 Hi, I'm Zor. Welcome to a new Zor education. So we will continue talking about heat transfer. In particular, we will talk about convection as one of the kinds of heat transfer. We have already considered during the previous lecture the conductivity. This is just another way of transferring heat from one object to another or from one part of the object to another. Now, let me just tell you from the very beginning that convection is a much more complex process than conduction. This lecture is about this complexity. I will try to convey this complexity to you. Now, this lecture is part of the course Physics 14 presented in Unizor.com. The website contains the course, basically, which is not complete as of right now. But gradually we are trying to put all the lectures, all the components in place. Now, the same website contains mass proteins, which is a prerequisite for this course, which I strongly recommend you to take. Because, obviously, I'm using a lot of mass during this course of Physics. And the website, by the way, is completely free. There are no advertising and you don't even have to sign in if you don't want to. Okay, so let's talk about convection in relationship to conductivity or conduction. Now, conduction is mostly considered for solid objects. So, if you have some kind of a solid object where the molecules are relatively fixed in place, the forces between the molecules or whatever else, they keep this solid object solid, which means molecules more or less are in place. However, they do oscillate. So, each one of them is oscillating when the heat is applied. And the more heat is applied, the more intense this oscillation is. Now, whenever we are transferring energy from a hot place, let's say this piece is hot and this piece is relatively cold. Now, this oscillation actually forces, it hits, it shakes, if you wish, the neighboring molecules. The neighboring molecules shakes its neighboring molecules and that's how the heat is transferred. So, in a way, you can consider this to be a microscopic transfer of energy from molecule to molecule. Now, the convection is a completely different process. Convection is mostly related to liquids and gases where the molecules are not really fixed within the same position, maybe oscillating around this position, but still relatively fixed. In liquids and gases, the molecules are relatively free to move around. So, instead of this molecule, which is hot, it has a lot of kinetic energy, just shaking a neighbor, it just flies this way. Now, it also flies this way or this way or this way, but for instance, if we are heating in a pot, the water, now the flame is here, right? So, this piece is very hot and the molecules of the pot, let's say it's a steel pot, so the molecules which are inside the steel pot, they are using the conduction, sorry, they are shaking the immediate neighbors of molecules of water. This is conduction because this is a steel, but as soon as the first layer of the molecules of water is heated, then it immediately can go to all the different directions and in this direction or going down, it doesn't really move anywhere, but all the way up, it heats up the upper layers basically forcing the cooler molecules to go down. Now, they are heating and they are going up in all other directions and that's how the whole process is being done. And this is what convection actually is. So, convection is the movement of the heat, not from the molecule to a molecule, but movement of an entire mass of molecules from one place, let's say in this case from the bottom of the pot to the upper layers of this. Of the water. So, it's a completely different process, but nevertheless it's also a type of heat transfer. Now, not only it's a different process, it's a much more complicated process. Now, let me go back to conduction. Now, with the conduction we actually derived a relatively simple formula of the Fourier law of heat transformation, which basically was something like this as function of, so this is the wall, this is the air and this is the room outside air. And the room actually is warmer, so the heat goes this way and if this is X, then the temperature is changing from the room temperature to the outside air temperature. But the way how it moves, basically the derivative, the speed of changing the temperature, is basically amount of heat which goes through the unit of time and unit of area at the distance, at the surface which is on the distance X. So, that's kind of an understandable and relatively easy. Now, with conduction, situation is much more difficult because as you see, even in the example of this pot which we are heating up the water, the movement is extremely complex. Now, it would be incorrect to say that the theory does not exist about this type of process. It does exist and it's complex. It's so complex that it's outside of the scope of this course. It's related to differential equations which are called, if I'm not mistaken, convection, diffusion, differential equations, something like this. It's relatively complicated issue, so we are not talking about this. However, for certain practical cases, we can really, using the same logic as with conduction, we can use the same logic in the convection case and come up with a relatively simpler formula, which kind of approximately reflects the reality of the real life. Now, let's just think about the following. For example, you have certain hot temperature here. Let's say this is the bottom of the pot with water. Now, this is the surface. Now, initially, the water has, let's say, room temperature. So this is something like 20 degrees Celsius. Now, we are heating this up and the temperature becomes significantly higher. Let's say 100 Celsius. So the water is almost boiling basically in this case. Now, I think it is intuitively obvious, and while experimentally