 Hello and welcome to lecture 5 of the lecture series on introduction to aerospace propulsion. In the previous lecture, I think we had covered up some of the very basic aspects and terminology associated with thermodynamics. We shall continue our journey further into understanding thermodynamic principles and basics of thermodynamics in this lecture and in future lectures. Now in this lecture what we are going to cover are basically to look at some of the other aspects of fundamental thermodynamics. We shall start our lecture with understanding what are known as quasi static processes and subsequently we shall understand the concept of energy and different forms of energy. We shall understand a very important term associated with energy known as internal energy and what is meant by internal energy and what constitutes internal energy. We shall then look at total energy and how do you calculate total energy for a particular system. We shall then understand concept what is known as enthalpy. Enthalpy happens to be a combination property that is it is a sum total of two or more different types of properties. Towards the end of this lecture, we shall understand what is known as the zeroth law of thermodynamics, what is the significance of zeroth law of thermodynamics and what is the outcome of the zeroth law of thermodynamics. As we shall see later on the outcome of zeroth law of thermodynamics is temperature. So, temperature measurement has the basis in zeroth law of thermodynamics. So, let us first look at what are quasi static processes. Now, when we understand or analyze a particular system thermodynamically, we would like the process to proceed in a manner such that the system remains infinitesimally close to an equilibrium state at all times. So, such processes are known as quasi static processes or sometimes also referred to as quasi equilibrium processes. And so this process in this case proceeds slow enough to allow the system to adjust itself internally, so that properties in one part of the system do not change any faster than those in other parts. That is during a quasi static processor, we would have change in properties which are same throughout the system. That is you do not have a different set of properties in one part of the system and another set of properties in another part of the system. So, quasi static process proceeds at an infinitesimally slow pace, so that properties in one part of the system is the same as that of the other part of the system. Now, let us look at what are quasi static processes in terms of examples. Now, the first example which is shown here is that of a piston cylinder assembly. The first picture that you see here shows a slow compression process. A slow compression process is one wherein the piston moves at an infinitesimally slow rate, so that the system remains in equilibrium at all times. So, this qualifies to be known as a quasi static or a quasi equilibrium process. Now, the second process on the other hand shows that the piston has moved a much greater distance as in the previous case. So, this really does not qualify to be a quasi static process because the system has proceeded in a very fast rate and therefore, properties in one part of the system will not be the same as the other parts of the system. So, it is not really a quasi equilibrium or a quasi static process. Now, let us look at another example again of a piston and cylinder assembly. In the first example that we shall see here, we would like to compress the gas which is held within this piston and cylinder assembly. So, what we do is that initially let us say the piston was at this location and then as you drop a weight on the piston, the piston and cylinder assembly moves down. So, the piston and cylinder assembly moves down causing the gas to be compressed or if we look at it the other way around, if the system was in its initial state at P 1 V 1 and T 1 which is at a higher pressure and lower volume and a different temperature. This was because there was a weight placed over the piston. Now, as you remove the weight from the piston, the piston moves up because the gas which was compressed will now get expanded. And therefore, the system will finally reach a state which is referred to as state 2, the final state which has a lower pressure and a higher specific volume. Now, if we were to analyze the system thermodynamically, what we would like to do is the following. So, instead of one single weight, we would like to have infinitely small weights which are placed one over the other. The total weight of these individual weights will be equal to the larger weight which we had seen in the previous slide. So, if you remove each of these weights one by one and because these are infinitely small weights, the system proceeds from one state to another state through a series of equilibrium states. So, these equilibrium states have been denoted here by these cross symbols. These each of them are referring to states which are in equilibrium. The reason is that we have an infinitesimally small weight which is removed from the piston causing the piston to move by an infinitesimally small amount. So, if your system were to proceed in this manner, this is referred to as a quasi-static process. And so in thermodynamics, we shall be analyzing different processes which are assumed to have taken place quasi-statically or in quasi-equilibrium. So, this is important because