 Hi, I'm Zvor. Welcome to Unizor Education. Today, we will talk about another form of energy, heat, which is basically inner or internal energy of the object. Now, this lecture is part of the course called Physics for Teens. It's presented on Unizor.com. I do suggest you, by the way, to watch this lecture from the website rather than from YouTube or somewhere else where you found it, because the website contains very detailed notes for each lecture. Plus, it's presented in a logical sequence of other lectures. So you will see where exactly this lecture is relative to the other other ones. And for those who would like to be challenged, there are exams for many parts of this course. Also, on the same website, Unizor.com, you have a prerequisite course, which is Math for Teens. I do consider mathematics as very important to be studied before you study physics, because physics actually is using a lot of math, especially something like vector algebra and calculus. All right, so back to heat. Now, this is the second part of the energy partition, if you wish, of this course. The first part was about mechanical energy, kinetic energy, potential energy. Now, this is the second part, which is related to heat, which is internal energy of the objects. And before going into discussion about heat as a form of energy, I would like actually to spend some time to discuss what exactly our objects or substances we are dealing with consist of primarily molecules and what are they doing inside our objects. It's very important to understand the nature of the heat. So, molecules. Let me start with a very simple object, like a drop of water, for instance. So, you have a drop of water. So, I will do, I will split it in half, and I will have basically half of that drop. Then I will take the half of this drop, I will split it in half as well. I will have a quarter. Now, if I will continue this process long enough, I will come to a point when the quantity of water I have, it's still water, but I cannot really split it in in halves, so each half would also be water. Now, this is the smallest part of the substance, in this case substance is water, is the smallest part which basically retains all the properties of the big one. The big one was the drop of water, and this is, well, you can say it's a drop of water, basically it's a molecule. That's what the molecule actually is. By definition, the molecule is the smallest part of any object or substance which retains the physical quality of the big one. If you will split it even further, you will not get water anymore. Now, we all know, for instance, the water contains two atoms of hydrogen and one atom of oxygen. Now, I do not talk about atoms right now, we will talk about this later on, but in any case, if you split this molecule, somehow, you will get basically this picture. This is hydrogen atom, this is hydrogen atom, and this is the oxygen. This is basically inner structure of the molecule, but we are not talking about this right now. We are talking about molecule as the smallest part which retains the qualities of the water. Okay, so we know what the molecule is. By definition, the smallest part of the substance or object which retains the qualities. Now, what are these molecules do? What are they doing inside this object? Well, it depends. In certain cases, molecules are really attached to each other into some kind of orderly structure and rigid structure. For instance, if you will take a look at the molecules inside some metal, for instance. If you go inside of iron or steel or something, steel actually is an alloy, but no matter what, it still has this crystalline structure inside, and all the molecules are very tightly attached to each other. They are not really significantly moving against each other. What they are doing is they are actually oscillating. So consider the same picture, but you have springs here. So that's how the molecules are arranged in some cases. Now, what actually happens in this case? We have the general shape of an object which contains these types of molecules and these types of connections between them. The general shape is always retained. So if you will put it in the gravitational field, for instance, the piece of iron will be still a piece of iron. If you will put it in space where there are no gravitational forces, it will still be the same piece of iron. So this is kind of a state of the matter, which we are called solid. So the solid objects are those objects where the molecules are only vibrating around their places, but their places are the the middle point of their vibrations still the same and it's fixed. Now, there are some other cases. The cases when the molecules are attached to each other, but not as rigidly as in this particular case. They are not just vibrating. They can actually move around each other, but not far away. The model which I can envision right now is, let's consider the model, the molecules are covered with some kind of a sticky subject like a strawberry jam. All right, so these molecules are really like little balls with a strawberry jam around them. They are sticking together actually, but not very hard. I mean, you can really shape them. If you will put them into, let's say, some kind of a vessel or reservoir and you have a gravitational field, for instance, they will all go down, but they will still stick to each other. But if you will change the shape to this one, they will be in a different shape and they will still stick to each other. So these are liquids. So the liquid is a different state of the matter, solid, solid and liquid. Now, if you will put it in, let's say, a space without any gravitational field, it will still stick together, right? But there are no forces which are pulling them into one particular site. So in most of the cases, they will they will just form some kind of a sphere because they are still sticking to each other. But there are no outside forces, so they will make a sphere. And there are different, actually, explanations of why this is sphere. We will, which we might address in the future when we will talk about properties of liquids, about surface tension, etc. But anyway, these are still sticking together, but not as hard as in this particular case. Finally, there is another state of the matter, which we are which we are talking about as gas. Now gas is when, basically, there is no strawberry jam. So these are individual molecules which are not really very much connected to each