 Now, although I have said that we would like to proceed in a unidirectional fashion, remember thermodynamics is one single whole, not H O L E, W H O L E. So, we cannot stop talking about some idea, some concepts which are going to be explained. For example, we are comfortable with temperature, but in this course we have not properly defined or formally defined what temperature is, but we have sketch state spaces with temperature in it and nobody takes an objection because we generally have some idea of what temperature is about. Now, here we have the word interaction. So, let us begin with what interaction is, but before that let me say that the whole of thermodynamics is related to a scheme like this, a system A usually and a system B. Classically you may call it our system and our surroundings, but one should remember that surroundings is a word which is in colloquial English and not really defined in thermodynamics. If you have to define in thermodynamics, it is a proper surroundings is a proper thermodynamic system with which our system, the primary system interacts. So, instead of system A and system B at show near, if you want to write system A or system and surroundings that is perfectly okay or you could even say primary system and secondary system. If you want to call system B at surroundings, be sure that it is defined as a proper thermodynamic system with appropriately defined boundaries. Do not be so general as surrounding is everything that is outside the system that is to lose a definition. Now we have understood what is meant by a process. So, let us say that the two systems work together. A system A executes a process from A1 to A2, system B executes a process from B1 to B2. Well this is a short form of saying system A is initially in a thermodynamic state A1. Finally it is in thermodynamic state A2. So, A1 to A2 by system A is the process executed by system A. And why do the systems change? Well they interact with each other. There is some sort of a give and take and this give and take is known as interaction. It is actually a transaction. You go somewhere buy something and then there is a two way interaction. The grocer gives you some grain. So, grain comes out of the grocer's shop into your back and you take out your purse from your pocket and give him some money. So, there is a material interaction, material transfer from the grocer to you and there is a crash transfer from you to the grocer. This is a sort of a once type of interaction. In thermodynamics, we consider interactions of the energy kind. So, what we are going to look at are energy interactions. And we are going to classify the energy interactions in two types. The classification of these will be the work type of interaction which is actually a primitive, but we will define it or redefine it properly. And the second one is the heat type which we will formally define using the first law of thermodynamics. So, that brings us to the first interaction which is the work interaction. Yes, I will hold on. So, the work interaction is actually a primitive for thermodynamics, but unfortunately the work interaction although is defined in other branches of physics. Generally, the idea is the work is force into displacement. You have force acting on something, I may be pressing something or even a gravitational force acting on this downwards. So, if there is a movement of the point of application of the force in the direction of the force that is the force into the displacement is the amount of work done. The directions are different. You take a vector dot product and so on. And we have worked all over for example, this bottle comes down with a mass m comes down by a height g. We say that the work done by the force of gravity is m into g into h. So, the general idea is force into displacement. In thermodynamics, we want to say that this is an interaction and according to our definition remember that whenever there is an interaction, there must be two systems involved system A and system B. A single system by itself will not have an interaction because that will be an isolated system if it does not interact with any other system. So, we have to have some sort of a proper thermodynamic definition. So, although we are not going to redefine the work, we are going to combine all the ideas of work and give a thermodynamic version or the thermodynamic definition of the work interaction. Let us see a few examples of the work interaction. Let us say some simple system. Let us say that we have a string or something like that which is our system. You could say a wire. This is our you know the Hooke's experiment I am showing. And let us say that there is a small loop. My system B is the whatever is trying to pull it. And since I am trying to pull it, there is a tension T which would be a property of the wire or the string. And under this pull, there is a small amount of displacement dx. And we will say that the small amount of work done by our system on the surroundings which is I or any machine or any entity which is pulling it will be minus T dx. T is in the negative x direction, dx is in the positive x direction. So, we generally write minus T dx. You take for example, an electric cell. The one which is inside your mobile for example, it has a positive terminal, it has a negative terminal. And this cell let it be, let that be our thermodynamic system. We connect a wire here and a wire here, positive terminal, negative terminal. And we connect something here which is our surrounding system system B. And let the potential difference from the negative potential rise from the negative terminal to the positive terminal BE. And let a charge dq be transferred completing the cycle through the other system. This could be done by withdrawing a current I for a small amount of time dt. So, in this case, you could have dw to be