Welcome back. Let's talk about Gastronomy the next section. About cooking via the heat transfer. Now when we talk about heat transfer in cooking, there are few concepts that actually we are going to do, including the mixing of food, including heating up something using your utensils, so that we can heat up the food. We can talk about the chilling so that the food can be raised to a high temperature, and we cool it down to a low temperature. Oh, we'll talk about freezing. Basically, you want to have the food frozen. Now all of this would involve the transfer of energy, and we are going to talk about one by one. Now energy, as we understand, can exist in multiple forms, such as chemical energy. Chemical energy usually are the energy which are stored in a substance where they keep all this molecules and atoms bound together. That's called chemical energy. Electromagnetic energy. These are the energy which can be transmitted in wave form so that it can go from one place to the other without any medium in between. And that's referred to as electromagnetic energy. Kinetic energy. It is very easy to understand. When you move around, you have motion, you have vibration. You migrate from one place to another place. That's referred to kinetic energy, movement is involved. Thermal energy is very easy to understand. It's talking about the change of temperature. So from very low temperature to high temperature. We say that that's a increase of thermal energy. And finally, potential energy. When we have any kind of, such as a bottle, when we move it from one place to the other. Basically we would have energy stored in them in a potential form that can be used later on. That is called the potential energy. Now, all this energy form, they are interconvertible. Essentially, it's following the law of thermodynamics. Energy can be transferred from one form into another form. Now, let's look at actually how heat energy is being transferred. When we say that heat, when it is in atomic scale, we are talking about how this kinetic energy of a molecule or a substance when they, being transferred from one place to the other place through movement of molecules. Now, so all this would involve the change of position, change of orientation, and change of their vibration frequency. And all this results in increase or decrease of the heat energy. Now in this whole process, when some substance is changed from low kinetic energy to high kinetic energy, we call that heat transfer. Now, of course what we're dealing with is including chilling, freezing, heating, and all these things essentially we use in cooking. Now, how about temperature? Temperature, in a way, is a very convenient way for us to measure what that kinetic energy is really is. We are referring to, what's the average speed molecule or substance. They are moving. Now, so the higher the temperature, that means they are more active, the faster they are moving. The lower the temperature, basically they are more stagnant, they are not moving around. And of course at the, a condition of absolute zero, that is in minus 273.15 Celsius degree. What we have is that basically, the molecules or the particles, they stay without any motion. And in that situation, we call it absolute zero. When we talk about heat and temperature in actuality, we talk about how heat flow from one place to another place. So I want to introduce to you a very simple equation, not that I want you to remember it, but that highlights some of the key elements of this so called heat and temperature, and the flow of energy. Now, basically the flow of energy can be highlighted in this, marked by Q. Q is amount of energy. And t is amount of time. So therefore, amount of energy transferred per unit time, that's referred to the rate of the heat transfer, or heat flow. And this is affected by a few parameter, including k, marked here, referring to the very characteristic property of a substance. This heat is transferring through. A refers to the area, that means how large area this energy is being transferred through. Delta T refers to the temperature difference, whether it's going from a very high temperature to a very low temperature, or they only differ by very little. So of course, the higher the difference between the two temperature, the faster the flow of the energy. Yes. And of course, finally, L. L refers to the distance between this two source. One is the energy source. The other one is the sink where the energy is flowing to us. Now, having that, I want to highlight that when we are cooking, we usually deal with a few ways that heat is being transferred. Usually, they are done by conduction, convection, radiation, vaporization, or condensation, and sometimes we refer to that case as a phase transition. All this would involve transfer of heat from one form to the other form. Now, with that, let's take a break, and we'll look at each of them and see actually, what's the principle behind them.