Okay, let's talk about the second form of heat transfer, convection. Now in convection heat usually is being transferred involving some sort of like bulk movement of the material. So in that case we have some substance which is originally at high temperature, and they are going to move from one place to another place. And at the same time this happens, heat energy is being carried with them and be transferred when it's in direct contact with another substance which is at low temperature. Now usually when convection takes place, it occurs in liquid or gases. Usually these are fluid like material. And inevitably we notice that when convection is taking place conduction is required. Because it takes direct contact between one hot substance and the cooler substance to be in contact so that energy can be transferred. It is very easy to understand that everybody at home they may have a heater which is using this convectional scheme. We would have this heater which is heating up. What happen is that you would have air being heat up by this heater. So when they're heating up what you have is that the air become having lower density. Having lower density they start to rise and move to the top. At the same time it creates some sort of like a vacuum. So that the cooler air will start to fill in and be heat up by the heater again. And the hot air when it goes up gradually become cooler. They sink back so as a way what we have is that we have this convection current going on and allowing this bulk of movement of fluid like substance involving the transfer of the heat from one place to another. Now in order to show you how this takes place, I'm going to invite my assistants to do an experiment for you. Leo and Angel. Now in this particular experiment what do we have is that we will have some hot water, which is highlighted in yellow with dye and we have cold water which is highlighted in blue.

Now what we're going to do is we're going to put this hot water and cold water together. But each time we are doing it in a slightly different way. In one case we put the hot water on top and in the other case we put the cold water on top. Let's see what happens. Angel and Leo. Now you see, they use a card to help to hold the solution. And now they are putting on your right-hand side, we have the hot water on top and the cold water at the bottom. And on the left hand side we have the cold water on top and the hot water at the bottom. Let's pay attention on the right hand side. Do you notice that the yellow solution in fact they remain yellow all the time? And the blue solution remain blue all the time. Basically that's no movement whatsoever, why? Because the yellow water, they are of lighter density, they stay on top. And the blue one, colder, they remain having higher density, they stay at the bottom. And that's no bulk movement of the solution. On the other hand if you look at the left hand side, what you see is hot water originally from the bottom it is having lower density. It's start to move up. The cold water blue on top it having higher density. It sinks back. So you may recognize in a minute or two they get all mixed up. It becomes a green solution. Homogeneous. So that tells you how convection takes place involving bulk movement. In fact this kind of convection process, we utilize this principle in a lot of other cooking location. For example when we are baking we again utilize the density difference of air. For example, in oven you can put some sort of food in the middle. Now what happens is that you have a heater which is heating up at the bottom. So what happens when it heats up the air become heated up, they will have a lower density. They will start to move up and they will be in contact with the steak and then the

heat energy is transferred from the air to the steak. And when they go to the top eventually they become cooled down and they come back. So eventually in the oven what we have is that we'll have collision of the air molecules with the steak with the collision of the air molecule with the oven surface which is heated up. So therefore heat is being transferred from the heated surface, the metal, to the air. And again from the air to the steak by direct contact conduction. And this movement of air results in the convectional current and it helps us to heat up food in oven. But of course in oven we would have other heat transfer events going on including radiation, which we'll talk about a little bit later. Now similarly we say that these two combine together in fact allows us to cook a steak inside a oven. Now how about in another situation such as in boiling. Boiling is very easy to observe. When you have water we say that you put a pot of water on a heated surface. The heated surface will transfer the energy by conduction onto the body of water. And on the surface when they are in contact. The energy get picked up they become hotter lower density and they start to move up. And on the surface of the water body when the density is higher, they start to sink back resulting in this bulk convectional movement of this body. And so what you have is that by this convectional current you're going to generate a some sort of like a heat transfer and eventually the entire body of the water become heated up. Now, of course, the advantage of that is you can maintain a very stable temperature by this active movement. Now, in fact, when we talk about boiling, you need to understand that boiling in fact is defined by the condition that we would have a particular liquid. When they go to a particular temperature that the vapor pressure equals to atmospheric pressure. Then that's the time that we call the boiling point and that's the boiling occur.

In fact, if it increases the pressure, what it means is that you need to have more energy introduced in order to allow this water to go through the vapor state. And so therefore what we notice is that if you have one atmospheric pressure, what we have as the boiling point is 100 degree. But, if you have higher atmospheric pressure that means it takes more energy to raise the body of water to that particular condition that they can generate vapor. And you'll find that the boiling temperature can go up as high as 160-180 degree when this ten atmospheric pressure. And of course the same thing happens if we have even higher pressure applied to this liquid body. We can utilize it as a way to help us to cook things more effectively. You remember that many people would have a pressure cooker at home. What they allow you to do is that would raise the temperature inside this particular cooker. And when you boil it because the pressure keeps on being built up so therefore you need to have higher temperature to keep it boiling. So therefore you can raise the temperature above 100 degree so that allows more effective cooking of the food. Boiling in fact can also be affected by other elements such in this case is salt. We say that when you're boiling something the temperature at the boiling point actually is 100 degree. In fact you can manipulate it by adding more sugar or salt. Because when you put salt and sugar there when salt and sugar reach the point of 100 degree. Do you think salt and sugar they would become vapor? No they won't. They keep on absorbing more heat and yet they are not being vaporized. So what happens is that it results in keep on elevating the temperature and as a result when you have salt and sugar added to a solution the boiling point usually go up. A few degrees, sometimes 101, sometimes 102. And we are going to demonstrate that to you in a separate experiment that you'll see later.

Now, of course at the same time we can also do deep frying. In this case when we are doing deep frying we are relying on another medium, in this case oil. We use the oil as a medium with a hot surface. When the hot surface in contact with the oil, the oil becomes heated up. And when they heat up, they have lower density. They start moving up. So now let's say you have some sort of meat in the middle. The meat will be constantly in contact with all this heated-up oil, which is moving upward. And then by direct contact conduction. Heat will be transferred to the meat. And of course, when they rise to the top the oil which is at lower temperature or higher density, they sink back. So again it creates a convectional current that allows us to heat up the meat by deep frying. Now all this, of course, it would have advantage because it allows you to generate very high temperature. Not in water 100 degree. Oil can go up to a 100, 200 degrees. Now so what happens is that at that time such a high temperature sometimes it results in dehydrating because water at that point they would all go into vapor. Because it's higher then 100 degrees. Now so that's what we do in deep frying. You basically put all the food which is made of water and dip it into the oil. On that I want to illustrate again, when we are boiling something in fact this convention current is heavily affected by what kind of medium they are in. Sometimes the viscosity of a medium can affect how the convention current can take place. For example, if you are making some kind of soup, which is very thick such as creamy soup or a porridge or so. They are very thick, convection will not be able to take place very easily because that requires bulk movement of the medium. On the other hand if it's in water, this bulk movement is much easier. So in the following section we're going to show you in another experiment that how the viscosity is going to change convectional current. And I hope that you see how it exactly occurs and what makes a difference when you're cooking some sort of food with

different viscosity.