we shall see little later that quasi-static processes can be also classified as reversible processes and so on. And therefore, it is important for us to ensure that the process proceeds in a quasi-static manner. So, the most of the processes that we shall be analyzing in thermodynamics are assumed to be taking place quasi-statically. And so, why are we interested in quasi-static processes? As engineers, we are all interested in quasi-static processes. Firstly, because they are easy to analyze and secondly, if you are looking at work producing devices or power generating devices, these devices generate maximum work when they are operating on quasi-static processes. And therefore, as engineers, we will be able to define a process which has taken place quasi-statically and this particular process will serve as a standard when you would like to compare actual process with reference to a quasi-static process. And this is the reason why we are all interested in quasi-static processes because that helps us firstly in easier analysis of a particular process as well as such processes will serve as a standard for comparing other processes or actual processes with it. And what we shall see now is, what are the different forms of energy and what are the implications of these different forms of energy? Now, as we are all probably aware, energy can exist in different forms. You could have energy in the form of thermal energy, mechanical energy, kinetic energy, potential energy and so on. So, you would have already come across many such forms of energy or terms associated with energy. Now, some total of all these forms of energy like mechanical, kinetic energy, potential energy, electric, magnetic, chemical, nuclear and so on, all of them put together is referred to as the total energy of a system which is usually denoted by symbol e. So, specific energy which is symbol small e is equal to total energy per unit mass. Now, in thermodynamics, we usually do not provide any information about the absolute value of total energy. Thermodynamics, we only deal with the change of total energy and it does not really matter what is the absolute value of energy. We are interested in change of energy from one value to another. So, we would always be dealing with changes in energy rather than the absolute value of energy. Now, what are the different forms of energy? So, energy can exist in different forms. These can be classified broadly as macroscopic energy and microscopic energy. Macroscopic energy refers to the energy that a system would possess as a whole with reference to some outside frame of reference. Examples of macroscopic energy are kinetic energy and potential energy. These are the energies that a system can possess with reference to some frame of reference. Microscopic energy on the other hand are those related to the molecular structure of a system and the degree of molecular activity and this is basically independent of the outside frame of reference. So, microscopic energy is the energy content at the molecular level and they do not really depend upon what with what frame of reference you are looking at. And the sum total of all the microscopic forms of energy of a system is referred to as the internal energy of a system. And internal energy is usually denoted by symbol u which is for internal energy and small u which is for specific internal energy that is internal energy per unit mass. Now, to illustrate this example to this point further, as I was mentioning macroscopic energy could refers to the energy that a system contains as a whole and it is with reference to some frame of reference. Examples of macroscopic energy are kinetic energy and potential energy and the example that I am going to show is about one simple example of the macroscopic energy associated with let us say a car which is climbing up a hill. Now, as we know that this particular car that is shown here by this cartoon has some amount of potential energy and kinetic energy. And these energy will change as the car moves up the slope because its potential energy changes and if its speed also changes then that changes the kinetic energy of that this particular system. So, the system we are considering here consists of the car and what is around it is the surroundings. So, the energy the macroscopic energy of this system which is in terms of its kinetic and potential energy will keep changing as the car moves. Now, as I mentioned internal energy on the other hand is the sum total of all the microscopic forms of energy and what are the different microscopic forms of energy these are referred to as the sensible energy, latent energy, chemical energy and nuclear energy. Sensible energy refers to that part of the internal energy which is associated with kinetic energy of the molecules. As we have seen earlier macroscopic energy looks at kinetic energy of the system as a whole and not at the molecular level. Sensible energy is the energy which is associated with kinetic energy of individual molecules and these are again of different types you could have rotational kinetic energy, translational kinetic energy, vibrational kinetic energy and so on. These are again associated with the molecular level of the system. Latent energy on the other hand is the internal energy which is associated with phase change of a system that is if the system changes from solid to liquid or solid to gas or vice versa. The energy that is