 Let us start. So last session, we have actually started heat transfer, and we have completed the conduction mode of heat transfer. So now we are getting into the other modes of heat transfer. And why we are studying the different kinds of modes of heat transfer separately, because every mode of heat transfer follows a different set of rules. So that is why we are studying the heat transfer of different types separately. All right, so all of you please write down the next mode of heat transfer is convection. OK, so convection heat transfer, I will start convection heat transfer with some example. You might have, for example, have you ever taken a hot utensil and then you poured cold water on it? I think many of you have tried that. So if you take hot utensil like this, then you have option to either put cold water in it like this and cold water just go away. And then newer cold water will come, get in touch with this hotter surface, and then will go away. This is one way you can cool this. Another way that you could have used was you take this hot plate and then you just keep it on a cold object. In both the cases, the plate will cool down. This is also cold and this is hot plate. And this is a cold fluid. And this could also be fluid or could be a solid as well. Where do you think the object will cool down faster? Case number one or case number two? OK, Shrestri, which one? One or two? The answer is that it is one. So in two, there is conduction going on. Here, it is an example of convection. Now let's see why. Why at one, the cooling is much faster. What happens at one is that the cold fluid come in contact with the hot object, then takes the heat from the hot object. This fluid also heats up and then goes away. And then a new cold fluid will come. Earlier, this cold fluid was there. Now, this portion has taken heat and gone away. Now, a new cold fluid is coming. Again, this is cold. This gets in touch with the hotter object and then takes the heat and goes away. So what happens is that temperature of the cold fluid is let us say Tc and temperature of hot fluid, hot object is Th. So Tc is remaining constant. Even though it takes heat, but then after taking heat, Tc increases because the fluid is absorbing heat, but then it is going away as well. A new object comes in, a new fluid comes in and it further absorbs heat. So because of that, Tc remains practically constant. And heat transfer will happen because of the temperature difference. Over here in case 2, hotter object, when it comes in contact with the colder object, temperature of hotter object goes down and temperature of colder object will go up. So the difference between hotter object and the colder object, it decreases at much faster rate in case number 2 because temperature of hotter is going down and temperature of colder is going up. So the temperature difference is decreasing from both ends. Whereas in case number 1, in convection, Th minus Tc is the temperature difference. Only Th is going down. Tc remains practically constant. So the case number 1 is a new type of heat transfer where what happens is, please write down, bulk movement of fluid over an object transfers the heat. So in the conduction, we never taken the case in which the fluid is moving. But several cases, wherever we want higher heat transfer, in that case, the fluid is actually moving. That is not an example of conduction, but that is an example of convection. Now in our syllabus, luckily, we don't have extensive study of convection. Just few examples of convection are there. So in case they ask you in your school exam with respect to give example of convection, then you need to talk about natural convection. Please write down. So what happens is natural convection is a process due to which naturally the convection current will flow. And let's see how it happens. For example, let's say this is C. It is C, huge water body is there. And this is the she-shore. So just to make you believe, I'll draw a coconut tree. This is coconut tree. And this is you. Fine. Suppose sun is there. Okay. What happened? Because sun is there, the specific heat of water and let's say this is specific heat of land. I know that specific heat of water is very large compared to specific heat of solid usually. So during the daytime, what happens is tell me whose temperature will move up quickly, the temperature of water or temperature of land. Which one will go up quickly? The sun is there. The temperature of land. The temperature of land will go up faster because specific heat of land is lower. Okay. And hence, temperature of land is typically higher than temperature of the water or temperature of the sea. Okay. So because of that, the air in contact with the land. Okay. The air that is in contact with the land, this air. Let's say that is AL. And this is in contact with water. Let's say AW. Which air will be lighter? Sorry, which air will have more temperature, which is in contact with higher temperature object. So temperature of air of land will be higher than temperature of air of water, right? Because of which density of air of land will be lesser than the density of air of water. Is it true? Are you able to understand? Yes. Okay. So what happens because of this? The air which is in contact with the land is at much higher temperature than the air which is slightly above. So because of that, the air which is lighter or density is less tends to move up. Okay. This happens because of gravity. This moves up. Now, because this air is moving up, because this air is going up, what happens is that pressure over here decreases because air is moving up, all right? And since pressure over here at the land side is lesser now, the air from the water side will start moving to this side because of the pressure difference. Okay. And this creates an entire cycle. Okay. So what happens is that when you are near the sea shore during the daytime, the air will tend to flow from sea to the land. Okay. And you can clearly see that the air which is cold is coming to cool the rocks which is near the sea shore and this cooling process is a convection process because there's a bulk movement. Is this clear to all of you during daytime what happens? Yes. Yes. Now let's see during nighttime what happens. During nighttime, suppose this is sea. This was a previous slide for one second. One second. Okay. I'm anyway going to send you this. Don't worry. This is your coconut tree. Same coconut tree. And now there's a nighttime and you're still standing here. Now what will happen is that during the entire daytime, the water was getting heated up and the land was also getting heated up. Okay. What happens is when the sun is set, the source of heating up the objects have gone now disappeared. So they will start cooling down. So during the entire day, water was heating up and the stones were also heating up. So at some point in time, temperature of water became equal to the temperature of the rocks finally and then they have started to cool down during the nighttime. Now tell me which will cool down faster? Land will cool down at faster rate because specific heat of land is lesser. So it will lose out. If it lose out X amount of heat, its temperature will go down faster rate because it's specific heat is lesser. You can write down X is equal to M s delta T. So delta T, which is temperature change will be X divided by M into S. So you can see that for same amount of heat that is losing away, change in temperature depends on S as inverse proportion. So if S is less, delta T for same amount of heat absorbed or lost will be higher. So now it is about losing heat. So rocks will cool down faster than water because of that the temperature of air near land will be lesser than the temperature of water. Sorry, temperature of air near water. So what will happen is that now temperature of air is higher over here. So this air will move up because of the pressure difference now air will go like this and just to complete the cycle it will take a turn like that. So this is again a natural convection. Now what will happen is that the colder air from the sea shore is going towards the water to cool down the water using convection. So during the night time the air will flow from land to the water. Is it clear? Any doubts you have? Please quickly ask. No doubt. Fine, so that's it with respect to convection.