to move this equilibrium only occurs when the induced field e prime is equal to the applied field e, so the resultant field in the material is zero the applied field is kind of cancelled conclusion a conducting material acts as a barrier to electric fields cannot pass through and by the way this is a principle behind fajita’s cage now let's expose a dielectric material to the same field e a dielectric material is an insulator, so the charges are not free to move or maybe a little bit and this little bit is crucial to understand this we need to look at how atoms or molecules interact with an applied electric field let's start at the atomic level as you probably know an atom is composed of a positively charged nucleus and a negatively charged electron cloud located around it if you look at the center of charges for the electron cloud which is negative and the center of charge is for the nucleus which is positive they are the same position yes an atom is symmetric now apply an electric field oriented to the right the nucleus will be pushed to the right and the electron cloud will be pushed to the left the positive and negative center of charges dissociate and move away from each other creating a dipole if the center of charges ends up at a distance d from each other we say that this atom has an electric dipole moment p is equal to q multiplied by d where q is a charge of the electron cloud that was displaced note that p the electric dipole moment is a vector it has a direction the atom is polarized now take this electric dipole moment and multiply it by the number of atoms in the material what you get is the polarization vector for that material this type of polarization is called electronic polarization other types of polarization can occur too like ionic polarization if a material contains ions and space charge polarization if it contains a few free charges let's see another type more intuitive called orientation polarization the perfect example for that is the water molecule in a water molecule the oxygen atom is more electronegative than the hydrogen atoms so the bonding electrons will be closer to the oxygen than to the hydrogen consequence the oxygen is charged negatively and the hydrogen are charged positively the positive and negative centers of charges are not at the same position that means that a water molecule has a natural occurring dipole moment and thus a polarization that is why in chemistry water is said to be a polar molecule in the absence of an electric field water molecules are oriented randomly, so the total polarization of a volume of water sums up to zero water molecules can move around therefore when exposed to an electric field they will orient themselves, so that the dipole moment vectors align with the direction of the field this will provide a net polarization to the volume of water whatever the type of polarization it will have an effect on the resultant electric field inside the material look at this by applying an electric field e many small dipoles appear in the material each inducing their own electric field in opposite direction to e these contributions sum up in the material to a global induced electric field e prime that opposes the applied electric field e after a transitory period an equilibrium forms the resultant electric field in the material is the vector sum of the applied electric field, and the induced electric field that is e minus e prime so you see because the directions of the fields are opposite the resultant field within the material is weaker than the applied field so you understand now that the atomic or molecular structural material will define its ability to get polarized by an external electric field and consequently impact the resultant field within it this property is called the susceptibility of a material it measures how much a material gets polarized per unit of electric field it is represented by the Greek letter chi in the cgs unit system the polarization of vector is p is equal to chi by the electric field strength in the metric system p is equal to epsilon zero by chi by e where epsilon zero is called the permittivity of vacuum epsilon zero is there for historical reasons but it's just a constant really built up from other fundamental constants related to empty space so we realize that the electric sustainability is intimately linked to the ability of a material to weaken the electric field passing through it that resistance to letting the electric field pass through is called the permittivity of the material epsilon the permittivity of a material is the permittivity of vacuum epsilon zero multiplied by one plus c electric susceptibility chi the dielectric constant is defined as being the ratio between epsilon and epsilon zero and therefore also equal to one plus chi consider vacuum in vacuum there's nothing that can get polarized right therefore the electric susceptibility is zero that means that the dielectric constant is one that means also that the permittivity is epsilon zero susceptibility permittivity and dialectic constant all mean roughly the same thing the capacity of a material to get polarized in other words to oppose an electric field that is applied to it now that you have a good grasp of what a dielectric material is let's illustrate with the most common application energy storage if you're not familiar with what an electric potential is, or with what a capacitors before going further you'll need these concepts to fully understand the rest of this video let's look at two parallel plates that I connect to a battery delivering a voltage v give a few milliseconds for charges to accumulate and distribute evenly on the plates now you have a voltage v across the two plates the capacitor is fully charged every of the charges on the plates generate a radial electric field if you superpose all the electric fields generated by all the charges you end up with a resultant field e between the plates which is uniform this means that the electric field strength has the same magnitude in the same direction at all points between the plates by deriving a relationship between a uniform electric field e between two plates which are separated by distance d with a voltage v it's pretty easy to demonstrate that e is equal to v over d suppose now that i add another identical battery in series this will provide enough energy to double the amount of charges on the plate consequently it will double the voltage across the plates voltage show capacitor and the amount of charge on the plate are proportional the proportionality constant is called the capacitance c of the capacitor q equals c v or if you prefer c is equal to q over v the capacitance tells you how much charge you can store in a capacitor for each volt you apply to it lets replace the vacuum between the plates by a dielectric material the applied field will cause a polarization of the dielectric material which in turn will generate a field e prime that opposes the applied field e the resultant field between the plates e minus e prime is now weaker the thing is that the field between the plates must still be equal to v over d the distance d between the plates hasn't changed so the voltage v must have decreased but v is fixed by the battery in order to restore the original voltage v across the capacitor the battery will have to push more charges on the plates by replacing the vacuum by a dielectric and for the same voltage across the capacitor more charges can be stacked on the plates in other words the capacitance increases and that's not all by stacking more charges on the plates the battery needs to do more work that means that more energy is stored in the capacitor I will derive formulas that provide the energy stored in a capacitor but for now let's look at the one that is useful to us e the energy stored in a capacitor is equal to one half c v squared where c is a capacitance and v is a voltage across the capacitor with this formula you can clearly see that by increasing the capacitance at a constant voltage the energy that can be stored in the capacitor increases that is why dielectrics are used in capacitors you now know that when you apply an electric field to a dielectric material you generate trillions of small dipoles within it each dipole can be seen as a spring that is extended by an electric force due to the electric field within the capacitor remember your mechanics when you extend physically a bunch of springs you are working on them, you are transferring energy to them and this energy becomes mechanical potential energy that is stored within the springs well here it is the same idea an electric field applied to a dielectric material pulls positive and negative charges away from each other creating dipoles that means that the electric field is working on the charges in other words the electric field is transferring energy to the dipoles consequently that energy becomes stored as electrical potential energy within the material because dielectric materials can be polarized they can store energy
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