confirmed, that the flow would be faster if the difference between these temperatures is greater. Now, if there is no difference in the temperature, basically the heat will not flow anywhere. Now, the more different these two temperatures are, the faster these hot molecules will be moved faster to the upper layers of the water. So the whole convection actually very much depends on the difference between these temperatures. Now, let me go to an extreme case. Now, if we are heating the water, gradually it will reach the boiling point. Now, whenever it reaches the boiling point, the temperature doesn't really go any further. So even if the temperature down is, let's say, 200 of the flame is 200 degrees Celsius, for instance, or 400, whatever, the temperature of the water will not be greater than 100 degrees Celsius because the water will start evaporating and the steam will go out. The vapors will go out and the excess of heat will go out. So the temperature will remain as 100 completely. The whole thing will be 100. Now, what is with this heat? The heat goes up all the way, right? Because it should go somewhere and obviously it will evaporate with the steam which takes all the excess energy with itself. But the temperature of the water will remain 100 degrees. Now, so there is some kind of a flow from 400 or from 200 degrees to 100. It gradually actually changes. Now, again, the temperature will basically direct how fast this heat will go. So obviously the amount of heat which goes through, let's say, unit of area during the unit of time should really be proportional to difference between temperatures of the source of the heat and, let's say, the ultimate destination of where the heat is. Probably a better example is when we are calculating this amount of heat which we are consuming for certain purposes. For instance, if you have, let's say, hot weather outside and you would like to maintain your room at certain temperature, you need air conditioner. And how do you calculate how much air conditioner you need? How much power this air conditioner should really produce? Or how much heat it should actually take out from the room to maintain the room temperature cooler than outside temperature? Or similar example, if you have, let's say, some kind of a hot pipe going through the room and you would like to find out how much air conditioner you need to neutralize the heating effect of this heating pipe to basically maintain the room temperature. So these are two examples which I'm going to, right now, to address. And that's where I will use this type of a formula. Now this is not equal sign, this is actually proportionality. And obviously there is a proportionality coefficient which definitely depends on what exactly, whether it's a water or air or something else, basically depends on the subject and its state. And I will talk about this a little bit more in detail. So let's consider these two examples. Now the first example is you have a room, you have a four meters hot pipe at 100 degrees Celsius and the room is supposed to be at 25 Celsius. Now the diameter of the pipe is 0.2 meters to know the surface, right? So based on this formula, and I actually put it here, that h in this particular case is equal to 40. So h is, how is it called? It's convection heat transfer coefficient, something like this. Yes, convection heat transfer coefficient, that's what it is. And in this case for the air, I took 40. Now this is jose per second per meter square per degree. I put Celsius degree or Kelvin degree doesn't really matter, it's the same, right? Now jose per second is actually watt, right? So it's watt per meter square and degree of Celsius. So that's what my convection heat transfer coefficient, which is here. So per unit of, now watt is jose per unit of time per second. So this is also per second. So what do we have now? Well, first of all, what is my heat transfer per unit of area? Well, that's 40 times difference in the temperatures, which is 100 minus 25, which is 75, 40 times 75, which is what? 300, 3000. Now 3000 of watt per meter square. Now what's my area? My area of the cylinder pipe is length times circumference, which is, if this is the diameter, I have to multiply it by pi by 0.2 and I have it as 2.512 square meters. So if I will multiply this by this, now this is amount of heat per square meter, which the pipe actually emits and the air takes with itself. And if I will multiply it by square meterage, I will have something like the total amount of heat per second will be 7536 watt. Now usually air conditioners are specified in BTU, which is British thermal unit per hour. So in BTU it will be, I have to divide it by 3.41 and the result would be about 2200 BTU per hour. So this is kind of an air conditioner which I need in this room to neutralize the amount of heat which is produced. So this is the amount of heat which is produced, so this is the amount of heat which I have to extract from the room to maintain the same temperature. So this is how this formula basically is used. Now where did I get this 40? This is a very murky situation, very, very unclear. Because look at it this way. Now for instance you have this pipe which is going through the room. Now if I don't do anything and just let it basically maintain this inside temperature of 100 and I would like to have room temperature at 25 and I put somewhere on the side the air conditioner assuming that they are not far from each other and the air is circulating relatively freely. Now what if I will put a fan against this pipe? Well the fan will take more, at the same time, the fan will take more hot air from the pipe. And which means that I will take more heat from the pipe during the unit of time. Which means my air conditioner must be more powerful because more heat will go from the pipe. So this