equal to edq. Or if you want, you can expand it as ei dt. But the main format here, just the way we had dw is minus T dx, here we had dw is edq. Continuing further, maybe we can look at liquid, system consisting of some liquid. And I put in a stirrer and try to stir it. So, as I try to move the stirrer by some d theta, the liquid is viscous and it tries to counteract by providing a torque tau opposing my movement. So, in this case, I can write my small amount of interaction is minus tau d theta. We will be wondering why I am not drawing this figure. Well, I am drawing this now. Let us say we have a gas in a cylinder enclosed by a piston. So, my system is the gas. And let us say it has a pressure P. The cross sectional area is A. So, there is a force F acting. F will be equal to P into A. And there will have to be a force acting on the other side F, so as to keep the piston in its place, otherwise it will fly off. And if I allow the piston to move by a small distance dx, the work done will be force P A into dx, which is P into A dx, which will turn out to be P dv, where dv is the change in volume. So, look at these four. I can give more illustrations. For example, a spring or a liquid film stretching or all that. We have looked at stretching of a string, discharging of a electric cell, stirring of a liquid, expansion of a gas. Notice everywhere it is some P dv minus tau d theta minus T ds when e d cube. So, what we notice is each one of these interactions is of the type dw is x dy, sometimes plus, sometimes minus. So, we notice the following in this. First this plus minus is we keep a note that there is some sign convention involved, otherwise we would not be consciously writing minus sign somewhere. The second thing we look at x and y. Let us go back. Let us look at what is x is and what y is. Here we will notice that tension and length x is change in length. So, the length x or l of this thing, these are properties of over system, which is that wire or that piece of string, which is a tau. Notice T and x are both properties of the string. What type of properties? T is an intensive property, x is an extensive property. You check for yourself. If you cut the string into anywhere, the tension anywhere is going to be T, but the length is going to be that much shorter. Similarly, e d cube, you will notice that e is an extensive property. A smaller or a bigger cell of a certain kind provides you the same potential difference across its terminal. Whereas, the amount of charge or current that it can provide, so called ampere hours type of thing, that depends on the size of the cell. A big cell will have larger of q possible. So, e is intensive, q is extensive. Come to tau d theta. Here we have a funny thing. We will notice that unless d theta comes, there is no tau. So, the torque nor theta has anything to do with the properties of the system. Here we will again notice that pressure is an intensive property, volume is an extensive property. So, when you notice this, you will say x, y sometimes x is an intensive property and y is an extensive property. Again, you will notice going back to our illustrations that let me just look at 3 and 4. See, if I allow the pressure to rise slightly, so that the force on the piston overcomes from the fluid, overcomes the force on the piston from outside slightly, the piston will move to the right, the gas will expand and dw will numerically be a positive number. If I allow the gas to reduce its pressure slightly, then the piston will move to the left, dv will be negative, volume will contract and dw will have a negative numerical value. So, this is a interaction, the work interaction, which is possible both ways, positive as well as negative. Whereas, look at this interaction, tau d theta. If you start with a fluid in equilibrium, that means uniform properties, unique properties, put a stirrer in it and try to rotate clockwise, torque will opposite, the value of dw will be negative. You try to rotate it anticlockwise, again torque will opposite. In any case, the value of dw will be negative, but always the interaction will be, the energy will be absorbed. You will have to expand work in stirring the liquid. You cannot put in a stirrer there and ask the liquid to stir the stirrer, that will not do. The liquid is incapable of stirring the stirrer by itself, because tau and theta are not properties of the system. So, here we will notice that interactions 1, 2 and 4 are two way, actually interaction two depending on the type of cell could be one way or two way. The cells used in our mobile phones for example, or car batteries, they are rechargeable. So, we have a two way interaction, but the cells which you simply buy to put in your torch or your shaver, those are not rechargeable. So, they can only be discharged, they cannot be charged. So, those are one way interactions. So, here we also notice that some dw are two way interactions and some are one way interactions. And we have also seen that there are depending on the type of the system, there are different modes of interaction. This is stretching, this is electrical discharging, this is stirring, this is expansion of a fluid. Depending on whether it is a magnetic material, you will have some other type of interaction. So, there are different modes of work interaction. In thermodynamics, we have to be careful because we have to have now a definition by which we can decide whether an interaction is work or not or what component of the interaction is work. If so, what is its direction and what is its magnitude? So, what we now come to is what is known as the operational definition. The question is this. Let us say that we have an interaction between system A and system B. There is some interaction, let me simply call it I. System A goes from state A1 to state A2 because of this interaction. System B goes