 Next definition, when we describe properties, we are describing a macroscopic characteristic of the system. So it's a description of something about the system as a whole. And we sort properties into different categories so that we can keep track of which ones are useful under different circumstances. The first distinction that we make is intensive versus extensive. That refers to the fact that an extensive property changes based on how much of the substance there is. Again, an extensive property changes based on how much of the substance, how much mass there is. An intensive property does not. Again, an intensive property does not change based on how much mass you have. So for example, if you imagine floating in the air above you, there was a big block of steel, and all of a sudden half of it were to disappear magically. If a property changed after half of it disappeared, that would have been an extensive property. If it didn't, that would be an intensive property. So for example, volume of the steel block would change when half of it disappeared, therefore volume is extensive. Weight would have changed after half of it disappeared, so weight is extensive. Density of the steel is likely to be unaffected if magically half of it disappeared. Density is intensive. Also the color of the block is unlikely to have changed, so color would be intensive. And then we add in a specific addition to that categorization. A specific property is an extensive property divided by mass. When you take an extensive property and divide by mass, you get out a specific property. And specific also means intensive. So once again, a specific property is an extensive property expressed per unit mass, and it is intensive. So for example, if you describe volume per unit mass, that result is specific volume. And what you get is an intensive property. Specific volume is an intensive property. It's kind of like density, in fact. It's the reciprocal of density. If you would like more examples, here are more examples. This is by no means an exhaustive list. This is just more fun exploration. We are using properties to describe systems. A system is a quantity of matter or a region in space chosen for study. When it is a quantity of matter, we describe it as a control mass. When it is a region in space, we describe a control volume. The system is the thing we're analyzing. Everything else around the system, the whole rest of the universe, is the system's surroundings. And the separation between the system and its surroundings is the boundary. For example, if we were analyzing the power produced by the gas engine in my Honda Accord, we might be considering what's happening to the gas as it expands as it is heated up. In that analysis, we would want to analyze gas inside of that piston-cylinder arrangement, but we could not establish a control volume because the volume of that piston-cylinder arrangement is changing. The piston's moving up and down, which is changing the volume. But the mass of the gas is unaffected by that process, so it would likely be more convenient for us to establish a control mass as the thing that we are analyzing. When we describe control masses and control volumes, what we are also doing is trying to limit our scope. And one way of doing that is by defining our system as being open or closed or isolated. That refers to whether or not we are considering the effects of mass and energy crossing the boundary. In an open system, both mass and energy can cross the system boundary. In a closed system, mass cannot cross the system boundary, but energy can. In an isolated system, neither mass nor energy can cross the system boundary. And note that there is no need to establish a force where mass can cross but energy can't, because the mass itself brings energy with it, so we cannot have a fourth possibility here. It's also worth noting that when we treat a system as being open or closed or isolated, we are not necessarily describing what it actually is. We are describing how we're modeling it. So for example, if I wanted to figure out how many ice cubes I should add to my Yeti thermos to bring my coffee down to a temperature that I can actually drink, after I add the ice cubes in and put the lid on, I could probably get away with treating that as an isolated system. Or should I say ice-related system? That's because the energy exchange is primarily happening internally, and there would be negligible mass going in or out of that Yeti thermos during that process. So by treating it as an isolated system, I am neglecting the effects of mass and energy crossing the boundary. It's not that it's perfectly insulated for real, it's the fact that I'm treating it as being perfectly insulated. Similarly, when I treat a system as being closed, it's not that it actually is perfectly closed. I'm saying that we can model the behavior of the actual thing as a closed system and probably have a relatively accurate result. Another way of thinking about this is the universe is made up of open systems, but we treat them as being closed or isolated to simplify our analysis. We are building a model of reality and analyzing the model. And, as my stats teacher once said, all models are wrong, some models are useful. The degree to which they are wrong is an important aspect of engineering. Bearing in mind what inaccuracies are present in your model, because there will always be some, and understanding how those inaccuracies are affected by the assumptions you make. If I wanted to figure out how much heat was required to heat my office and all of my windows were open, then treating it as a closed system would likely be an inaccurate result. When we are analyzing a control mass, that is, we are defining our system as being a quantity of matter for study, what we are doing is effectively treating it as a closed system because no mass enters and increases the mass nor leaves and decreases the mass. Control volumes, that is, regions in space, are more commonly