associated with this particular phase change is referred to as the latent energy. Now, chemical energy refers to the internal energy which keeps the molecules bonded together to it to themselves. So, internal energy associated with the atomic bonds of a molecule is referred to as the chemical energy and this is energy that is released or absorbed when either bonds are broken in a chemical reaction or new bonds are formed during a chemical reaction process. Nuclear energy refers to the amount of energy that is associated with the strong bonds within the nucleus of an atom and this energy is tremendous. As you perhaps aware that nuclear fission or fusion reaction produces tremendous amounts of energy that is because the energy associated with the bonds within the nucleus of an atom is very tremendous and therefore, the nuclear energy refers to that particular energy which is associated with the bonds within the nucleus of an atom. Now, we will explain this by examples which are shown here showing the different forms of microscopic energy which form the sensible energy. I mentioned that sensible energy refers to the energy which is associated with kinetic energy of individual molecules. So, examples of such kinetic energy associated molecules are the molecular translation that is motion of the molecules, molecular rotation or the electron spin or the molecular spin or it could be electron translation or that is movement of the electrons across around the nucleus and you could also have new molecular vibration. So, all these individual forms of microscopic energy form what is known as the sensible energy. So, this sensible energy refers to or gives us an idea about the amount of energy which is associated with kinetic energy of individual molecules. Now, the other forms of energy are the latent energy, chemical and nuclear energy and so, internal energy is the sum total of all these forms of microscopic energy. So, we will keep referring to internal energy because that places very significant role in thermodynamic analysis of systems and this internal energy basically refers to the sum total of the sensible energy, the latent energy, chemical energy and the nuclear energy. So, every system has certain amount of internal energy associated with it and that is basically comprising of these individual energies which are at the molecular level or sum total of all these microscopic forms of energy constitute the internal energy of a system. Now, if you were to look at the advantages of microscopic forms of energy or macroscopic forms of energy, macroscopic kinetic energy it basically refers to an organized form of energy. So, microscopic kinetic energy is the organized form of energy and it is more useful than the disorganized forms of kinetic energy of molecules. Now, there is an example which is shown here. We can see what is shown here is that of a dam wherein water is discharged into a turbine which generates a power output. So, what is shown here are a turbine wheel which is placed here at the exit of the dam or if the turbine wheel is placed inside the reservoir. Now, this reservoir of the dam contains a lot of energy associated with it basically because of the molecular motion or kinetic energy of these molecules. So, there is a lot of energy associated with it, but it is disorganized form of energy as you can see all the molecules are oriented randomly and therefore, this disorganized form of energy would not help us in any way because it does not generate any work output. So, if the turbine wheel was to be placed inside the reservoir even though the reservoir has lot of microscopic kinetic energy of the molecule, it does not generate any work output because it is disorganized form of energy. On the other hand, if you were to place the turbine wheel at the exit of the dam, this disorganized form of energy gets converted to organized form of energy. That is you get the macroscopic kinetic energy of this water which is coming out of the dam which is basically conversion of the microscopic form into macroscopic form and this macroscopic kinetic energy is what you can convert to useful work output. So, you get useful work output from this system because of the macroscopic kinetic energy. The macroscopic kinetic energy is the organized form of energy as we had seen in this example and therefore, it produces useful work output as compared to the disorganized microscopic kinetic energy of the molecules. So, we are all engineers as engineers we are always interested in the macroscopic form of energy because that is what gives us the useful work output. Now, let us look at how we can calculate kinetic energy is potential energy and the total energy associated with the system. You probably have already understood some of the aspects of kinetic energy earlier on. So, kinetic energy is basically the product of mass it is square of the velocity divided by 2. So, m v square by 2 is kinetic energy of a system and kinetic energy per unit mass is v square by 2 that is in either kilo joules per kilogram usually referred to in kilo joules per kilogram on a unit mass basis. Potential energy on the other hand is the product of mass acceleration due to gravity and the elevation from the reference. So, here z refers to the elevation of the system from the reference. So, product of mass acceleration due to gravity and z gives you the potential energy of a system in kilo joules or on a unit mass