from B1 to B2 because of this interaction. And we want an operational definition which will allow us to determine one. Is I work or work type of interaction? If so, what is its magnitude and what is its direction? And the direction is associated with the sign convention. We will come to that. All over thermodynamics text books say that the work interaction is that part of interaction which can be converted by primitive means. That means means which need not be dependent on any thermodynamic ideas like stretching of a string, compression of a spring, raising of a weight or twisting of a solid rod. All these are sort of primitives. They do not depend on any thermodynamic entities. No temperature is involved. No entropy, no internal energy is involved. So, work interaction is defined as some interaction which can be reduced by primitive means to the raise of a weight because the idea is raise of a weight is a primitive work interaction requires work to be done. So, anything which is equivalent to raise of a weight that is mass in a gravitational field should be work interaction. Now, saying it in so many words is okay but let us see how we will implement it in an operational definition. So, let us look at the operational definition of work. Let us see what we do. We now try to implement the operational definition. Step one, what we do is we have these two systems A and B with an interaction I. What we will do is we will try to replace B. I will have this system A which goes from A1 to A2 and it will say I am doing an interaction I. But instead of B, I will try to set up another system C, say C1, such a way that the only external effect of that would be raise of a weight mass m, let us say mass m1 in a gravitational field G. I am not writing G1 if you want you can write. It gets raised by height h1 not lower and C1 is a primitive system that means does not depend on any thermodynamic effects. You could have ideal pulleys, you could have frictionless inclines, you could have wheels and axles, you could even have 100 percent efficient transformers, electric motors and things like that. Because we know that in principle these are possible. If you provide enough time, enough money and enough interest you can make the efficiency of an electric motor well as nearly 100 percent as possible. Because we know friction cannot really be reduced to 0 and resistance cannot be always reduced to 0. But you could try and in principle there is nothing wrong in mechanics or electricity and magnetism which prevents the efficiency of an electric machine to be 100 percent. So that is okay. This should be primitive and there should be during this interaction there should be no change of state. That means if it executes a process it should execute, C1 should execute only cyclic processes. So by that what we mean we are able to properly define and set up C1 in such a way that A believes that it is doing interaction I with B by undergoing this change of state. But that interaction can be completely reduced to the rays of a weight and then we say that if C1 can be set up then first I is work 2. Now this is about the direction. We say that A does work on B or system A does work on B or system A does work on system B. A is the donor of work, B is the recipient of work. And third one we say work done by A on B if you want you can write A B is plus M1 G H1 and work done by B is minus M1 G H1. Notice that this magnitude M1 G H1 is the magnitude of the interaction I and hence except for the direction or sign plus here and minus here there should be no difference between W A and W B. You cannot say W A has interaction I A and W B the same interaction becomes I B which is different from I A in magnitude. That is not allowed interaction cannot vanish in between. Now it may not always be possible to set up C1 like this. So if C1 cannot be set up go to step 2. What is step 2? We do the same thing but we look at it from the point of view of system B. System B executes a process from B1 to B2 has an interaction I with respect to system A. So we do not change that B does not see anything different. But that interaction we try to provide by means of a system C2 whose only external effect would be the raise of a weight M2 in a gravitational field G by a height H2 again the raise. And then we say that if C2 is equal to C1 then C2 can be set up first. I is work B does work on A and third work done by B on A is plus M2 G H2 and work done by A on B is minus M2 G. Now we go to step 2. The step 3 says is if neither C1 nor C2 can be set up. I forgot to add here that C2 again must be a primitive system fully defined no black box and no change of state. That means if the state changes if it executes a process that must be a cyclic process bringing it back to its original state. Then the conclusion is I is not completely a work interaction. This is the operational definition of work. Now if you want a slight modification to this when I say it is not completely a work interaction it does not necessarily mean that it is totally a non-work interaction. If it is not completely a work interaction then one can go back to this definition and see whether a part of I can be converted completely to work and if it can be completely converted to work we will say that component of I which can be completely converted to work either through step A or through step B is the work interaction. There may be some other part which cannot be completely converted to the rays of a weight. So that is some other component of the interaction that is not a work interaction. I will take an illustration later when I come to the end of this session or during the discussion session which would make this clear. Now there are some things here. C1 and C2 have to be primitive systems that mean contains stuff which is definable and is completely covered and handled by