System, Surroundings, and Boundary; Control Volume, Control Mass

Okay, let's talk about this very, very important now. So far, we have covered something much more simple. Right now, I'm going to cover stuff that will be very, very important for the rest of this course, okay? And the first thing I want to talk about is the system. The system is basically—it's actually a fancy name—but it's the fluid molecules or particles or matter, whatever, inside of an arbitrary volume in space. So what I do is I have myself a particular volume, right? This is so closed like this; I have a particular volume, and I really care about it, okay? And I call this the system, right? And the stuff that I don't really care about, I call this environment or surroundings, so that is the outside of what I care about, okay? So, for instance, the system can be this room that I'm in currently, you know, preparing these notes. Okay? So in here, for instance, I have the AC on, I have all sorts of energy transfer, heat transfer, etc. We'll talk about these, but at this point, I want to just talk about what a system is. System is, it's something that I, you know, I care about. This is what I care about. So let's write it over here. It's the fluid molecules—I mean it can be solids as well—but that's not of interest in this particular course; that's why I'm focusing on the fluid side of things, okay? Or you can call this matter as well, inside and inside a volume, okay? In space—in space doesn't mean outside of the world; it means, you know, outside of in some place, okay? So this is my system. I'll give more concrete examples at this, you know down the road. And this outside of it is called the surrounding, and some people call it environment, okay? So it's everything outside the system. And the intersection of both those two is this right here, where they intersect; it's called the boundary. Nothing fancy—you know this too already, right? So it's the edge of the system or the surface that separates the system from the surroundings. There's a bunch of different, you know, things. But one thing I want to talk about kind of, you know, interesting is can be real or imaginary, okay? Whenever I talk about imaginary, you know, I have to talk about that for a minute. So I mentioned that, hey, this is closed, right? It doesn't have to be closed, and I'll give an example in a minute, okay? This can be fixed; the boundary can be fixed, but it can be moving. I'll give an example moving too, okay? So I'll come back to this. You know, I'll give an example from real one, I'll give an example for imaginary, I'll give an example fixed, I'll give an example for moving, okay? But for now, you know, let's just the, you know, get that. Because I also need to define you the control volume and control mass to be able to talk about these things, okay? So let's talk about the open systems. Open systems, okay? And I call this control volume. So when you look at the chapter titles, for instance, in many thermodynamics books, you're going to see that the analysis for open systems—first law for open systems, first law for closed systems, etc.—so let's first define those open systems. So this is a system, okay. So it seems it is a system that basically, I pick a particular volume in space that I care about, and I do not make much restrictions on it. The mass can come into it, and energy can come in and outside my boundaries. Okay? Cross the boundary. And the boundary—I call this control surface. Just another fancy terminology that I don't use a lot in this course, but I use a ton of times in the fluid mechanics class, okay? So open systems is a control volume, a system where both mass and energy can cross the boundary. And I gave the example of this room—so currently the AC is running, so obviously, a cold air is coming in. So the energy of the room—I don't want to explain too much because I still need to cover some stuff—but in the energy changes, right, of the inside the room, and the mass is changing because air is coming into the room. I have windows, but they are not completely insulated, is it? You know, so there will be some kind of leak. So this is an example of an open system. And if you don't know whether the system is a closed system or an open system, assume it's an open system—it's the most general version of it all. Okay? Obviously, the analysis of the most general case is harder than the special cases, which holds true here as well. The second one that I want to talk about is the control mass. So this is a special case of control volume—I want to highlight that right off the bat—so this is special case. So special case of control volume, okay, where no mass crosses the boundary. It's called a closed system, okay? So let's talk about what it is and what it is not, okay? So, it says that, hey, I have this open system, and I'm saying that I don't have any mass leaving or entering the system. Okay? That's what it means—I don't have any mass transfer to it. But I didn't say anything about energy, did I? So energy can still transfer into my control surface, through the control surface, sorry. Okay? So, you know, I'll give an example of this in a moment. It will be making more sense, and I think it is making some sense at this point, right? So the mass doesn't transfer—that is what I'm talking about. I even have a more special case of this, okay? So it's a special case of the control mass or closed system. And, now, what I'm saying is I'm going to call the system isolated system, okay? And I'm going to have, in this case, special case of closed system. Right? It's a special case of closed system. But also, the special case is this: no energy transfer. So here I restricted the mass transfer, right? And here I don't have mass restriction. I have mass restriction because it's a special case of a closed system, plus on top of it, there's no energy transfer. So what this means is no mass, no heat, no work crosses the system boundary. Okay? So it's, I want to highlight one more time that you know, the analysis will be completely up to you. It depends on a system that you select. If you select the system differently than another student, you may get a different result. But that doesn't mean it's wrong. You're looking at different, you know, you're focusing on a different area or volume. So it's important that I define my boundaries very well—quite important. You will see soon, okay? At this point, it may not making too much sense, but it is, okay? And I want to know about what is in it, what is in the system, how are the boundaries—are they real or are they imaginary, are they moving or are they fixed—what can pass through the boundary. Very important, right? It will determine whether I'm here, here, or here. Easiest, middle difficulty, or wow, this is going to be a challenge. I'm serious, okay? We'll go over it, we'll make a bunch of assumptions and do our analysis here. Okay? Let's see. What are the forces that’s applied to my boundary that will determine whether I will have a fixed or moving as well. Or fields, right? I have a, you know, gravitational field; I have Iike taken it into consideration or not? Will not be negligible? So this a bunch of different considerations; we'll talk all about that. Okay? Let me give you an example, promised an example. So what am I go ahead and give you an example as I promised it, right? So let's say I have a sink, okay? This is an example