 Now, what is the property of a system? Again, thermodynamic property of a thermodynamic system is any, remember we said state of a system is defined by listing the relevant characteristics of a system. Each and every one of those relevant characteristics is known as a property of a system. The property of a system is a relevant and irrelevant characteristic may be considered as a property but it is of no use, it will just stay there. Even if it varies, we are not going to worry about it. Which one? This is, this property is, I can continue the whole series of lecture without using the word property, provided instead of property I say a relevant characteristic of the system. So a property is nothing but the short form for a relevant characteristic of the system. You take any thermodynamic discussion, wherever you see the word property, replace it by a relevant characteristic of that system. It is perfectly okay, only thing is book will become bigger. This is only a short form and we are going to come to a number of short forms like this. But we must be clear as to when it is a short form and when it is an operational definition. Property is a short form. Characteristic is a primitive. Combination of this list of characteristics and their quantified numbers is the state of a system. So naturally, any of those quantified value change, you say the system has changed its state, it is a simple asset. Now this is a short form, that is why I have used 3, 9, equal to, that is a habit but I may not be perfect in it. The next thing we do just to be consistent with the traditional textbook is the classification of property. Properties are classified in a number of different ways and hence a property may be classified under various heads together. It is not possible that a property is of type A as well as type C. Another property is type B as well as type C. One way of classification is to define, consider them as primitive properties, basic thermodynamic properties or derived property. Illustrations of primitive properties, volume, pressure, velocity, charge, electric potential, etc., etc. Whatever is required and is defined in other branches of science, chemical concentration, density, all these things are primitive properties. We do not have to define them in thermodynamic. Sir, density we have defined in fluid mechanics, perfect. Use the same definition here, it is a primitive process. Basic thermodynamic properties as I said is temperature, entropy, to some extent energy but really out of energy the so called thermal internal energy U because these are defined in thermodynamics. Now in thermodynamics as we go on developing things, we come across a number of combinations of properties. Those could be algebraic combinations or those could be differential combination. An algebraic combination is this combination U plus P into V. This we come across so often particularly when we work with open thermodynamic systems is that we need to have a short form enthalpy for it. This is a derived property. Again the whole of thermodynamics can be rewritten without using the word enthalpy and the symbol H provided wherever we see enthalpy we write a combination of U, P and V whenever we see H replace it by U plus P. But nobody will do that because this is so convenient. So convenient that you will sometimes find tables of enthalpy and not tables of the internal energy which is a major part of enthalpy. There are other properties. For example for fluids, pressure, volume, temperature are the main characteristics. So way volume changes with pressure in a relative way and since as pressure increases volume decreases just to make it a positive number we put a negative sign there and we do it say isothermal. So what is this? This is the isothermal compressibility. Kt alpha t, beta t whatever. Similarly we will have the isentropic compressibility. There reciprocals, the isothermal bulk modulus and isentropic bulk modulus. Similarly keep pressure constant but we are interested in how the volume changes with temperature again in a relative way. Usually as temperature changes volume increases so there is no need for a, so this is, sorry isobaric expansion coefficient. It is also a derived property. So rather than talk of isothermal compressibility we can talk about derivative of V at P with respect to P at constant temperature divided by V with a negative sign all this instead of that isothermal compressibility. So these are very useful short forms and all these are derived properties. Other derived properties are Gibbs function, Helmholtz function and many, many cross derivatives. Many of them without names but derived properties nevertheless. The other way of classifying property, did I write one here? Yes is extensive or intensive but with a difference. When you classify or try to classify a property as primitive basic thermodynamic or derived, any property will fall in one of these three classifications. Any property which you mentioned should either be primitive or basic thermodynamic or derived. There is no fourth classification. That is not true for this classification. We will now define whether a property is extensive or intensive but there could be properties which will be neither extensive nor intensive. So it is not necessary that a property be either extensive or intensive. And we proceed like this. Let us consider a system and let us let it have some property. Let me just call it phi. The question is whether phi is extensive whether phi is intensive or neither. Operational definition. Again I will emphasize this operational definition initial few times because in thermodynamics at least the way I would like to present it is all definitions which are not short forms should be operational definition. Partition this system into two parts. Just consider an imaginary surface going to the system, splitting it into two parts. Need not be equal in volume or anything. You measure the same property for one part let that be phi 1. You