other. And whenever you have certain space available for them, they will start moving in all the different directions and they will fill up the whole space. So the gas is something which has the weakest links between the molecules and the fact that they are moving really makes it to take the whole volume available for it. So solids, liquids and gas are three main states of the matter. I mean, there are some others like plasma, for instance, we are not talking about this. The major three major states for our purpose are solids, liquids and and and gas. Now in all three cases molecules are not standing still. They're still moving within certain limits. This one for the solid they're just moving around their point of equilibrium like on the springs. Now in this case, these are moving without actually losing the contact. So one particular ball covered with strawberry jam can move to another one. It will still be part of the whole liquid, the whole of the body of the substance. And if there is some kind of a gravitational field, it will still go down and fill up this particular vessel. And finally the gases. The gases are always the weakest connection among themselves and they're always taking the whole space. And obviously they are completely chaotically moving, much more chaotically obviously than liquids and liquids are much more chaotically moving than the structure of the solids. By the way, in case of solids, you are not necessarily having this kind of a structured order of the molecules. So it's not necessarily of this kind. It can be for instance something like this. Let's talk about the same kind of model. So you have the balls covered, not in the strawberry jam, but in glue basically. And the glue is basically hardened. So you don't have really a structured symmetrical structure like this one, but you still have a solid shape. They're still moving against each other. And maybe even moving a little bit if glue is not really completely hardened. But still the general shape is preserved for these solids. And the difference between this structured order and non-structured order of the solids is in terminology. These ones are called crystals or crystalline type. And the ones which do not have such an orderly structured is called amorphous. Okay, these are just terminology. Everything is by the way on the internet in the notes for this particular lecture. All right, so we have covered this type of things. What's next? Heat. Okay, now we are ready to talk about heat. What is heat? Okay, the heat is intensity of the moving of the molecules inside the body. So whenever you are heating up for instance this solid, these molecules are shaking much harder. They're still maintaining their neutral position, but the amplitude is increasing. You don't see it obviously because we are seeing on a micro level and this is on a level of molecules which we need special instruments to do it. But that's what happens. In case of liquid, again, this strawberry jam is basically trying to keep them together, but they're still moving much more intensely as the heat coming. And finally, the gas is chaotic movement of all the molecules within the volume and they're moving much faster. So the intensity of the moving of the molecules, intensity of the molecular movements inside the object or inside the substance is actually the manifestation of heat and that's what we call heat. Now, these molecules are shaking or moving around, etc. So whenever we touch something which is hot, it means the molecules inside of that body are moving very, very fast. And the molecules inside our finger, for instance, are moving much slower. And then when you touch them, what happens? Well, if you just take two, for instance, different volume of gases in one volume, you have gases which are moving very, very fast. The molecules are moving very fast and another slow and then you open the border between them. What happens? Well, those which are fast start moving towards those which are slow. They're trying to mix together and gradually their moving becomes uniform, relatively uniform, because those which are faster will start hitting those which are slower and increase their speed correspondingly. And eventually the distribution of these speeds will more or less even out. So that's exactly what happens when you're touching something. The molecules which are moving very fast are touching on the border the molecules which are moving slow and these molecules which are slow becomes faster. So that's how we are feeling that something is happening on this border between our finger and the hot surface. And eventually these molecules which are moving very fast coming through the finger and they are hitting the receptors, the nerves, etc. etc. and the signal goes to the brain and that's we feel actually that this is hot and this is cold or something like this. Now, if it's cold the other way around, obviously, if you're touching the cold thing then our finger has faster molecules and they are touching those which are slower. So the heat actually goes towards the slower ones decreasing the speed of the molecules inside the fingers. And again we are feeling some difference. All right, so this is basically the most important part of this lecture. Heat is an intensity of the molecular movement. So it's like a definition basically if you wish. That's what I wanted to make sure you all understand. So you can talk about kinetic energy for instance of individual molecules. Obviously different molecules have different kinetic energy but since they're all hitting each other eventually it's more or less evenly spreading within the object. So if you for instance start hitting a metal rod on one end, the fast molecules will hit the slower ones making them moving faster and they transfer this movement, this energy further and further. Even if you consider this relatively rigid structure of the solid, if you start shaking one particular molecule it will eventually shake the whole system because the movement is transferred from one spring to another, from one connection to another. So eventually the, well I don't want to say the term temperature, I will still try to determine it a little later. But anyway the heat is dissipating. Okay what's next? Now, and we were talking about how energy is transferring from one part of the object to another. Okay now what happens for instance with different states of the matter? If we will start hitting for instance the solid what happens? Well