 Hi, I'm Zor. Welcome to Unisor Education. Continue talking about electric field. Certain properties of electric field will be discussed today. Now, this lecture and the next one, which is actually a problem solving, are preparation for the third lecture, which will be about capacitors or condensers. So, it's two short lectures of preparation and then there will be a little bit more physical part of this. Now, this lecture is part of the course called Physics 14, presented on Unisor.com. I do suggest you to use that website to go to this lecture, because the lecture is basically part of the course. There is some kind of a logical way of going from lecture to lecture. All material is interrelated. So, there is some sequence, which all these lectures are supposed to be taken. Also, there is a mass for teens prerequisite class, because mathematics is very important in physics. And a lot of things related to whatever I'm talking about are actually mathematics more than the physics. Okay, so what we are talking today is about something which is called permittivity. Such an interesting word, obviously related to the word permit to allow something. Okay, so let's start from the step one. Step one is the Coulomb's law. So, the force, which exists between two charges, is proportional to these charges. Let's say object A has a charge qA, object B has a charge qB. It's inversely proportional to the distance between them. And there is a coefficient here of proportionality, which basically is needed for allowing the dimensions. So, these two are measured in coulombs. This is, let's say, meters and this one in newtons. So this coefficient of proportionality is just needed to align all these dimensions, measurements, and obviously the dimension of this constant K, which sometimes is written as KE, to specify that it's electricity related. It's called Coulomb's constant, and its dimension is newton, to get the newtons on the left, meters square to neutralize this, and divided by Coulomb's square to neutralize this. So this is the dimension of K. All right. Now, I was talking about this when introduced the Coulomb's law. This R square is very important, and there is certain logical justification why it's R square. If you imagine a point object charged with certain amount of electricity, and it's in space, which means that whatever forces are going from this particular point object to attract or repel other charged objects are going radially from this point to all directions from the point in a three-dimensional world. If you imagine that this is some kind of an energy which is being dissipated, or the force which is being spread around something, what is it spread around? Well, it's spread around a sphere, right, in a three-dimensional world. So you can always imagine that whatever these forces are, they are going and they are they are radially emitted from this point object, and at any given distance are, it's actually distributed among the whole sphere of that radius, of radius R in this case. Now, the area of the sphere is 4 pi R square. So it's kind of more naturally, I would say, to express this in slightly different fashion. So certain amount of energy or forces, whatever, is divided and distributed on the whole surface of the radius R. So that's why I would prefer, and I think it looks more natural, to use 4 pi R square, which basically just changes the constant, no big deal. And for this particular case, the constant, for some historical reason, was entered in the denominator rather than in the numerator like in this. And the constant is called epsilon zero. So it's just another constant, which is definitely related to kA. So k is actually the same as 1 over 4 pi epsilon zero. This is the constant, this is the constant, it's pure arithmetic, nothing more than that. Or if you wish epsilon zero is equal to 1 over 4 pi kE. Now, that's fine. We have basically modified very, very slightly for purely aesthetic reasons, no more than that. So now 4 pi R square represents the area of a sphere of a radius R, where all these forces are distributed to. Okay, so this is my first kind of, not even the complication, just slight deviation from this formula to this formula. Nothing to it, just arithmetic. But it leads us to the next experiment. Now, whatever the situation was described in this particular case, it was based on experiments, obviously. So the person by the name Coulomb, probably, in which actually owner the constant was called, and probably some other people made experiments and they found that this is inversely proportional to R square. Then some other people, maybe more familiar with mathematics, or less, doesn't really matter, decided that this is a better formula. But in any case, all these experiments were conducted probably in the air. Then somebody else decided to change the experiment. And they have started conducting the same experiments not in the thin air, but on one hand, maybe in vacuum, that that's probably the same thing as a thin air. But on another hand, let's say in a sandbox. So what if you have a sandbox around this thing? And this is in the middle of the sandbox, and you're measuring the force, which basically goes from the source to a probe object, and it's all within the sand. Well, let me tell you, the results will be different. F will be different. And it looks like material, which this is immersed into, the material through which the electric field is penetrating, does matter. There are some materials which are significantly weakening this force. So what it appears is that this formula is good in vacuum, basically. Now, in air, it's almost okay. I mean, the epsilon zero is probably well