measure the same property for the other part let that value be phi 2. If you find that then we call it an extensive property. If you find phi equals phi 1 equals phi 2 then we call it an intensive property. There is a minor assumption here but which we will I will explain after a few points. For example mass of a system. Quite often it is a relevant property partition the system into two. Mass of one and mass of two. Mass of the total system is m. Extensive or intensive? Because m1 plus m2 is m. Similarly, consider temperature of a system. Intensive. What is the assumption that we have made here? We have assumed equilibrium but we have not yet defined equilibrium. We will do that soon. Let us get rid of these definitions. The third one which is a minor thing so I will keep it here is what is known as a specific property. A specific property is nothing but an intensive property divided by mass of a system. So specific volume, specific energy, specific entropy, specific enthalpy, these are all specific property. One exception density is also a specific property reciprocal of specific volume but that is not per unit mass it is just a reciprocal but we consider it to be a specific property. But because it is different we do not call it specific mass. Specific mass will really be one. We just call it density. It is good as a separate name density but it is a specific property. Now let us again go back to our illustration and that brings us to this question. Question 3. What is a thermodynamic state space? Very few have even attempted that. That is what I noticed today morning. So now let me go back to my illustration of this. Gas in the LPG cylinder in our kitchen. And we said that we determined its state as 14.6 kg 27 degree C and 18 bar for mass, temperature and pressure respectively. Now if you look at it in a, you know in a nebulous way, isn't it similar to saying I have a point in a three dimensional space saying x equals 14.6, y equals 27, z equals 18. Something similar to that. So when we come to that, let us look at a state. Our thing was a gas in a cylinder and we said the state was represented by mass was 14.6 kg. What was it? T was 27 degree C and the third one was pressure is 18 bar. If we consider this analogous to the coordinates of a point in three dimensional space, the analogy is perfectly valid. You can consider each one of these as say equivalent to simply x equals 14.6, t equals 27, t equals 18 in geometry, sorry x, so z here. That is perfectly analogous. And this analog is g, it brings us to a thermodynamic state space. Thermodynamic state space is a, you may not call it geometric, but equivalent to a coordinate system in which each coordinate represents a thermodynamic property. For example, here this could be m in kg, this could be p in bar and this could be t in degree C and we could say 27 degree C, 14.6 kg and 18 bar. So complete this rectangular parallelo prepared and somewhere here we will have, this is the state as represented by this. This representation is known as thermodynamic state. In thermodynamic state space, each coordinate represents a property. So that state of a system is represented by a point in thermodynamic state space. We may not, you may not have used this, but you and your students have drawn dozens or hundreds of PV diagram, TS times. Those are nothing but projections of the thermodynamic state space on the relevant plane. You have not perhaps defined it, be conscious of it, but here it is a reasonably simple system where we had three coordinates because there were three relevant properties. So it is a three-dimensional thermodynamic state space. But we should not be scared for a complex system there will be seven coordinates. So it is a seven-dimensional thermodynamic state space. We are not going to show it, we are not going to measure things in it. Even three-dimensional space we can see it, we can feel it, this is three-dimensional. But when it comes to a projection in the bowl, I am showing a two-dimensional projection in some sort of an isometric stuff. But if I were to project it from top, I would get the temperature mass or mass pressure or temperature pressure projection. Now the next question. First thing is, is this idea clear of what is meant by a thermodynamic state space? It is an n-dimensional space where n is the number of relevant properties or relevant characteristics or nth properties. Each coordinate represents one property and a point in thermodynamic state space represents the state, one state of our system. Now the next question is two questions actually. Question one, is it always possible for us to represent the state of a system by a unique point in thermodynamic state space? It may be possible, it may not be possible. For example, you take this water, you will say mass, temperature, maybe volume, if I am drinking it, it will change and it is pressure. But then you say that look, there is gravity and there will be a hydrostatic pressure gradient from here to here. But then you will also say that look, I am not going to worry about that pressure gradient. It is not going to matter. This is strong enough to take care of the slightly higher pressure here. So you will say that although the pressure varies, that variation I can neglect giving me a unique value of pressure. But now suppose I open it and put a stirrer in it, start stirring it, electric stirrer. I will have a nice meniscus parabolic. The pressure in the center will be significantly different from the pressure near the periphery. And if I want to represent the pressure by a unique value, I will not get a unique value. Similarly, if I am holding it my hand, if you try to measure temperature, the temperature near the surface which is near my hand is likely to be say 35 C, whereas the temperature in the center is likely to be say 29 C. So even temperature will not be unique. So now we classify our states as those which are representable by a point in thermodynamic state space