these molecules are shaking more and more intensely. And eventually this shaking can actually destroy this nice structure. I mean if heat is really very very intense then the shaking of these molecules can actually break the links and these links become, they still exist, the attraction still exists between the molecules but you're ripping them up all the time and new ones are actually formed. So this is melting. So when you heat the steel for instance it will melt into liquid. Now what happens with liquid for instance if you will warm it up? Well again these molecules start moving faster and faster. These are basically the balls as I was saying covered with strawberry jam but if they're moving very very fast some of them can actually break this link between them. This strawberry jam is sticky but not sticky enough and they will start evaporating and gradually this liquid can be transformed into the gas. So that's how different states of the matter are transforming one to another as we are hitting the objects. And there is obviously the reverse if you start freezing the gas for instance you can freeze the halium into liquid halium and then liquid for instance the water can be frozen into ice. By the way ice also has this crystalline structure. So you have transformed a substance or an object from one state of the matter to another by using the heat by putting more heat or or getting heat out from the object. So the transformation between the different states of the matter is important and is related very much with the speed of the motion of the molecules which is actually a heat. Okay done that. What else is important? Let's consider we applying heat to the same steel rod which we were doing this but not as much to melt it completely. Well just let's say from the room temperature to another I know like the temperature of the boiling water right. What happens if I will take the steel rod from the room temperature and put it into hot water? Well let me tell you a practical example. If you have a can which you have to open by unscrewing it what happens. I mean it's really very difficult but especially if you took it from the refrigerator right. So you have a glass can with let's say pickles and you have this lid which screwed on and you're trying to unscrew it and it doesn't really happens with bare hands because it's really hard. So you put it under a hot water only the lid all right. And then all of a sudden it's much easier to open it. Why? Okay so what happens is the following. Again let's go into the internal structure. You have the structure and then you start shaking these molecules around. Now whenever you're shaking them they are a little bit spreading apart. So the size of the object is slightly increasing with the heat. I mean different substances are increasing in size differently by different percentage or different degree or whatever else. For instance glass is really kind of difficult to increase in size by hitting it up. But the metals for instance are really increasing in size noticeably. Now how can we for instance notice that? Now if you know whenever you're having the rails. So there is one rail they put and then another rail they have a little gap in between. Why? Because in hot weather the rails expand so if you will put it completely touching to each other they will bend and we don't want bent rails right. So this is just an example of really practical case when things do expand with the heating. Okay they change size so usually usually not always but usually with the temperature increasing the geometrical size of the object is increasing and decreasing in an opposite direction. There is actually one very interesting well you can call it exception from the rule. But this exception is so important that the whole life on earth actually depends on it. So let's consider the water. Water is a very special liquid by the way. So let's consider the water and we are gradually bringing down its temperature. Now as you bring down the temperature from let's say a room temperature I'll use the Celsius for instance 20 degrees Celsius. You bring it down to four degrees Celsius. So what happens? Well it probably decrease in the volume just a little bit but then if you freeze it a little bit further it actually increases the volume. So this is something which is the minimum I would say and then whenever it's converted into the ice ice actually has a greater volume than the water at four degrees. Why is it important? Well if it has a greater volume and I don't know why I mean it's just that's how it is. Now if it has a greater volume it has less density. So this is more dense than this and what happens when the lake for instance is covered with ice? Ice is on the top because it's less dense than the water underneath. Now if it was the other way around if the temperature as the temperature goes down the volume goes down down down and which means that the density goes up and up and up with the same mass. Then the ice as soon as it forms it will go down to the bottom and then again and again until the whole lake would be basically completely frozen and if it's frozen and no fish there etc etc. So this preserves actually the life in the water during the winter time when the ice is actually formed on the bottom. Okay so this is a very important thing. Size changes with the temperature. Now this actually opens the door for us to use this change of the size as the measurement of the heat which is inside this particular object or substance. How can we do it? Well actually it's kind of simple thing. So now we're talking about thermometer which helps us to measure the temperature. Now what is temperature? Well temperature is actually in plain words its intensity of the movement. So we rarely need the entire amount of energy in the object. I mean maybe we do but in most cases we are not interested in this. We are just interested in how hot this object is if I will touch it. So what does it mean? It means I have to really measure the intensity of the movement of the molecules of this. Okay what is the intensity of the movement of the molecules? For instance you can measure the mass and the speed and that's why kinetic energy of every molecule of this object. Then you add them up divide by the number of molecules and you have an average amount of energy kinetic energy per molecule. That's actually a good measurement of intensity of the movement right of the temperature. The average amount of kinetic energy per