 So far, in our thermodynamic adventure, we have talked primarily about closed systems. In closed systems, we are not allowing for mass to cross the boundary of our system, and as a result, we don't have to worry about mass crossing the boundary of our system. We've talked about material properties, both for ideal gases and for real substances for which we have property tables, and we have developed a pretty good skill set for setting up an analysis on a problem. Now we will start to consider open systems. When we describe an open system, we aren't neglecting mass crossing the boundary, and we have to be able to account for it. That will affect our analysis in a couple of ways. First of all, we're going to have to be able to account for the ability of the mass to change. The mass changing during the system could affect things in our system. So since mass could change, we're going to have to develop a skill set for describing how it changes. That skill set will be the mass balance. And the mass balance is exactly like the energy balance, in that it's not so much an equation as it is just a listing of the possibilities. If you have a system and mass enters it, either the mass stays there or it doesn't. Those are the list of possibilities. So when we describe the mass of a control volume, we can say that the change in mass of the control volume is equal to the mass entering the control volume minus the mass exiting the control volume. If we have, if we consider Thomas, who has three apples, if we give him four apples and he gives away one apple, his change in apples is three apples. We could say that his amount of apples at the end of the process is six apples. The mass entering would be four apples, the mass exiting would be one apple. That will be a new tool in our toolbox to describe how the mass changes. The other major modification to our skill set is going to be accounting for the energy of a moving mass because when we consider energy entering or exiting our control volumes in our open system analysis, we have to account for the fact that energy could enter or exit as heat transfer, work and mass. So when we talk about our energy balance, we have to consider the energy associated with any mass crossing the boundary as well. When we do that, for example, if we talk about the energy entering a system, we could talk about heat transfer, work or the energy associated with an entering mass. When we describe the energy associated with an entering mass, we describe its specific energy and we abbreviate it with a theta. Theta represents the energy of a moving or flowing mass. That theta term starts out looking the same way as the energy of our systems looked when we were talking about control masses. We start off with the combination of the microscopic and macroscopic energy in the same way, meaning that we're talking about the specific internal energy plus the specific kinetic energy plus the specific potential energy and then we add to that a specific flow energy. And that flow energy essentially represents how much energy it takes to displace the fluid in front of it. So if we are talking about a box full of water for some reason and we have a pipe sticking out the box and there's water entering that pipe and therefore the box, each unit of mass of water has to displace the mass in front of it in order to move. And at very small distances as the limit of the size approaches zero, we have an approximately isobaric process and as a result of that, that flow energy looks the same as isobaric boundary work. It is pressure times specific volume. So because we have internal energy plus pressure times specific volume, we can make the abbreviation here for specific enthalpy. That specific enthalpy plus specific kinetic energy plus specific potential energy is going to be how we refer to the specific energy of a flowing fluid over time, which we abbreviate with a theta. Lastly, though it isn't new, we have to talk a little bit more about steady versus transient analysis. When we talked about closed systems, for the most part they were undergoing transient processes. That isn't entirely the case. We considered a couple of steady processes, but the steady processes aren't quite as interesting as the transient processes. For a closed system, if no mass is crossing the boundary and nothing is changing with respect to time, there's not much to do. We just have a rate of work entering and a rate of heat transfer exiting or vice versa. Remember that when we talk about steady versus transient analysis, what we're really doing is accounting for time. If we are allowing time to have an effect on our analysis or if we are neglecting the effect of time or any effect it could have, the way I like to think of this is as a table. We consider three different types of establishing a system. We have open systems, we have closed systems, and we have isolated systems. In an open system, both mass and energy are quote allowed unquote to cross the boundaries of our system. That is to say we are accounting for the energy and mass crossing the boundary of our system. The first simplification we can make from an open system is by neglecting any mass crossing the boundary. When we do that, we are treating it as a closed system. We are neglecting any mass crossing the boundary. If we take it one step further, we can treat it as an isolated system where we are neglecting mass and energy crossing the boundary of our system. The real world is made up of open systems. When we simplify to closed or simplified to isolated, we are neglecting some aspects of that reality to simplify for the purposes of setting up a model. Similarly, when we describe reality, there are lots of things that can affect properties. When we neglect the effect of time specifically, we are simplifying reality and treating it as though time has no effect, what we call a steady analysis. In a steady analysis, we have neglected the effects of time. We describe a process that is steady as being in steady state. Now in many cases, you can actually split a real analysis into a transient and steady component. For example, if we were to consider the temperature of my office increasing as I'm trying to record a video and I have the door closed to block out any cats from coming in and meowing at the microphone, the temperature of the office will increase as a result of the fact that I'm producing energy within the office and it's not able to reject as much energy as is coming into it. Now as the temperature of the office increases, there is more temperature difference between the inside and outside of the office and as a result of that, there is more heat transfer from the inside to the outside. Eventually, the temperature of the office will reach a point where there is as much heat transfer in the outward direction as there is energy produced within the office. So if I were to plot the temperature of the office over time, after I close the door, the temperature would increase and eventually level off. So if I wanted to describe the energy in and around the office, I could consider the transient component or the steady component, both have their advantages and disadvantages. The steady state analysis might be something like, what temperature does the office reach? At that point, we could figure out how much heat was rejected because we know that we have to be rejecting as much heat as we are producing. We could also consider something like how long would it take for the temperature to reach x degrees? That would be within the transient component or impossible. So the transient versus the steady is really a question of what matters in the problem that we're considering. When we talk about open systems, we are typically talking primarily about open steady analysis. In this table, there is a lot of meat for analysis here. There are a lot of processes that we consider that are open and steady. That's not to say that there are no processes that are open and transient, but as we're learning especially, there's going to be more opportunities to analyze things that are open and steady. Similarly, when we analyzed closed systems, we talked primarily about transient analysis. And that's not because there were no opportunities for steady closed analysis. It's just that there weren't nearly as many interesting problems to solve. When we talk about isolated systems, we are talking a little bit about transient analysis and a little bit about steady analysis, but the magnitude of interesting problems within the isolated category is so small to almost be ignored entirely. So don't be led too far into associating steady analysis only with open systems and associating transient analysis only with closed systems because remember, there are the occasional useful problem in the open, transient, and closed steady side. As a result of that, we are going to be starting with our same energy and mass balance and simplifying to steady state any time that occurs. The next couple of examples that we consider are going to be steady state analysis of open systems and they are a classification that we refer to as steady flow devices. Steady flow devices are devices in which something flows steadily. We have steady state analysis with a fluid flow and that is a category of analysis that is quite useful to us, so useful in fact that I think it'll have its own video explanation. When we talk about a rate form of the energy and mass balance, what we're doing is taking the energy balance or mass balance and dividing all three terms by dt. That produces the edt on the left and edot in and edot out on the right. Remember that our dot notation indicates rate of change with respect to time, so dedt is equal to edot in minus edot out is just the energy balance in terms of rates. If we call the top line the magnitude form and the bottom line the rate form, we recognize that the top form is more useful for transient analysis and the bottom form is more useful for steady analysis. So we will often make this simplification for a steady state analysis problem because on the left hand side the edt becomes zero. It becomes zero because we're neglecting the effects of time therefore nothing can change with respect to time including the energy of our system. Similarly, the mass balance when divided by time simplifies into dm dt which is zero if it's steady state is equal to m dot in minus m dot out.