and those which are not. Those which are representable by a point in thermodynamic state space are known as states in equilibrium. That is our definition of equilibrium. So a state in equilibrium, again mind you this is a thermodynamic state or thermodynamic state of a thermodynamic system because unless system is defined you cannot define its state. In thermodynamic equilibrium, so when I say equilibrium it is by definition thermodynamic equilibrium. Means can be represented by a point in thermodynamic state and that means unique values of if it cannot be represented in thermodynamic state space it is not a state in equilibrium. And we will follow this definition of thermodynamic equilibrium throughout this course. This is our definition. So you want to know whether the state of this system is in equilibrium or not. Try to measure properties. If you get unique values for relevant properties then you can represent it by means of a point in thermodynamic state space. This could simply be some property 1, some property 2, some property. This is state s in equilibrium, unique point. How do I show a state not in equilibrium in thermodynamic state space? I cannot really show. You will get some cloudy nebulous stuff not in equilibrium. In fact this is also a very crude depiction. You should just have some grey area there. Any discussion on this? Any comments on this? For any objection to anything which I have said? Let me, yes. State which is not representable by a point. Can we consider as a state which is representable by a point with some assumption? What you are saying is, you know this, again I take the same illustration. Pressure is not uniform. But we have an illustrative exercise. Suppose my job is to just heat it, warm it up. Then is that pressure variation going to be of relevance to us. So when I say relevant property, you should also consider the fact that if the property may be changing to some extent, is that variation going to be of relevance to me during my process. So if I am just storing this water in the fridge, that is of no relevance to me. But the same water if it is at a high top of a dam and is made to flow down the penstock to a turbine. Then naturally that difference in pressure and that difference in height is of relevance to me. Which one? No, but I can have my big system starting from the top of the inlet to the penstock to the exit of the penstock. Then it is one single system but that pressure variation is something which is of relevance to me. So in that case that is not a unique value of pressure. So I cannot show it as a state in equilibrium. But the illustrations of non-equilibrium states are states in which significant churning or stirring is taking place. States in which we are heating or cooling or states which are or systems which are changing states so fast that it is almost impossible to do a proper measurement of some relevant property. Yes madam? No, volume is a relevant characteristic which should exist for any system. But the boundaries can change shape, boundaries can come near and go away. So volume can change of a system during a process. But if it is in equilibrium. Which one? I have not yet used the word control volume in this course yet. Well we will define that later. Unfortunately the two words control volume and control mass are both confusing because we are not controlling the mass there. We are isolating the mass in our system for a closed system. For open system the use of the word control or use of the words control volume as an equivalent is unfortunate. If you consider a system like a turbine with fixed initial plane, fixed exit plane then the volume remains fixed. But you take the cylinder gas in an IC engine, you know as the processes are executed the volume changes. But traditionally we call it a control volume just as a equivalent nomenclature for open system. But rather than control mass control volume we will be using closed systems and open systems. However to be consistent whenever it comes to open system the subscript used will be CV like ECV, VCV etc. But we will not be using the phrase control volume to the extent possible. Even if we use it that will be equivalent of an open system nothing more, nothing less. Because there is nothing special about volume it can increase, decrease, it can go in two parts if a droplet breaks into two all sorts of things can happen. Now one thing was about equilibrium. So let us finish the discussion of equilibrium. The idea of equilibrium is there in other branches of physics. For example in mechanics you have mechanical equilibrium. In chemistry you have chemical equilibrium. Our thermodynamic equilibrium apparently encompasses all these. So it turns out that a system which is in thermodynamic equilibrium will also be in mechanical equilibrium, will also be in chemical equilibrium. But a system in mechanical equilibrium and a system which is also in chemical equilibrium does not automatically be a system in thermodynamic equilibrium. If you remove the requirements of mechanical equilibrium it will remove the mechanics of chemical equilibrium from a system in thermodynamic equilibrium. One aspect remains that we will later consider as thermal equilibrium. We need not define it now. But just because by the time students come to you they will also have studied mechanical equilibrium sigma f0 and sigma tau0 per sigma moment 0 whatever. Similarly chemical equilibrium some vague ideas about chemical potential they may know. So such questions come up. So you should tell them that our definition of thermodynamic equilibrium is unique point in thermodynamic state space. And they will say sir what is the link? Then we will say that look our thermodynamic equilibrium is a super set of chemical and chemical equilibrium. Maybe in some branches of physics there will be some other equilibrium. Remove all those requirements. But if you say something in mechanical equilibrium, something in chemical equilibrium, something in