 Equilibrium is the condition of balance. That means that there are no unbalanced potentials in a system. For example, inside of this isolated system described below, if I had an even temperature distribution, that means that it would be in thermal equilibrium. So the box on the left is not in thermal equilibrium, the box on the right is. In addition to thermal equilibrium, we also consider mechanical equilibrium that's mechanical potentials like forces. We also consider chemical equilibrium, which is going to be the potential for chemical reactions to occur. And when we have phase equilibrium in addition to that, that is no unbalanced phase changes that are trying to occur, those four make up a shorthand term called thermodynamic equilibrium. So when something is in a state of thermodynamic equilibrium, it is in thermal equilibrium and mechanical equilibrium and phase equilibrium and chemical equilibrium. So this box from earlier could be described as not being in thermal equilibrium, but we could also say it is not achieved thermodynamic equilibrium because not all four equilibrium states are met. A process is the transformation from one state to another. And it's important to note that for our purposes, the process just refers to the connection between states one and two. It doesn't actually refer to that specific path. So all of these connecting lines between one and two are referring to the same process, but they have different paths. The process is just from state one to state two, if that distinction makes sense. The specific line is the path. Some of the common process paths that we define are isothermal processes, isobaric processes, isochoric processes, isentropic processes, isentropic processes, etc. And when you see a type of process described as being iso and then something, that's just a way of saying that that property is constant. So an isothermal process is a process that occurs with a constant temperature. Again, an isothermal process is one that occurs with a temperature that remains constant. An isobaric process has a constant pressure. Furthermore, you could refer to a line of constant pressure on a graph as being an isobar or a line of constant temperature as being an isotherm. An isochoric process is one that occurs with a constant volume. An isentropic process is one that occurs with a constant entropy and an isentropic process is one that occurs with a constant enthalpy. We haven't talked about entropy nor enthalpy yet, so it's okay to be unfamiliar with those terms for now. But those processes are all describing a constant property and they're described as iso or isa and then the thing. A cycle is a special type of process or sequence of processes that get back to where it started. That is, a cycle must return to its original state. So in this process diagram from P1 and V1 to P2 and V2 to P3 and V3 to P4 and V4 and back to P1 and V1 is a cycle. We often consider thermodynamic cycles because that cycle allows us to accomplish something. For example, a heat engine is a type of cycle. It is a power cycle and a heat engine converts heat into work. My car engine is a heat engine. It performs cycle that allows me to take in heat from the combusting fuel and produce work, mechanical work or power as a result. A refrigeration cycle is a similar cycle. It is a sequence of processes that allow me to take in work and accomplish a heat exchange process that would not happen naturally or rather that would not happen without an investment of work. You are taking in power and pushing heat in a direction it does not naturally want to go. Some special types of processes are referred to as steady state processes. A steady state process is one where you can neglect the effects of time. Just like with the distinction between an open system and a closed system being whether or not you consider the effects of mass, treating something as being steady or transient refers to whether or not you are allowing for time. That is, if time matters, if you are including the effects of time in your analysis. When something is treated as being steady or steady state it does not change with respect to time. When something is transient, then time matters. Things are changing with respect to time. A steady flow process is a process that has a flow that is steady. It doesn't change with respect to time. It's okay for properties to change with respect to position within that system as long as they don't change with respect to time. Next definition. When we talk about density, we will describe a variety of properties that all refer to the same general concept. How much matter there is in a given space or how much space an amount of matter takes up. Density is an expression of mass per unit volume. Specific volume as a specific property is an expression of volume per unit mass. They are the same general concept because specific volume is the reciprocal of density but one is more useful than the other in situations where you're trying to factor out mass. For example, if you were dividing an entire analysis by mass then writing out specific volume would allow you to take volume and express it on an intensive basis because it is a specific property, specific volume. Now, specific gravity, despite the fact that it has the word specific in it is not quite the same thing. Specific gravity would imply to you that you're dividing gravity by mass which isn't what we're doing. Specific gravity, abbreviated SG because we are particularly clever when it comes to abbreviations is a density of a substance expressed divided by the density of water at standard temperature and pressure. So for example, if we were reading the gauge on a manometer it might be useful to express the specific gravity of that gauge fluid because it would immediately communicate to the reader or the viewer how dense that substance is compared to water. A specific gravity of 2 means the density of that fluid is twice that of water. A specific gravity of 0.8 means it is slightly less dense than water. Its density is 80% of that of water. For something to float in water it would have to have a specific gravity less than 1. A specific gravity of something higher than 1 would sink. If something has a specific gravity higher than 1 that would imply that it would sink. And similarly to specific gravity another concept that will come up with that is similar is specific weight. Specific weight is abbreviated with a gamma and it is the weight of something divided by mass, divided by volume. So if you were expressing the weight of something as its mass times gravity and you were dividing by volume that is density times gravity. So it's like density but it also includes acceleration. You'll see specific weight as a concept more frequently when we get to fluid mechanics. Next definition, a pressure is a normal force exerted by a fluid per unit area. We will write that symbolically as pressure is defined as force per unit area. For our purposes we are only ever going to be considering the pressure of something at an instantaneous point or the pressure of the entire system all at once. That pressure exerted on the inside of something or on the outside of something is expressed as a normal force. The fact that the force exerted by a pressure is normal to its surface the fact that the force exerted by a pressure is normal to the surface to which that pressure is applied means that if we were considering say the pressure inside of a vessel the force exerted by that pressure is always normal or orthogonal to the surface to which the pressure is applied. Normal or orthogonal is like perpendicular but in three dimensions. Perpendicular is to two dimensions as orthogonal is to three dimensions. If you were looking at a plane, a normal force is perpendicular in this direction and this direction. When we talk about units of pressure we are going to be describing it primarily in kilopascals and psi but you may encounter the atmosphere, the bar, the tour as well as heights of a column of mercury. Those are pretty common ways of expressing pressure especially in applications that are a little bit more field based. Remember that any conversion you need between units of pressure remember that the majority of units of pressure that we encounter can be converted between by using the conversion factor sheet on the inside of the front cover of your textbook. Next while a pressure will have a force applied to a surface that has a direction meaning that there is a force vector the pressure itself is a scalar quantity.