other branches of physics. There could be idealizations like frictionless pulleys, non-stretchable strings, non-dissipatively chargeable and dischargeable batteries, frictionless pulleys. I think I said that 100 percent efficient electrical machines, generators, motors, transformers, etc. What is not allowed is saying that it is a black box. Say that look I will set up such a system. No, you cannot do that. You have to specify it completely and second thing is in C1 and C2 no change of state is finally allowed. During the interaction there may be some local change of state but finally when the interaction is complete system B goes to B2 or system A goes to A2 you should have absolutely no change of state for C2. Third one is notice here as well as here in step 1 as well as in step 2. I have put a plus sign, I have put a plus sign here and a minus sign here. Again in the one I have put a plus sign here and a minus sign here and this is our sign convention. The sign convention here is that work done by a system is positive but mind you this is a sign convention. This is just for the matter of consistency and mutual understanding. I could have said work done by a system is negative and consistently use that. All that will happen is some form, some sign will change in the further derivations but finally in any thermodynamic problem the final answer quantitatively will be the same, qualitatively as well as quantitatively will be the same. In fact this is the first of a large number of sign conventions that we are going to come across right from here, right up to energy, right up to definition of heat, right up to definition of entropy. There are many places where we have to define direction and we will say one particular direction is positive another particular direction is negative. Why? Well we have to be consistent and let us define it that way but if you want to define it the other way well you are welcome but declare it that that is your definition and consistently use it. Do not confuse it by mixing up the two sign conventions. Now after this the next thing is because we are going to evaluate work there are also something to remember that there must be for W interaction two systems actually for any interaction two systems must be involved system A system B or you could say a donor system and a recipient system. One system by which we have to be consistent by itself cannot undergo any interaction and interaction means there are two systems involved and this is where our idea the thermodynamic idea or thermodynamic definition of work differs from some other work interaction. For example you go to mechanics you throw a ball skywards. Let us assume there is no air resistance or anything like that what happens? The ball goes up under the influence of gravity the speed reduces finally at the apogee it becomes 0 and then it starts falling back. If you are a good fielder you can catch it. Now here at any time there is the force of gravity acting on the ball. So there is a force there is a point of application of the force and that point of application is being displaced. If you throw a ball down from the top floor of a building just leave it falls down. You will say gravity is doing work on the ball but where is the recipient system? In thermodynamics this is not a work interaction. We will say okay the ball is your system if work is being done which is the other system which is receiving it since it is not receiving it we will say there is no work interaction involved here. However instead of ball let me hold this bottle. Now I am holding this bottle and the bottle is in equilibrium. Bottle is my system it is in equilibrium because nothing is changing it is at a given height. There is a force of gravity acting downwards and but it is not falling because the system B which is me or my hand is providing an appropriate equal and opposite force primitive Newton's third law of motion. Now I slowly allow the bottle to come down okay bring it down slowly. Now what has happened? The force acting on the bottle is downwards sorry force acting on the bottle is upwards because of my hand force acting because of gravity is downwards. So what is the interaction? The bottle has done work which is because of the force down. Who has received the work? I have received the work. If I push the bottle upwards I say I have done work on the bottle. If I push the leave the bottle slowly and bring it down the bottle has done work on but if I just leave it and let it fall down no work is done. That is the difference between the idea of work in mechanics and idea of work in thermodynamics. In thermodynamics remember this is the most important thing. Two systems must be involved a donor and a recipient. This is true not only for the work interaction but this is true for any interactions of the energy kind or any other kind. Any interaction in thermodynamics we are looking at energy interactions but any interaction work kind later on we will define heat kind. Those all will require two systems a donor system and a recipient system. The next thing is well we have defined work now we come to the next topic evaluation of work. Now remember whenever you evaluate the work done by a system notice the following systems must be involved. Two or perhaps more in case of complicated stuff. The second one is we should note that there are different modes of work and the third one is we have to take care of the details of the work and the process. By modes of work I mean the following. Let us consider a system which is made up of a fluid but which is an electrolyte. I will sketch a system for example I may have a cylinder piston arrangement. We know that those of you who use inverters which have a battery the specification is do not leave the battery set inside a room or an unventilated room because when the battery discharges