some other cosmological equilibrium does not automatically mean thermodynamic equilibrium. Because when we come to zeroth law we will define what is known as thermal equilibrium which is unique to thermodynamics. And all other equilibria along with thermal equilibrium will lead to thermodynamic equilibrium. This is just a link because the word equilibrium exist in other branches of science. And when we teach thermodynamics you have to take care of these things because they have an idea of work, they have an idea of equilibrium, they have an idea of something else. So that has to be linked up with what we talk in thermodynamics. So when we talk of equilibrium here it is not just sigma forces to be 0 or sigma and sigma moments to be 0. It is thermodynamic equilibrium which operational definition is unique point in thermodynamic state space. Only then the state of our system is in thermodynamic equilibrium. Now we come to one of those clear things in thermodynamics which does not have the status of a law or has not been given the status of a law but is a premise. And that brings us to state postulate 1. When we said that we want to define the state of a system, we have to list out the relevant characteristics of properties, required properties of the system. Question comes up is how many properties are needed to uniquely define the state of a system. This question is one of those big questions which has not yet been very properly answered. This is the domain of the state principles or the state postulates. But one premise which has been shown to be true, not derivable is the first state postulate and that says that the state of a thermodynamic system can be defined using Kumding primitive properties. The premise not really explained and emphasized in many, many books on thermodynamics. This means that without using temperature, without using entropy, of course energy is a common boundary. One can define uniquely the state of a system. So that means if I say that my system is this water, temperature is a thermodynamic variable. It is not a primitive. Primitives are volume, mass, pressure. So that means if I define its volume, if I define its mass, if I define its pressure, that means its temperature is uniquely defined. Similarly for a gas or a fluid which is a very common system and it is important to emphasize this on to students because when you talk of a fluid or a gaseous system and when you start talking of temperature, the mental set is, unless the temperature is defined, how have I defined the state? But then you argue that if it is an ideal gas, you agree PV equals MRT, then PV and M you have defined, isn't T defined? Then it scratches the head. But then it is a surfer way, more complicated system, it will be different. No, it will not be different. Why? Why I can't tell you but it turns out to be so and that is the state postulate or the first state principle. For some reason it has not become a law but it is a premise. How many of those properties because a system will have a number of primitive properties? How many of those that is still open? We will have some answer to it after we do our study of the work interaction. That will also not be a complete answer but just now we say only primitive properties and remember that it can be, need not be. We are not forced to define terms of primitive properties but if need be system, the state of a system can be uniquely defined using primitive properties only. It may not be the most convenient way of doing things. We know that for a gas pressure and temperature is very, very relevant. So rather than volume which can very easily change, we prefer mass pressure and temperature because of convenience but if forced to saying no, I don't like temperature. I don't like entropy. System can be defined in terms of mass pressure quality. However complicated the system be, this is true and I think I would have succeeded in my efforts in discussing thermodynamics with you if you note this down because this is not very clearly presented in many, many books on thermodynamics. And this is what you should tell quite at the early stage. And what is the consequence of this? The consequence of this is mechanics developed a lot without thermodynamics being involved. Fluid mechanics developed a lot without thermodynamics being involved. Cosmology developed fantastically without thermodynamics being involved. That is because primitive variables, primitive properties define the state of the system. We don't have to really worry about temperature or entropy. It's only when the heat interaction came into being that we had to define temperature, we had to define it. The next one is simple. Let me complete this. By a time means at this stage in our discussion. So the next topic is thermodynamic. We had a question what is a process? Quite a few of you have written down the correct one that is a change of state is a process. And that is our definition that a thermodynamic process is nothing but a change of state. Again, full form, a thermodynamic process is the short form for the big phrase a change in the thermodynamic state of a thermodynamic system. It's again a short form. So what is the minimal realization for a process to be defined? We must have a system which executes the process, something called an initial state of the system and something called the final state of a system. So you can't define a process or you haven't defined the process unless you have done the three things. Define your system, define quantified its initial state, quantified its final state. And that means the minimal realization or minimal depiction of a process in the thermodynamic state space will be initial state 1, final state 2. That is the minimal requirement. Sir, are there any final state of the system? No. Process means initial state and final state of a given system. What is the name of the process? Process means initial state, final state plus nothing in between. That's the minimal. What you