chosen for open systems because mass is freed across the boundaries. An isolated system would also likely be a control mass. So there is a correlation between open and closed and isolated and control mass and control volume, but it is not necessarily a direct causation in all cases. The way you frame your system can also make understanding a problem a lot easier. If I asked you a question like, how much heat is being produced by this engine, that seems like a complicated calculation. You might be tempted to go in and analyze all the combustion happening and figuring out how much heat is being rejected from the gas to the container or to the cylinder walls as it's undergoing its combustion process and then how much of that heat is transferred into the cooling system and how much of that is going into the radiator and how much of that is actually being dissipated and where the heat is being dissipated by and from and all the convection effects of the heat being pushed into the air by places other than the radiator and all the radiative effects from all the metal being hotter than the atmosphere. But we don't really need to do that. If instead we were to surround the engine with a control volume and consider what's going in and out because we know that energy cannot be created nor destroyed, if we figure out the difference between what's coming in and what we know is going out as other things, the leftover must be how much is leaving as heat. So in this case, I have heat exiting, I have exhaust gas exiting, and I have power leaving in the drive shaft. The inputs are fuel and air. If I were to figure out the energy available in the fuel and air, figure out how much leaves as exhaust gas, how much leaves as power and I assume that none of that energy is actually stored within the engine over time then the difference between the two must be how much is leaving as heat. Going back to the checkbook example, if you bought three video games at GameStop and a controller and you knew that each of the games cost $50 and you knew that your total before tax was $180 then the controller must have cost $30. You don't need to know the cost of the controller if you know everything else in the problem because you can figure it out. If we were analyzing a gas, it might be convenient to describe our system as a control mass, not a control volume. Furthermore, we would probably simplify our analysis by neglecting any gas escaping around the piston. That simplification from reality into a model by treating it as a closed system is probably reasonable. If we were analyzing water being accelerated through a nozzle we could figure out how much kinetic energy is increasing as a result of decreases in pressure, how the enthalpy of the fluid drops in order to accommodate that increase in kinetic energy. We could perform that analysis by treating the nozzle as an open system. We would do that by defining our system as a control volume, establishing an imaginary boundary at the inlet and outlet of the nozzle and treating the inside of the surface of the nozzle as our third boundary, closing our volume. If we were looking at an air compressor that was operating steadily, taking in power from a motor and increasing the pressure of air, we would probably want to treat this as a control volume because we have mass coming in, we have mass going out and it would not be appropriate for us to assume that no mass crosses the boundary. Therefore, we would treat it as an open system. If we were analyzing a water heater, we could have the option of treating it as a closed system or an open system or an isolated system depending on what's actually happening. If there's no water going out and no water coming in, then you could make an argument that it is a closed system. If you're assuming that the water heater has turned off and it is cooling over time, you could describe that as a closed system. If you were trying to figure out how the hot water that was in a water heater was going to mix with the cool water if no heat is added and no heat is lost and no mass crosses the boundary and figure out the resulting temperature of the mixed water, you'd be treating it as an isolated system. If you were allowing water to leave at a high temperature and water to come in at a cold temperature and figure out how much heat had to be required in order to increase the temperature of the water, you'd be treating it as an open system. Defining the parameters of your problem helps you identify what assumptions you can make to simplify the scope. Generally speaking, it is useful to describe what's happening even if you feel like it's obvious. That will help you figure out what assumptions you need to make in order to simplify reality to a model. For the purposes of my homework assignments, I will often make you establish given and fine statements even if they are incredibly obvious to help you get into the habit of parsing out what you know and explicitly listing what you're looking for. Next definition. A state refers to the condition of a system as described by its properties. A state could refer to a point in space, it could refer to a point in time, or it could refer to a point in space and time. For example, if we were considering the expansion of some gas and we knew the mass and temperature and volume at the beginning, we knew the mass and temperature and volume at the end, we could describe those as two distinct states and apply properties to those states. Another thing to note on the discussion of state points is that it takes two independent intensive properties to fully define a state point. That means once we have identified two independent intensive properties, that state point is fixed. From that information, we can describe the rest of the independent intensive properties. If I know the temperature and pressure of air, I can figure out what its density is, because the state point is fixed. It takes two independent intensive properties to fully define a state point.