molecule is a good measure. The problem is we can't measure it our best. What we can measure is something a little bit simpler which gives us basically a relative understanding of how intense the movement of these molecules actually is. Let's take for instance a classic thermometer which has some kind of a reservoir with let's say mercury. You know quicksilver and this is the level of the mercury and this is a relatively thin tube. Now why is it thin? Very important. If I will increase the temperature of the mercury, mercury is liquid. Liquid will expand and if this is a thin tube then it will expand under relatively noticeable lengths. The thinner the tube is the same expansion of this mass would actually take it higher right. So that's very important because if you don't have this for instance if you have very like a glass right and you have this mercury. Now obviously if you will heat it up it will increase the level because the volume will increase but very very tiny increase. But if you have a thin tube here it will increase much more noticeably. Okay so this is a good instrument to measure the temperature of the mercury inside. So for instance we just put it on the table and we have some kind of a mark obviously on the on this tube and we know it goes up or down which means the temperature is higher or lower. How can I measure temperature of something else not this particular thing? Well you remember that whenever you are touching something the molecules which are faster force the molecules which are slower to move faster. Well then it depends actually which body is more massive. Obviously if you have a very small body and a very big body then whatever is happening in the big body would be the most important part and the average after you will add this small attachment. So if you have a big and this is the heart and the tiny which is cold what happens? Well if you touch them then this cold will become almost as hot as this one. I mean there is something which is maybe lowering down the temperature of this but very very insignificantly. So considering this is the big body and this is a small one the resulting temperature of this thing would be practically the same as this one. So I will take this device which is very small one actually and I will touch with this device a big one. Let's say I would like to measure the temperature of the body so I have this thing thermometer to to put it into mouse or under arm or whatever and eventually the temperature of the body will influence this particular thermometer to have the same temperature and then I can again look at the level of this mercury in the tube to find exactly what exactly the temperature is. So all I need right now is to quantify the whole thing. Okay now how can I quantify it? Well first of all we have to find some base points. Now in the Celsius system of measuring the temperature the two most important points are the point of freezing when the ice actually is converting into water water into ice the melting ice kind of a temperature. It's by definition a zero degree of Celsius. Now another base point is boiling water which is by definition a hundred Celsius. So all the it is okay let's consider this is zero this is 100 divided in 100 little pieces and we will have degrees. So the temperature is measured in degrees of Celsius. Now obviously each degree can be broken down into tenths of degree or whatever. Now if the temperature goes down or the temperature goes up whatever it is doesn't really matter we have the whole scale of temperatures. Now this is basically the temperature the way how to measure temperature it's all over across the world except United States. In United States well and its territories the measurement is in degrees of Fahrenheit. Now the Fahrenheit also has two main points zero and 100. Now 100 is almost temperature of the body well at the time when it was invented they considered the temperature of the body to be a hundred degrees of Fahrenheit and then they had some kind of a solution some some kind of a salt I don't even remember which one. The point of freezing of that solution it's not water some kind of a solution chemical solution was taken as the zero point and again they did exactly the same thing they divided the scale into hundreds of degrees etc. Obviously it's a different scale I mean different things were taken as the basis and the one degree Fahrenheit is not exactly the same as this one but there is a very simple formula if you subtract 32 from Fahrenheit and multiply by five nines you will get Celsius. So that's how it's converted now in which case zero Celsius is basically 32 Fahrenheit and finally there is another measurement scale it's Kelvin degrees of Kelvin. Now degrees of Kelvin are very much like Celsius in what is a degree so the size of one degree is exactly the same as in Celsius but the zero point is different. Now the zero point in Kelvin is the absolute zero. Now what is absolute zero but if you go out to space there are no stars in in the vicinity nobody actually gives you any source of energy well the temperature over there is called the absolute zero and absolute zero in Kelvin is minus 273.15 degree of Celsius so that's the conversion basically so you have to add to Kelvin you have to add 273.15 to get into Celsius. So these are three different scales now the Kelvin is used in scientific research because the Kelvin degrees are very nicely accommodated in formulas of physics and thermodynamics etc which we will talk about. Now the Celsius is again across all the countries except the United States and the United States is using Fahrenheit. That's basically all I wanted to talk about this is an introductory into what actually heat as energy is. There are no formulas as you see for a change usually I have some formulas around not this time but no there is one formula yeah this one. Anyway so this is basically an introductory into heat as the form of energy. I do suggest you to go to this website and read the notes for this lecture. It's kind of a also relatively concise explanation of the same topics and it's very important to know exactly what we mean when we're talking about heat as energy when we're talking about temperature as basically intensity of the movement of the molecules etc and again everything depends on molecular movement as the source of this heat energy. It's internal energy which is concentrated inside the object or substance. That's it. Thanks very much and good luck.