basis potential energy is equal to g times z which is product of acceleration due to gravity and elevation from the reference in kilo joules per kilogram. Now, if you were to calculate the total energy associated with the system now if you assume the absence of any magnetic electric and surface tension effects then the total energy of a system consists of kinetic energy potential energy and the internal energies. So, total energy E is equal to u plus k e plus p e which is equal to u plus m v square by 2 that is the kinetic energy plus m g z which is the potential energy. Now, the same equation on a unit mass basis that is energy per unit mass is equal to internal energy per unit mass that is u small u plus kinetic energy and potential energy both on unit mass basis. So, that is equal to u plus v square by 2 plus g z in kilo joules per kilogram. So, this is equal to the total energy of a system. Now, usually we come across systems closed systems whose velocity and elevation of its center of gravity would be remaining constant during a process and such systems are referred to as stationary systems. So, if a system is stationary then there is no change in kinetic energy of a system there is also no change in potential energy of a system. So, the change in total energy of such a system would be equal to the change in internal energy of a system because there is no change in its kinetic energy as it is stationary and there is no change in its potential energy because its elevation from the reference does not change. So, the total energy of a stationary system is equal to the rather the change in total energy of stationary system will be equal to the change in its internal energy. Now, what we shall see next is a property which is known as a combination property that is a property which is a combination of two or more different properties is referred to as a combination property. Now, one such property which is very important in thermodynamic analysis and which we shall refer to several times during this course is known as enthalpy. Now, enthalpy is a combination property which is a combination of internal energy u and product of pressure and specific volume p v. So, the combination of u and p v is very often encountered in the analysis of open systems or in control volumes. So, enthalpy is usually denoted by the symbol h and h is equal to u plus p v on a small letter scale it is per unit mass that is small h is equal to small u plus p v in kilo joules per kilogram and if it is not per unit mass then total enthalpy is denoted by capital H which is equal to capital U plus p v in kilo joules. Now, enthalpy is also often referred to as heat content and in many textbooks you would see that enthalpy is often referred to as heat content of a particular system. And a process during which the enthalpy remains a constant is known as an isenthalpic process. If you recall during the previous lecture I had mentioned towards the end of the lecture that if during a process enthalpy remains constant it is known as isenthalpic process at that point we were not sure what enthalpy means. So, enthalpy as I have just mentioned is a combination property it is the sum of the internal energy and the product p v. So, enthalpy forms a very important role in analysis of systems of engineering importance like for example, if you are analyzing an aircraft engine which we shall do little later enthalpy will form a very important aspect or very important property in thermodynamic analysis of such engineering systems. Now, let me give an example here which explains the importance of enthalpy. Now, we are looking at a control volume here which is denoted by these dotted lines and because there is mass influx into the system and a flex from the system this is an open system and it is also referred to as a control volume. So, the flow enters the control volume with a certain internal energy u 1 and certain pressure p 1 and v 1 and it leaves the system with an internal energy u 2 pressure p 2 and v 2. So, enthalpy of the system changes from h 1 at inlet which is equal to u 1 plus p 1 to h 2 at inlet at the outlet wherein h 2 is equal to u 2 plus p 2 v 2. So, there is a change in enthalpy of a system of this particular system and analysis of this thermodynamic analysis of this system will involve understanding change in enthalpy of this particular system. And so, we shall be using this concept of enthalpy and during several process analysis which we shall be doing during this course. Now, what we shall look at next is a very important law very fundamental law of thermodynamics which is the zeroth law of thermodynamics. Now, zeroth law of thermodynamics states that if two bodies are in thermal equilibrium with a third body they are also in thermal equilibrium with each other. So, this is zeroth law of thermodynamics and it serves as a basis for validity of temperature measurement which we shall see little later. Now, replacing the third body with the thermometer the zeroth law we can restate as two bodies are in thermal equilibrium if they have the same temperature reading even if they are not in contact that means that if there are two bodies both of them show the same temperature reading even if they are not in contact they are said to be in thermal equilibrium because each of them is individually in thermal equilibrium with the thermometer. And therefore, thermometer helps us in measuring temperature because