are thinking of is what about the path, what happens in between. I say I have an initial state which is at 1, I have a final state which is at 2. So that means I was here at 11 a.m. I measured the mass, volume, temperature, pressure of this water, my system. And that was state 1. I came back after lunch at 2 p.m. I found that mass had changed, temperature had changed, pressure had changed. At least one of them had changed because if nothing had changed, I don't even know whether a process has taken place or not. My initial state and final state is the same. That's okay. Initial state can be a final state but for a process I must say this is my system, this is my initial state, this is my final state. We were discussing about whether a process which I again emphasize is nothing but given system specified initial state and a specified final state. This is our definition of a process whether it needs to have a path or not. And the answer is no, it need not have a path. However, it is possible that a system may have a path. Now when will a system have a path? Now at this stage I will restrict myself just for simplicity to two dimensional state spaces. And let us say that these are some properties. Let me simply call X1 and X2. If you are more comfortable, write them as PV, TS, whatever, whatever, whatever. And the minimal realization of a process is initial state 1, final state 2. Shown them distinct, accidentally they could be the same. That's okay by us. The two states need not be distinct. Suppose we say that a path exists. What does it mean? When can we represent it by a path? We should ask ourselves and ask the student to imagine what happens during a process. See there is a change in the state of the system. For a general process state 1 is not state 2. That means at least one property changes. So at least one property goes from its value at the initial state to its value at the final state. How does it go? They have already come across a unit step up and a unit step down in all their dynamics and integration and Fourier series. So a sudden jump, a discontinuity is something which they are comfortable with. I mean they are not uncomfortable with. Let me put it like that. So it is possible that suddenly it could go somewhere between 1 to 2 at some time when we are not looking at it. Or it is possible that whenever we look at the process that is in the illustration I took in the morning 11 a.m. it was 1. At 2 p.m. it was at 2. I come at 11.30. I notice that it is here. Then I come at 11.40. I notice that it is here. And then somebody else says no I came here in between and it was here, here, here. And if I have enough evidence that it went through such a locus. That is any time I observed it. I observed it in a state of thermodynamic equilibrium which could be plotted by a unique point. And if all those unique points form a continuous locus beginning from the initial state and ending at the final state. Then we will say that a path exists. And because there is an initial state and a final state it is usually a good idea to show it by an arrow. Because there is nothing special about 1 and 2. It could be some process from 2 to 1 also. 1 and 2 are nothing great. a, b, x, y whatever you want to write you can write. But this is a very special case. Intermediate state, equilibrium. There is a continuous locus. If this happens, this is a very, very restrictive thing. Remember that means the changes taking place in that one, at least one property are so slow that an observer is unable to determine that things are changing. Such a process of this very special case is known as a quasi-static process. Otherwise it is a non-quasi-static process called x1, y1 does not matter. For example, a non-quasi-static process I will not be able to show by a path. However, just for convenience, as a matter of convention, we show it by a dotted line joining the initial state and the final state. This is a non-quasi-static process. Static means they have, students have realized the difference between statics and dynamics. Static means something which is unmoving. Dynamics means something which is moving in mechanics. Similarly, static means unchanging. And quasi-static means it is not really unchanging because if things are unchanging process will not take place. There is no change in state. Quasi-static means pseudo-unchanging or almost unchanging. But the real, this is just a definition. A quasi-static process is a very special case of a process where all intermediate states are states in equilibrium or to a great excellent approximation are states in equilibrium as can be determined by our measurements. And there should be a continuous locus. You cannot say that from here to here it is quasi-static then there is a sudden jump and from here to here it goes. This is not quasi-static process because there is a discontinuity in between. Let me try to erase this. Let me try to erase this. Good. Now for a non-quasi-static process the dashed or dotted line is only a link of convenience from 1 to 2. It does not represent any intermediate states. So for example 1 to 2 like this and 1 to 2 like this through A and through B are distinct quasi-static processes. But here 1 to 2 like this. These are just for convenience I am showing so as not to clutter 1 to 2 through A, 1 to 2 through B. But intermediate states do not mean anything to us. The dotted line and the arrow is only to link the initial state and the final state. How you link it that is left to you. The way you sketch it has no meaning. But the way a quasi-static process is depicted in the thermodynamic state space is relevant because each state in between is intermediate state in equilibrium as represented by that locus. That is the question of definition. So representation and classification of thermodynamic processes, quasi-static processes is done. The next thing is a cycle. What is a cycle? A cycle or a thermodynamic cycle is a process such that initial state is the same thing