coefficient is very much dependent on conditions. Again if there is some kind of an extra movement of the air which is produced by some kind of a force, for instance by fan, then it's completely different. It might be from the range, that's what I want to say, the range of this coefficient might be completely different in different conditions. So that's why this is basically tabulated for many different cases like forced flow of air, not forced flow of air, forced in some particular way. Then it depends also on how this particular pipe is arranged because if it goes horizontally it's one thing, if it goes vertically it's another thing. Because if it goes vertically then the air would dissipate differently than if it goes horizontally. And it also depends on the gravity obviously. So somewhere up in the mountains it might be different. So that's why it's very difficult to deal with convection. It's very fluid I would say. I mean it depends on so many different circumstances that it's kind of difficult to do this type of calculation. So this is very, very approximate. And don't count on this number 40 to be a reality. Well it's close to reality under certain circumstances. I took it from basically some practical textbooks. However, there is no guarantee that in some particular cases, in some particular pipe it will be the same. And now my second problem which I wanted to talk about. This is about air conditioning considering we have a weather, warm weather. So let's say outside I have 40 degrees Celsius. Inside I would like to have 25 degrees Celsius. Now let's say for simplicity I have a glass wall which separates outside from inside. So if this is my room, so this is my glass window and everything else is insulated. So this is the heat, 40 degrees. And this is what I would like to have inside. So in this particular case let's assume that the area of this glass wall is 20 square meters. In which case I have to multiply my 40. This is my convection term heating, terminal heating transfer rate, whatever it is. So it's watts per square meter per degree of Celsius. I have to multiply by difference between temperature which is 40 minus 25. And I have to multiply by area. So this is my wattage. So this is 15 times 20, it's 300, so it's 12,000 watt, joules per second. This is amount of heat which I am consuming from the window. And obviously this is amount of heat which I have to extract if I would like to maintain the same temperature of the room. So this is basically how my air conditioner should be calculated. And this is in BTU it's equal to approximately 3500 BTU per hour. Which is not very powerful air conditioner, so it's not a big deal really. So this is applications, but all these calculations they seem to be like correct calculations. But don't forget it all depends on this. Which is convection heat transfer coefficient. Which is very much poorly defined, let's put it this way. For instance again if you have a vertical glass window the air dissipates differently than if the source of the heat is horizontal. Or in some other shape like a pipe for instance. And it all depends on again whether there is some kind of extra movement of the air inside extra circulation etc. However convection is extremely important obviously for instance the weather depends on the convection of air. The ocean currents basically are results of convection because they are usually going from warmer to colder area. So convection is extremely important and extremely often occurs in nature. But all I'm saying is you really have to approach all these calculations related to convection very carefully. And primarily because of this coefficient of heat transfer in the convection. It's very much difficult to calculate and to basically tabulate. Because there are some tables where saying under these conditions the convection heat transfer coefficient is such and such. Now liquids obviously have one coefficient, gases another coefficient, different liquids have different coefficients. It all depends on density, on viscosity of many many many different factors. And that's what really complicates the whole issue. Plus another very important thing is and that's very important actually in liquids. What if you have something like well the same pipe let's say and you have to really put some kind of a liquid around it to cool it down for whatever reason. Now it all depends how this liquid behaves because if it moves it's one thing. If it doesn't move then the convection would be different. Now the movement also is different. Sometimes the movement is along a straight line. If it's a liquid for instance it goes in a straight line like one flow with a river like a river. But however there might be turbulent movement when the water moves something like this. With gases actually it's more complex even than that. So these are very complex areas for exact calculations. However my purpose was to give you basically a feeling how the convection actually occurs, what problems, what difficulties are. And under certain circumstances we can actually do the calculations. Maybe approximately but still we can do the calculations of the heat transfer. And again this is just one of the examples of the heat transfer along with conduction. And the next lecture would be about the third type of heat transfer called radiation. It's completely unrelated to these because these are kind of mechanical movements of the molecules. In case of radiation it's completely different so we'll talk about this next time. It's electromagnetic fields etc. Okay I do recommend you to go to the website, take a look at the text which is accompanying this lecture. It's very useful and again I do suggest you to take the whole course, not just one particular lecture on theunisor.com. Other than that that's it. Thank you very much for today and good luck.