it establishes a thermal equilibrium with by within itself as well as with the system. So, zeroth law of thermodynamics forms the basis for temperature measurement. Now, let me explain this point further. Now, what is shown here are three different bodies denoted by A, B and C. Let us say the temperature of these bodies are T A for body A, T B for body B and T C for the body C. Now, let us say body A and C are in thermal equilibrium that means T A is equal to T C and body B and C and thermal equilibrium that is T B is equal to T C. So, as per the zeroth law or as a consequence of the zeroth law because A is in equilibrium with C and B is in equilibrium with C as per zeroth law A should be also in equilibrium thermal equilibrium with body B that means T A should be equal to T B. So, zeroth law helps us in explaining thermal equilibrium between these three bodies. Now, there is a reason why this particular law is referred to as the zeroth law. Well, this zeroth law was proposed only in 1930s. In fact, in 1931 whereas, the first and second law of thermodynamics were defined much earlier in the late 1800s. Now, this is called a zeroth law because it is a much more fundamental law than the first and second laws of thermodynamics and it should have actually preceded the first and second laws of thermodynamics. But since we already had a first law and a second law of thermodynamics, this particular law was renamed as the zeroth law of thermodynamics and all the temperature scales that we see in existence today they are based on certain reproducible states like it could be the freezing point which is the also referred to as the ice point or it could be the boiling point of water which is the steam point. On the Celsius scale which I am sure you are all familiar with, we usually measure temperature in Celsius even though that is not the SI unit of temperature. On the Celsius scale, ice and steam points were assigned 0 degrees and 100 degree Celsius respectively. So, these were the temperatures which were defined when Celsius scale was formed long ago. Now, in thermodynamics we would like to have a temperature scale which is not dependent on properties of a particular substance. For example, in the Celsius scale it depends upon properties of water which was ice point and steam point. Whereas, in thermodynamics we would not like to have such a scenario we would not want to have a temperature scale which depends upon the property of a particular substance which requires us to maintain a certain pressure and other properties of that particular system. So, we would like to have a scale which is independent of properties of one particular substance. So, the thermodynamic scale, temperature scale is also referred to as the Kelvin scale. We shall understand Kelvin scale in detail little later as well. Now, the lowest temperature on the Kelvin scale is 0 Kelvin and as we shall see little later in third law of thermodynamics which states that there are temperatures below 0 Kelvin is not possible or not feasible. So, thermodynamic temperature scale is the scale which is independent of any particular property of a substance and one such property we shall see little later. Now, there is a temperature scale which is quite identical to the Kelvin scale. This is known as the ideal gas temperature scale. Now, this ideal gas temperature scale involves a constant volume thermometer which is filled with gas which could be either hydrogen or helium and at low pressures the temperature of it is based on the principle that at low pressures temperature of a gas is proportional to its pressure at constant volume. So, this forms the basis of measurement of temperature using the constant volume temperature that is basically that at low pressures temperature of a gas will be proportional to its pressure if the volume is held constant. Now, in an ideal gas temperature scale what we see is that the temperature of a gas in a fixed volume varies linearly with pressure and that occurs at low pressures. So, the relationship between the temperature and pressure can be expressed as temperature T is equal to A plus B times P where A and B are constants and these constants are determined experimentally for a particular gas thermometer. So, if you were to find these values A and B you can estimate temperature of a system if you know the pressure at a particular instant. So, constant volume gas scale involves measurement of pressure of a particular gas and then inferring temperature from this measured pressure. So, it is necessary for us to find the values of these constants A and B which are determined experimentally and once you know A and B and also the pressure you can infer temperature from these parameters A plus B P. Now, if you were to determine these values A and B the constants A and B then you need to measure the pressure of the gas at two reproducible points. For example, ice point and the steam point which I had explained earlier are two reproducible states of water that is ice as we know now has a temperature of 0 degree Celsius and steam point is basically 100 degree Celsius for water. And so, if you were to measure pressure of a gas in a two different reproducible states and then assign suitable values of temperatures to these two values it is possible for us to find these constants A and B. And so, if you have just two measurements then you are able to find these constants A and B. So, once A and