represented the air. But if you conduct the very, very exact experiments, then the vacuum and air will differ. But significant difference is whenever we are putting something really significantly different between the source and the probe, like silicon, for instance, or sand, or marble, or porcelain, or something like that. So different substances are differently permitting, permitting, you see, that's permittivity. Different substances permit the propagation of the electric field differently. And this leads us to use, instead of epsilon zero, which is actually a constant derived for the vacuum, to something slightly different. So the more exact, I would say, or more universal way to express the intensity of the field is this. I put the suffix a, which means absolute. So there is something which is called epsilon zero, which is the permittivity of the vacuum. That's how this constant actually is called. So it's a permittivity of vacuum. And there is something like absolute permittivity of any particular material. So every material has its own permittivity constant. Now, instead, people decided to do it slightly differently. r epsilon zero r square, where epsilon r is epsilon a divided by epsilon zero. So if you divide absolute permittivity by the permittivity of the vacuum, you will get the relative permittivity. Now, this is the constant, which is like universal constant. And this is specific for each material. So it's a relative permittivity, which is a comparison between the permittivity of, let's say, silicon or sand relative to the permittivity of vacuum. And this is the kind of final formula, which I would like to offer you. And there are tables. I mean, for each material, there is its relative permittivity, which is sometimes called dielectric constant. So for each material, there is this relative permittivity or dielectric constant. Now, dielectric is basically a negation of electric. That's what prefix die means. So it's the resistance to propagation of the field. It's very close to something like viscosity of the liquid. There are different liquids with different viscosity. Water has maybe lesser viscosity with some kind of a gum or whatever else. They have much greater viscosity. It's difficult to go through this with some object. It resists the movement. So different liquids, because of different viscosity, different differently resists the movement. Same thing here. This dielectric constant determines how this particular material resists the propagation of the electric field. So if you have between objects A and B a material with high resistance to propagating electric field, the material with a greater level of dielectric constant, greater value of dielectric constant, then as a result, the F would be smaller, which means intensity of the field is reduced. Now, I was telling you that this lecture is a preparation for the lecture about capacitors. And capacitors do use this particular property. They put some material with higher level of dielectric constant between two charges. And that allows to diminish the electric field between them. And therefore, allows to accumulate more charges on these two objects so they don't really sparkle. There is no sparkle between them because the field is weak because there is this material with a large dielectric constant. So that's basically the preparation, this permittivity is preparation for condensers. And my purpose was to explain that there are different materials with different epsilon r, different relative permittivity or different dielectric constant. It's the same thing. It's the same two two terminologies which are related to the same thing, epsilon r. So the higher epsilon r, the more resistance the field actually has to propagate. And that's why there is less chances to have a sparkle between two objects if there is material between them with high dielectric constant. Well, basically that's it. A couple of examples. Okay, for silicon, for instance, this is something about 12, which means that the electric field will be 12 times weaker if silicon is between these two objects. What else I know? Air is very close to zero zero something close to vacuum basically. So that doesn't really provide any kind of real insulation. But there are certain materials with dielectric capacity of like 100, for instance. These would be probably the perfect insulators of electricity. They can almost like shield the charge and do not allow electric field to propagate outside of it. All right, another interesting thing. This dielectric constant is obviously defined for every material. However, if you change certain things about this material, for instance, temperature, or maybe slight change in chemical composition, if it's something like a salt or something, it all changes the dielectric constant. And by measuring the field with certain instruments which we have, we can actually measure the properties of the media, the medium between these two charges. For instance, we can measure the temperature, because if you will take, for instance, I don't know, the liquid, for instance, and the liquids has certain dielectric constant, but it's obviously changing with the change of the temperature. Not a lot, but changing. So if you can measure the change in the intensity of the electric field, it's, in effect, allows you to measure the temperature of the liquid. Okay, and basically that's it. That's all I wanted to talk about. Again, remember this word permittivity. So it's a relative permittivity, and another name for this is dielectric constant, which is playing a very, very big role in all our electrical equipment. That's it for today. Thank you very much and good luck.