as final state. That means in our nomenclature 1 is the same thing as 2. So I will show you various cycles. State of equilibrium back to the same state. This is a cycle. If I turn the arrow around it will be a different cycle. Because the path is, although the same path it is traversed in the different direction. It is like one of those ring roots. Andheri to IIT to Ghatkopar to Andheri was 336. But Andheri to Ghatkopar to IIT to Andheri, the reverse cycle was 337. Another possibility is 1 is the same thing as 2 but I will just show it like this. This is a non quasi-static cycle. I know it changed the state. It was somewhere I cannot make up. Finally it came to the same original state. Is this a cycle? It is a cycle. It is a process. I said that in a process the initial and final state need not be different. So that means we come to a situation where I have my system at a given state, state 1 at 11 o'clock. I go for lunch, come back at 12. I find it is in the same state. What do I conclude? I can either conclude that the state has not changed at all or it must have executed a cycle. Just by this information looking at the initial and final state, I can come only to this conclusion. Either no change of state that means no process executed. It was just sleeping there as it is. Or it must have executed a cycle and exactly come back where it is. Only by doing some other experiments looking at the nearby systems, you can make out whether it was executed a cycle or not. No, this illustration, path is not defined. It is a non-quasi-static cycle. This is a quasi-static cycle. The path is meaningful. This is another cycle. Both black one and red one are quasi-static cycle. But here this cycle is non-quasi-static cycle. I can assert it to be a cycle because I have checked the initial state. It was a state in equilibrium. I checked the final state. That was also a state of equilibrium and all relevant properties had the same values. So, they both states occupied the same point in my thermodynamic state space. Sir, suppose you major properties of state 1 at 11 o'clock. Major properties again at 2 o'clock. The properties are same. It has undergone a process in between, suppose. Two possibilities. Either it has not executed any process at all. Suppose it has undergone a process which is a cycle. Yes. But then by this definition you cannot say whether it has undergone a process or not. Yes. Because I have… So, that means path must be known or path must be defined. No, not necessarily. It might have executed a non-quasi-static process coming back to the same state. That brings me to the realization which you should have that a cycle whether quasi-static or not must have at least one point of equilibrium. Because the initial state, I say initial state must be final state. That means I must have a method of checking that initial state is final state. That means my initial state must be completely defined, a state of equilibrium. Final state should also be completely defined, the state of equilibrium. Cycle means initially final. So, at least one point on a cycle must be a state of equilibrium. In the case of quasi-static cycle, there are many points. I can start this quasi-static cycle from any point and come back to the same point because all intermediate points are known. But a non-quasi-static cycle, I know I started here, I ended here. In between what happened, I do not know. Now, we go back to closed, open and isolated. Now, a system executing a process is defined as a closed system, no possibility flow. A system by itself, I said in the morning, I am changing this because a system by itself cannot be closed or open. It is only during a process that it can be a closed or open. Actually, perhaps maybe after another 50 years, we will change the definition. We will say a closed process and an open process. A closed process is one in which mass does not cross the boundary, another one which mass crosses the boundary. But we will define it as closed system and open system. Or say during a process, a system remains closed or is caused a closed system if there is no possibility of mass flow across its boundary. That just does not mean initial mass equals final mass in volume. The mass which was initially in the system remained there throughout. It is not that part of it went out again. We call it an open system a-float. And we call that system an isolated system. I am encroaching on the next one. No interaction of it, no N. In this case, no mass energy is possible. In this case, mass movement is possible, energy movement is possible. These are our standard definitions. I do not think we have to worry much about that. Which one? We have not yet defined it. An adiabatic system is a very restrictive. Adiabatic is an adjective which if you look up your, our scheme of things, it comes in 5. What does adiabatic mean? 5.4. Till then we will have to wait. Now the last point in this I suppose, yes, before we go to work interaction is thermodynamic interactions. A very simple need thing. A thermodynamic interaction is nothing but, but again I am, it is something which we will say energy in transit or energy flow between two systems. You would have noticed that in the morning when I defined system, I did not use the word or I did not define a related term called surroundings, which is quite often defined in many books. That is for the simple reason that the surroundings is and must also be a thermodynamic system in its own right. So rather than system and surroundings, I would prefer to consider them as system A and system B. Because unfortunately surroundings gives us a feel that everything which is outside the system is surrounding. And if you say so, then students get under the impression that the surroundings is our physical environment or the system's physical environment and we need not worry about its boundaries. That is not true. When you say surroundings, what you really mean is immediately surrounding or system which is directly