B are known the unknown parameter now is the temperature. So, temperature will now be equal to A plus B times P and since A and B are known and the pressure is known for this particular instant you can find temperature from the simple equation A is equal to T is equal to A plus B P. So, this is the basic principle behind the ideal gas temperature scale which is a constant volume thermometer. Now, if we were to assign those two values that I had measured mentioned earlier equal to 0 degree Celsius and 100 degree Celsius then the temperature scale that we get is identical to the Celsius scale. In Celsius scale the two reference points are the ice point and the steam points. Now, if this were to be the case that is if you were to have 0 and 100 as the fixed points then it is possible that for A which is equal to 0 if you were to assume that the absolute pressure is equal to 0 then the temperature that we shall get from the scale will be equal to minus 273.15 degree Celsius and this temperature is regardless of the type and the amount of gas in the vehicle in the vessel of the gas thermometer. So, this means that it is possible for us to find a point where in A is equal to 0 you can assume A equal to 0 which means that the pressure is 0 and the corresponding temperature there would be minus 273.15 degree Celsius and as we shall see in the example later on this is independent of the nature of the gas in the vessel and the quantity of the gas in the thermometer. So, let me explain this point little further using this example. Now, what is plotted here are pressure on the y axis and temperature in degree Celsius on the x axis. Let us say the gas thermometer has different gases gas A, B, C and D and we have fixed two or more measurement data points but we just need two points one is the 0 degree Celsius that is the ice point and one is maybe 100 degree Celsius that is the steam point. Now, for this particular gas we have maintained ice point at 0 and then we find the corresponding pressure and then we also maintain the same thermometer at the steam point and find the corresponding pressure at that particular temperature and if we were to join these two points and extrapolate this line the point where it meets on the x axis is minus 273.15. If you repeat this experiment for several other gases what we shall see is that all these gases will finally merge towards this particular point minus 273.15. So, this means that this particular temperature scale is independent of the type of gas that is used and that is a big advantage for us because thermodynamic temperature scale you would not want a scale which is dependent on properties of a particular substance. So, there is a scale which is independent of the type of gas which is used in this particular scale. So, this ideal gas temperature scale is one which is independent of the type of gas that is used. So, the example here shows four different gases and you measure temperature well you measure pressure for two different temperatures which can be fixed or which can be reproduced. Now, if you were to use 0 degree Celsius and 100 degree Celsius as the reference points then this scale is very similar to that of a Celsius scale and. So, what we do in this ideal gas temperature scale is to measure pressure at two temperatures which can be reproduced at 0 degrees and let us say the steam 0.100 degrees and once you get two measurement data points you can join those two lines and extrapolate it and what we shall see is that extrapolation of these lines will take us to one particular temperature which is minus 273.15. So, minus 273 the significance of this temperature that we get of minus 273.15 is that this is the lowest temperature that you can get using a gas thermometer and. So, this we can call as an absolute gas temperature scale because we have assigned a value of 0 if you assign a value of 0 to the constant A then the temperature that you get will be minus 273.15 degree Celsius. So, this is very convenient for us because in this case we need to only specify the temperature at one point to define an absolute gas temperature scale and. So, temperature will now be equal to B times P. So, if you just know B and you measure pressure you can actually calculate temperature from there. Now, the standard point fixed point for temperature scale has been agreed upon as the triple point of water. Triple point of water is the temperature at which all the three phases of water that is solid liquid and gas phase coexist. That is you would have a system which has ice water as well as water vapor all the three of them coexist in equilibrium and this occurs at a temperature of 0.01 degree Celsius or 273.16 Kelvin. Now, if you recall I had mentioned earlier that the Kelvin scale begins at a temperature of 0 Kelvin. So, this is the lowest temperature that is possible to be achieved and as we shall see later on that at 0 Kelvin temperature which is also known as the absolute 0 the entropy of a system becomes 0 and what is entropy will be clear in later lectures and. So, from the entropy principle also we shall be able to derive the Kelvin scale which we shall do later on. So, 0 Kelvin is equal to minus 273.15 degree Celsius. So, 0 Kelvin on the Kelvin scale is corresponding to minus 273.15 degree Celsius on the Celsius scale. Now, the absolute gas temperature scale is not a thermodynamic temperature scale because there are a couple of problems