that I also give in my fluid mechanics class, but it's going to do just fine because it's applicable here, okay? I have a faucet, right? A fancy faucet, right? And in my faucet, I have hot water coming in. So this is a hot water faucet. And I have hot water so let's draw this with red it's kind of indicates it's hot. So let's say that the level of the water is here, right? What will happen to the temperature of the water? It's going to go up, right? But wait—I did this on purpose. First, you have to decide what we're talking about. Are we talking about the temperature of the universe? No, that's not going to change. Are we talking about the temperature of the entire house? No, that's not going to change because I turned the hot water coming into my sink. See? It's very important to determine my system, okay? And let's say that this is my system—I'm being very particular about it. I care about the water within my system over here, right? So this is my system. So now, if I can ask you what is going to happen to the energy of the system over time, it's going to go up, right? Because I'm putting hot water inside of it, okay? Let's also determine the surroundings—the surroundings is here. This is the region that I don't really care about, right? So let me now ask you about this. Let's look at the boundary. So I specified the boundaries with dashed lines right over here, right? So is this real or imaginary? Well, this is real, right, because if you think about the sink, this is real. How about here? It's kind of imaginary, right? You can see that there's no boundary there, but I take it as a boundary, right? So this is an imaginary boundary. Okay? So the question now is, is this a control, let's go up here, it's a control mass? No. Why? Because water is coming over here, water is leaving from the drain, so there is no way. Mass is crossing the boundary, you can clearly see. Look here. The mass is crossing the boundary, mass is crossing the boundary over here now. So then it's obviously not going to be an isolated system. So this is a control volume or open system, which will be a little harder to analyze, you will see. Okay? So I have a question. So what will happen to my system over time over here? Is it going to go up like this or it's going to go down like that? Well, the answer is it depends. If the flow rate from the faucet is higher than the flow rate from the drain, the system is going to expand. So you can see now that the system or the boundary can move. See? I said that it can be real and imaginary I demonstrated it. Now I'm demonstrating that the boundary can be fixed or moving, right? It can clearly go up or down depending on the rates of the flow rates from the faucet as well as the drain. Okay? So now let me ask you another question. Can I make this a control, you know, I know it's a control volume, but I don't want to deal with it; I want to deal with the control system, excuse me, closed system or control mass, okay? Can I make it that way? The answer is yes. If I turn off the faucet, if I turn off the drain, then there will be no mass coming into my system. There will be no mass leaving my system, so I'll be good to go to analyze this as a control mass or a closed system. And again, you can clearly see the analysis will be much more simpler, right? There's nothing coming in, nothing coming out, so it will be much simpler. So, I need to look into this, okay? So I gave you one, you know, kind of like a large example that illustrates a bunch of stuff that I want to talk about. But I also want to talk about something else, because, you know, this you will get, you know, exposed to very often in the thermodynamics class. So, piston right here. So, I have a piston over here, okay? Let's assume this is a piston, and it's completely sealed, right? And let's say that I have air at 20 degree C. Doesn't matter, I'm giving an example. So, let's say that I'm heating it up. So I have some kind of heat, you know, I'm heating this up, right? I'm on a burner, and the system is, I'm going to pick that as here. Let's say this is my system. You can see. You don't have to, in an exam or any other setting, you don't have to explain in detail what is a, you know, system. So, just draw a dashed line; I understand that that's the system, okay? And let's say this is the surrounding, right? So, what is kind of, you know, why we do it is this. If I heat it up, what will happen to this piston? So think about that for a second. Or let's go back a step: what will happen to the pressure when I increase this? Well, the pressure is trying to increase, right, because I'm heating it up. And if I heat it up, so the pressure is going to push this up, right? Because now, what it means is the pressure will try to, you know, equate on this side and on this side. As the pressure is higher over here, it's going to push it up, as the pressure is lower over here. So, it will balance itself. So, once the temperature goes up—actually, I should have drawn this much better because now I squeezed myself into a bad spot here, but that's okay. So, you know, like, the piston is going to go all the way up or something, you know. Yeah, so this system, the temperature is up. So, let's put this here: the T went up because I heated it up. I have basically an expansion in here. So, hey, we'll cover these things, okay? I'm not throwing you a bunch of different fancy terminologies just yet, okay? So it's a moving control mass. Why did I go now—why did I go control mass? Well, I told you. This is sealed, so there's no air—still air, it's the same air. The temperature is up. Now, let's say the temperature is 22 Celsius, right? It's still the same system; it's right here. This is my system, right? It's the same system, but now I have the volume has increased due to the, you know, the pressure tried to increase, and it balanced by increasing my volume as opposed to the pressure, okay? Now, you know, I want to also throw away another terminology in here, but this is very important, okay? I will only, in this course, deal with simple compressible system, okay? And you may think about, okay, what's going on here, compressible and all? Well, it's a fancy name, but basically, I'm not dealing with no electrical, okay? Nothing against my electrical engineering friends. I'm not dealing with any magnetic effects, okay? I am not really interested in gravitational effects, and you may be saying, "Whoa, where am I? Am I doing in space?" because you said that pick this in space, so am I literally in space? The answer is no, no, no. What I'm saying is the effects of gravitational fields will be much more smaller than the compressible effects that we're dealing with, okay? And this is a good assumption; it's not a bad assumption, all right? So I'm not making something that is unrealistic in real life, okay? I am not dealing with any motion effects, okay? I'm not dealing with surface tension, very important for fluid mechanics class—not in this one. I'm not dealing with it because it's not going to be significant. If I have a such a small, like, you know, very, very thin and long like this, maybe the surface tension will be important, okay? And I also will deal with single phase in these kinds of analysis. We will not relax that going forward, okay, from single phase to multi-phase, okay? I think that's all I have to say about this particular segment. Thank you for watching this video too.