or charges some gases are likely to be involved and those could be poisonous to the human being surrounding it. So you keep the battery always in a ventilated place. Let us say that there is a fluid may be partly liquid partly vapor we do not have to worry about it and there is a positive electrode there is a negative electrode. It is possible that the gas will evolve and the piston may move up and down there could be charging and discharging. So there is because of this there could be an electrical type of work because of this there could be expansion type of work ELEX and if you want you could even put a stirrer and you could have the stirrer type of work. This is an illustration where three modes are involved some of them two ways some of them one way. So whenever you evaluate or attempt to evaluate a work interaction you have to sum it up over one to number of modes. Do not ever be under the idea that DW always means PDV that is a big mistake many people tend to do. DW is always the sum of work interactions over various modes. In a special case it is possible that only the expansion work mode exists and hence you in that particular case you may have DW equals PDV. So we must check that two systems are involved here you will notice that for the expansion work to take place there must be some other fluid system or a weight or a spring must be there. For stirrer to work there should be someone to operate the stirrer that is the other system there. For charging discharging there must be for charging a cell or a generator for discharging something which runs a motor or a fan or an electric iron or a lamp or a may be a PC laptop has to be there. So any interaction will require two systems we look at the modes of work and the third one is the details of the process. Now if the process is from 1 to 2 I am just showing say x 1 y 1 again if you want you can replace this by any two properties of your convenience. Let us say this is the initial state 1 this is the final state 2 then we can say W 1 2 will be in principle integral 1 to 2 DW and this is the final and then you can depending on the mode you can write this DW expansion plus may be 1 to 2 DW electrical plus whatever you have. Let us take this for example this means integral 1 to 2 P dV. So this will be plus now the question is well we are looking at the details of process when can this be evaluated what is the answer this can be evaluated only when for any V between 1 and 2 P has a unique value. What does it mean? It means that if you are looking at the P V part of the state space if this happens to be state 1 this happens to be state 2 whatever be the process at least the P part must be uniquely defined and in that case that means the process must be a quasi static process. In that case this can be under the shown to be the area under the curve all that we know so things to remember is a component can be evaluated only when the process is quasi static. So for example in case of P V a mode of work where properties are involved the quasi static process will be shown on a state space diagram. So this is a process diagram but this is shown on a component of the state space whereas take the other component for example the stirring component when it is stirred we know that neither the torque nor the movement I am taking dw stirrer is minus tau d theta which is minus tau omega dt where omega is the angular velocity. So here the angular velocity neither the angular velocity nor the torque is a property of the system. So what we do here the process diagram is shown as a time charge against time 0 to whatever is the maximum time t process you will show tau and you will show omega as a function of time. For example the torque may vary like this this is tau as a function of time and omega also could vary something like this this is omega as a function of time but this also means that at any time t it is not exactly a quasi static process but at any time the parameters involved have unique values. If the process is non quasi static all that we will know is well this was the initial state 1 this was the final state 2 well we do not know where the system was in between. For this non quasi static process w expansion cannot be evaluated as an integral whereas here it can be integration is possible. Here because at any instant of time during the process both tau and omega are properly defined it can be evaluated but if there is a very jerky stirrer then you will not know at any instant what exactly is the omega what exactly is the tau and it is something similar to a non quasi static process you cannot evaluate it by integration not being able to evaluate it by integration does not mean that there is no work interaction there is or there may be work interaction we may be able to compute its value out using some other method for example knowing some other interactions knowing something more about the end states and so on. We will have some illustrative examples in that now there are certain standard stuff here which I will go over quickly and that all of you know how to demonstrate that w 12 by whichever mode depends on the path and other details of the process like whether it is quasi static or not. Hence dw is not exact differential hence we write integral of dw from 1 to 2 by whatever modes as w or w 12 and we will never write this as delta w 12 this is wrong never write it as w 2 minus w 1 that is meaningless just w 12 no delta nothing before that and since this dw is non exact often it is represented by d cross w or d prime w w or even some people refuse to use the d which they keep for mathematically exact differential and use delta w for us it is not necessary to use this for us we will continue to use dw but while being conscious of the fact that it is not an exact differential. My students they are all sitting there reading something