associated with this particular temperature scale. One of the problems occurs at very low temperatures. So, as you reduce the temperatures to very low values there could be condensation of the gas occurring within the constant volume gas thermometer and condensation can change the pressure and other aspects within the constant volume gas thermometer. So, this is one of the problems that at low temperatures at very low temperatures there could be condensation of the gas and the other problem occurs at very high temperatures. What happens at very high temperatures is that the gas could dissociate and probably ionize both of which can change the properties of the gas. So, dissociation and ionization can occur at high temperatures. So, this absolute gas temperature scale or thermometer is valid only for a certain range of pressures and temperatures. So, at low pressures there is problem of condensation at high pressures it may cause ionization and dissociation of the gas within the thermometer. So, it is important to understand that the absolute gas temperature scale is not really a thermodynamic temperature scale because a thermodynamic temperature scale requires us to start at 0 Kelvin's as you know that 0 Kelvin is the lowest temperature that is possible. But if you were to use the absolute gas temperature scale at such low temperatures it will definitely cause condensation of the gas within the volume. And as we had seen in the example earlier that we were extrapolating the graph to achieve this minus 273.15 degree Celsius. But if you were to actually do that for the gas for any particular gas real gas then you would notice that that particular gas is condensing. And condensation of the gas is definitely not good because that can change the properties within the control volume and therefore the equation T is equal to a plus b p will no longer be valid. But absolute gas temperature scale definitely has its significance in the range of temperatures in which it can be safely used. So, there is a certain range of temperature in which you can use the absolute gas temperature scale and in that range the absolute gas temperature scale works well and without any problem. Whereas, for defining an absolute gas temperature scale it requires certain modifications and change of definitions which we shall see when we understand the concept of entropy and third law of thermodynamics and the consequent development of the Kelvin scale during that. So, let me recap what we had looked at in this lecture. What we had understood were certain basic parameters associated with thermodynamics, the quasi-static processes we started our lecture with understanding water quasi-static processes. Quasi-static processes are those wherein the process is assumed to occur or progress at infinitesimally slow rates so that the system is in equilibrium with itself at each instant of time. After quasi-static processes we looked at the concept of energy and the different forms of energy. We understood that energy can exist in microscopic form or in macroscopic form and useful work output from a system can be obtained if the form of energy is in macroscopic in nature. Microscopic energy on the other hand does not really give us any useful work output because it is a disorganized form of energy. Now, the sum total of all the microscopic forms of energy is known as the internal energy and total energy of a system constitutes the sum total of the internal energy plus the kinetic energy and the potential energy of a system. Subsequently, we looked at the concept of enthalpy. Enthalpy is a combination property which is the sum of internal energy u plus p v and this plays a very significant role in thermodynamic analysis of several engineering systems especially those dealing with open systems or control volumes and many of the engineering systems are indeed control volume systems and therefore, enthalpy plays a significant role in its analysis. Towards the end of the lecture, we looked at the zeroth law of thermodynamics and as an outcome of zeroth law of thermodynamics how we can develop a temperature scale, how you can define temperature of a system if you understand the concept of zeroth law of thermodynamics, how you can define an absolute gas temperature scale and approximate the absolute gas temperature scale within ideal gas temperature scale. We also understood that there are certain limitations with an ideal gas temperature scale especially at low temperatures very low temperatures and at very high temperatures but in spite of these limits, we can still use an ideal gas temperature scale for a certain range of temperatures. So, this is what we had looked at in this lecture and in next lecture that is lecture 6, what we shall look at are specific heat, what we shall define specific heats of a system and we will see that there are two types of specific heats specific heat at constant pressure and specific heat at constant volume. We shall understand what is been by heat transfer and what are the different types of heat transfer and we shall also look at work and the thermodynamic definition or thermodynamic meaning of work and what are the different types of work that are possible. What we shall see in the next lecture is that heat and work are two different forms of energy interaction that a system can have with its surroundings. So, we shall look at these aspects in the next lecture that would be lecture 6.