Dielectrics and Dielectric Constant

in this lecture we're going to talk about dielectrics and the dielectric constant which was observed when we spoke about capacitance and capacitors now let's recall what a capacitor is a capacitor is simply a plate that has the ability to store charge and this electric charge can then be used to do useful electric work such as power a motor were light bulb now let's look at a parallel plate capacitor a parallel plate capacitor is composed of two parallel plates of the same size that create a difference in electric charge in other words if we take this uncharged parallel plate capacitor and we connect our circuit to a battery such as this battery what happens in the following once we connect these guys the anode releases electrons the atom of our battery releases electrons these electrons travel into our into our first plate and our electrons accumulate on this plate at the same time electrons from this plate leave this plate and travel to our kathal our cathode is where reduction takes place and reduction requires electrons and that's exactly why electrons leave this place and travel to our kathal now eventually when this guy's fully charged the amount of volts or voltage of this battery would be identical to the voltage in this so if this battery say has 1.5 volts this parallel plate capacitor when fully charged we'll have 1.5 volts the electro-motive force will be 1.5 volts so if we take away this battery and now we place a light bulb or a motor instead the battery this parallel plate capacitor will be able to do electrical work and will power our glow or make our light bulb glow or make our motor function now these parallel plates must be separated by some distance D if these plates were not separate by any distance define our distance zero that means our plates will be touching and if our plates were touching then the electrons on this plate would be able to flow from this side to this side and that means no electric charge will be created no voltage is created and if no voltage is created no work can be done no electrical work can be done so it's important that there is this distance between them and this distance will ensure that no electrons flow from this place to this place as long as the substance separating these guys found in between these two plates is an insulator is a nonconductor in other words the plates are separated by distance T the substance that separates the two plates should be an insulator non conductive because if we placed a conductor remember conductor allows electrons to flow while a nonconductor does not allow electrons to flow so on so if we place a conductor that means electrons would be able to easily fall from this side to this side the point is to put a substance in between these guys that does not allow electrons to flow so we need to put an insulator and this insulating substance is called a dielectric and every substance has its own specific dielectric constant a value that relates to that substance to the atoms that compose that substance and we saw that when we spoke about capacitance the capacitance of a parallel plate capacitor is given by the following equation capacitance is equal to dielectric constant K times the area of our plate times this epsilon naught a constant that depends on the Coulomb's constant divided by the distance between them now this dielectric constant refers to the insulating material that separates the two parallel plates so let's look at the purpose of the dielectric now the dielectric has three main purposes our first purpose is the following it allows charge to separate thereby creating a change in voltage remember if there was no dielectric constant if these guys were connected or if the material separating these guys was a non insulator or a non conductor or a conductor that means that electrons would be able to flow that that means charge would not be able to separate and that means that no change in voltage will develop across these two plates so the first and main purpose about dielectric our insulating material is to make sure that charge does not flow from this side to this side now the second and third purposes are the following the second purpose is it allows capacitors to store more energy than usual and the third purpose is it puts a limit on the voltage created by capacitors so let's look at number two and three in detail because they are important so let's examine point number two more closely let's see why dielectrics allow our capacitors to store more energy so let's look at two cases in our first case we have a parallel plate capacitor that's separated by some distance T and this distance is completely filled in with a dielectric and insulating material now within this dielectric we have molecules that have a dipole moment now we'll see what dipole moments are in a second first recall that if this capacitor is not charged like it is in this case that simply means that we have not yet incorporated a battery into our circuit and that means that this plate in this plate having that charge of zero so if there's no charge separation that means there is no electric field and there and if there is no electric field that means that there is no voltage difference between this side and this now we just said that our molecules within our dielectric have dipole moments what that means is the following suppose iron molecule is composed of two atoms atom X and atom why what dipole moment means is that one of these atoms is more electronegative than the other atom so say Y is more electronegative that means it's going to pull electrons closer to its side and that will give it a partial negative charge and pulling electrons away from X will give X a partial positive charge if that means our molecules within the dielectric will be oriented in some random manner in which our molecules will have a dipole moment now what happens once we incorporate a battery to our circuit well electrons from the anode or the negative side will begin to collect on this plane while the electrons on this plane well this guy will lead to our cathode where our reduction reaction takes place and so at the end when our talent plates are fully charged this site will have a net negative charge and this side will have in a positive charge now the quantity of charge will be the same difference will lie in the sides this side will have negatives that this side will have positive and now because there is a separation of charge that means there's an electric field and in an electric field our molecules will orient themselves according to our electric field in other words these guys that were randomly oriented in this situation because we didn't have an electric field orient themselves according to our field now because our field is doing work on these molecules orient them in such a way as to orient them along our field lines that means our electric field does work to orient the dipoles so energy is also stored it or anything our dipoles and the energy is given by the following formula where our energy storing our dipole moments is given by the negative of the vector of that dipole multiplied by our electric field created multiplied by cosine theta which is the cosine which is that angle with which these guys are aligned and now that means the following that whenever a dipole moment or whenever an electric field is created due to separation of charge energy is stored in in terms of electric potential as well as within the nine pole moments so a top of the energy stored in our voltage difference there's also energy stored in orienting these dipole moments these dipole molecules so the final purpose of dielectrics is the following some dielectrics have a maximum voltage in other words if this voltage is achieved our dielectric will break down and it will begin to conduct electricity to conduct electrons so if this voltage or above electrons will begin to flow from this side to this side allowing our capacitor to uncharged allowing electrical work to be conducted and this becomes very important whenever using certain devices because if your voltage goes above what you want your voltage to be that means your electrons can begin to move from this side to this side and electrical work could then be transformed into thermal energy or say mechanical energy now this maximum voltage is known as the dielectric strength and this dielectric strength has the units of volts / meters or

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Keywords: dielectric, dielectrics, dielectric constant, dielectric strength, potential energy, physics, lectures, physics lectures

Id: XEWfw7CHST8

Length: 10min 29sec (629 seconds)

Published: Mon Dec 10 2012