and they are all solving the problems they know that if I see in an answer book something like delta w what do I do I draw a red line and I do not evaluate that answer book any further I think at least one of you would have faced that at least in the first place after that you learn but that is the way to enforce learning not only on our student but also on our self you know if I sometimes I have data went to as a what is wrong with me this is not the way to work so I will stop working there wash my face with a very cold doubt of water and maybe go and have a hot cup of coffee. Hopefully my brain will start working again now something important because we are going to come to this again and again and particularly there is a state postulate which is related to this we will see it later and that is what is known as complexity of a system we know that depending on the type of a system a system may any number of possible work some some one bit later on particularly in the light of the second law of thermodynamics we will notice that two way work modes are of significant importance. So we will say that we will define let us say n 2 w is defined as the number of variables work system now definitions if n 2 w is one there is only one two way work mode a system can have such a system is known as a simple system n 2 w is greater than 2 we call it a complex we will also notice that there is a small but important class of systems for which the number of two way work modes is 0 we will take an illustration of this soon let me call this a rudimentary system something which is still simpler than simple is rudimentary system so this is not the order you could say any n 2 w number of two way work modes is 0 rudimentary system number of two way work modes is 1 a simple system number of two way work modes is 2 a complex system some illustrations and some further sub classification for example you take a fluid a simple fluid non dielectric non diamagnetic the only two way work mode the pdv type expansion or compression so such a system is called a simple compressible take for example may be an idealized version of your battery in your mobile phone cell or battery in your mobile phone this is recharging can be charged discharged that is the only two way work mode we will call this a simple electrical system you cannot bend it you cannot expand it similarly for example a wire stretched for a spring which you can only compress or extend this will be a simple elastic you can create examples of complex systems for example I showed you an earlier illustration here let me go back here suppose this is a dielectric fluid you can charge discharge it this could be a two way work mode you could expand and compress that fluid this could be a two way work mode stirrer obviously stirring a fluid is a one way work mode so such a system is a complex system because there are two two way work modes one electrical charging discharging the second one expansion and compression you could create other complex system for example without bringing into anything you take for example a bubble bubble inside a liquid the gas inside the bubble what are the work modes it can expand and compress while keeping the area of the bubble the same by changing the shape or without changing the volume it can change the area of its surface doing surface tension type of work the surface can expand the surface can contract so when it comes to droplets and bubbles you have to consider that those systems appropriate systems as complex systems because expansion compression is possible and extension and contraction of the surface is also possible there is a pdv mode of work and then there is a sigma da type of work so that also is a complex system you could have sometimes mechanically complex systems also for example I mentioned that a stretched wire is a simple elastic system you take a spring this is not a spring and unfortunately I do not have one with me here but from your lab you can take an illustration a simple spring made up of thin wire I think you will find it inside a ballpoint pen or something you can stretch it you can compress it so fdx positive xdf negative that is one two way mode of work but without changing the length of the spring you can twist it and you can untwist it you can do work by increasing the twist decreasing the twist it is a tau d theta type of work so look similar to a stirrer but it is a two way mode of work do not be under the impression that any tau d theta type of work is one way a fluid is involved that is why a stirrer is stirring is a one way mode of work but if you have a elastic spring for example the spring which is used which is a spiral spring nowadays we do not use it but earlier our wristwatches or our table clocks used to have spiral springs now perhaps you will have to go to some old home to have a clock which needs to be wound so that is a tau d theta type of interaction which is a two way interaction so if you have a spring which can be expanded and compressed as well as twisted and untwisted those will be two simple mechanical modes but it will be a complex system because there are two two way modes of work similarly I have not taken an illustration but if you take a piece of soft tire which is magnetizable and demagnetizable then that would be and if that is the only two way mode of work then you can consider it to be a simple magnetic system but if you lay it out as a rod so that you can compress it or extend it then well the elastic work is also another two way mode of work so now you will have a complex system with one mode of work of magnetization and demagnetization may be BDH or something like that and TDX the extension and compression and if you make it into a coil and make it a spring then maybe you will have three two way modes of work so in principle you could have even simple systems