interacting with our system, the primary system we are going to consider. So now I mentioned that when we talk of thermodynamic interactions, for us these are, students know what energy is. The ability to do work, blah, blah, blah, whatever they have learnt, we will redefine that. But such energy in transit between two systems, going from one system to another system, is known as an interaction in thermodynamics. So a thermodynamic interaction is an energy in transit and because they also know that there is something called conservation of energy. So if energy is lost by a system, must be gained by some other system and we know that if there is a loss and gain for a system, its state will change. So the first premise or the next premise here is energy transfer, energy transfer for energy exchange between system A, system B leads to change of state of A plus change of state. Because one system has gained energy, another system has lost energy, so the state would change. Energy we know is a relevant property of energy exchange. So interactions causes change of state. This is at the heart of our thermodynamic studies because interactions are energy interactions. And now of the three laws of thermodynamics, two laws except the zeroth law. The zeroth law is a special state. The first law and the second law have this relation. They relate interaction to the change of state. For example, our final form of first law, the change of state is represented by delta E. Interactions are represented by Q minus w. We have not defined anyone of these quantities yet, but they would have known some such form of first law of thermodynamics. So tell them that look, this law of thermodynamics relates change of state to the interaction as represented by Q and w. Some may write Q plus w, some may write Q minus w. That is the time convention we will settle on that. Even the second law, delta S is related to dQ by T, again heat interaction. And it says delta S is greater than or equal to dQ by T. It is a relation, it is not an equality. That is the specialty of the second law of thermodynamics. You can simply say that quite often one of these A and B is our system of primary interest. In which case the system B, which is a system of secondary interest is known as a surrounding. Rare case, rare case where the energy of a system changes because it gains energy but the state does not change. I have not come across any incidence of that kind. The state of the surroundings will change but it will not manifest significantly because maybe it is a large system. But it has gained energy so its energy had changed, so its state had changed. Its pressure temperature may not change. Something else may change. In fact that is the thing which we should guard against. That student feel that surrounding its state may not change. Put it, do not leave it as surrounding, call it a system B and just say quite often it is known as surrounding. Why not just call it the immediately surrounding system? It is a thermodynamic system. Why do not you want to call it a thermodynamic system? So call it immediately surrounding system, call it system B and then say that in a large number of thermodynamic textbooks it is usually called the surrounding. But give it the proper status of some thermodynamic system which it is because we can apply the laws of thermodynamics to it. So it is a thermodynamic system. Now that brings us to the initial discussion, essentially definitions. No new principle involved and some definitions and we have done the definitions of various properties, state of a system, properties of a system, thermodynamic state space. We have classified the systems during a process as closed, open, isolated and mind you as teachers you should realize that how to define a system, where to lay its boundaries and how to treat the boundaries. Boundaries can be movable, boundaries can change shape, size. That is all left to us. Consequently while modeling the process of a system we can define the boundaries in such a way that in model A the process can be modeled as system A, system B where both system A and system B remain closed and another model where system C, system D consisting of the same total system where both system C and system D are open. An illustration is suppose, I have it in my room you take a bottle containing water. I will not do it here but I will just show you what I tend to do. You take an empty bottle and connect it on top of this after removing this thing but with a proper leak proof connection. The experiment is turn it around. And you know there is a change of state. Initially this, if you say this is my top system contained only air, my bottom system, system B contained only water. Final state is this contains only water and this contains only air when everything falls down. One way to model is to consider the top bottle at system A the bottom bottle at system B in which case during the process both the systems are open system because some mass is transferred from B to A and some mass is transferred from A to B. But I can redefine my system saying the inside of the bottle whatever is the water part is my system A and whatever is the air part is my system B. Now during the process if I assume that no air dissolves in water and no water vapor goes into air they do not mix with each other then what is the status of A and B open systems or closed systems? They are closed systems, okay. So this is an illustration that the same process can be remodeled in two different ways. In one case having open systems in another case having closed systems. So remember when we say a system is open or a system is closed it is the way we are modeling the system and the process. So we should say let us model it as an open system or let us consider it and lay out our boundary as that of an open system and in the other case we have laid out our boundaries in such a way that during the process both the subsystems A and B will be closed. Is that clear?