simple means mechanically simple systems as thermodynamically complex systems because they have more than two way modes of work. Let me come to a situation where we have a rudimentary system illustration well I do not have you sitting in front of me so I cannot immediately ask you the illustration of a rudimentary system but I will give you an illustration of a rudimentary system take for example this forget that there is a button here and I can press it let us consider this to be absolutely solid and so rigid that I cannot twist it I cannot extend it compress it nothing any amount of force of any kind produces no displacement of any kind I cannot stir it I cannot do any two way mode of work so this is a system which is a rudimentary system okay if you say this is a useless thermodynamic system go home and from your medicine cupboard take out your clinical thermometer if you feel hot headed or if you feel feverish that is what you take out and you measure your temperature now that clinical thermometer looks like this but it is a glass with a capillary inside and a bulb of mercury connected to the capillary at one end it is a solid system can you twist it and twist it no it will break can you expand it compress it no there is no two way work mode involved but it is a good thermodynamic system because it helps us measure a property called temperature that is perhaps one of the most rudimentary systems so illustration is the clinical or in fact for any not necessarily the clinical any mercury in glass thermometer another one in your heat transfer labs you would have used thermocouples it is a bead connecting to dissimilar metals solid simple system no there is no two way mode of work so it has to be a rudimentary system no two way mode of work but useful for us for measurement of temperature bead of a thermocouple it is possible for experimental purposes a to create a system and make it rudimentary for example consider the fluid so our system is the fluid and we know that there is a two way work mode involved this is expansion work making our fluid system a simple compressible system but then all that we have to do is fix the piston sealed at one place prevent any change in volume now what is for such a constrained system is there any two way work mode possible no so this is another illustration where a simple system is constrained to behave like a rudimentary system such systems are also important particularly when it comes to 0th law and further discussions we will make use of such systems but this brings me to the almost the end of the work interaction I have finished in number 9 illustrative examples of various types of systems but exercises I will push along with problem solving in thermodynamics which will we will do at the end of today the 4th session but let me have try to have interaction with one center at least and that is home institute of technology and management center number one two five eight twisting and untwisting a spring is not a two way work mode so please clarify it sir your question was about the twisting and untwisting of a spiral spring twisting and untwisting of a spiral spring as is used in our classical wristwatches and clock mechanisms or even in old phonographs is a two way work mode because that spring can be charged or energized by doing work on it the spring discharges releases its energy and runs either the mechanism in our clockwork or the turntable in an old phonograph so a you know the winding and slow unwinding of a spring is a two way work mode of the tau d theta kind tau d theta is a detail of modeling but it is a two way work mode and if it is a axial spring it is stretching and unstretching is also a two way work mode and depending on the design of the spring you could have axial movements as well as twisting untwisting movements making it a two way work mode type of system over to you whether the two way work mode system affects the energy interaction on both the systems involved within the process or both the both the parts of the system see any interaction affects both the systems which are taking part in that interaction this will be clear when we study the first law an interaction does not go waste ever an interaction always leads to either a further interaction or the change in the state of a system a change in state of perhaps both systems which are involved so both systems the recipient system as well as the donor system do get affected so there is one more question as you are told that the pan as you are shown in your hand that you are called as rudimentary system but can you tell us that the ink is coming out of that pan when we are writing then still could we be able to call it rudimentary see the pen which I have in my hand is not a pen there is no ink in it at all it is a solid thing there may be some electronic inside it but there is only one stylus which has some sensitive issue with the laptop screen that is all I agree that if I have an ink pen then as I write it will not be I do not know whether I have to consider it is a rudimentary system or not but a plane is a fountain pen or even a ballpoint pen is a reasonably complicated system let us look at the refill as a system let us not complicate it now the refill turns out to be an open system because as the process of writing goes on some material flows out of it on to the paper there are no significant thermal interactions but if your hand is warm then it is possible that the ink in the refill will become thinner and may be more of it will flow out we can if you want we can analyze it in detail by considering the ink as a open thermodynamic system that is possible in fact you are giving me a good idea to set up a exercise in the next issue of things thank you for that and by the way I think I should take over here because it is tea time we will meet again after tea time.