 But before getting into property we need to first understand what is temperature and what is heat and how they are different from each other. Can you tell me what is temperature? What it is? Measure of hotness or coldness. What do you mean by that? Measure of hotness means what? To give it a specific value, heat is just an energy but when I say temperature, it specifies the value. No but heat can also be specified right, heat is energy so it can be written in the form of joules. It is relative temperature. Flow of energy. Heat is a flow of energy, heat flows or heat energy goes from one object to the other object. Just like water flows from one point to the other point. Why water flows from one point to other point? Because of the pressure difference right, because of the pressure difference water flows and because of the temperature difference heat flows. So you can drop parallel with let us say flutes and try to understand it. And if you go to the micro level as in what it is exactly. Suppose gas molecules are there in this room. I am talking about what is heat. What happens to the gas molecule if the temperature increases? If the temperature increases the kinetic energy of the gas molecule increases. So what happens is that when a gas molecule hit your hand, then it will transfer its little bit of kinetic energy into your hand and you will feel the heat. So if there is lot of transfer of energy from the gas molecule on your hands you will feel that more amount of heat is there. Now temperature is a measure of indirectly to the measure of how much is the kinetic energy in the gas. More is the kinetic energy in the gas, more it can transfer. So temperature in a way is a measure of kinetic energy of the gas molecules. Now what is pressure? Pressure also happens because of gas molecules. What is that? When the force gets accepted, when the gas molecule collide they will not only transfer the energy, they will transfer the momentum also. That transfer of momentum creates the force, rate of change of momentum creates the force and that creates the pressure and the transfer of energy creates the flow of heat. Now the problem is that we cannot go about looking at the gas molecule and then try to say that okay temperature is, I mean rather than temperature, you best way was that you just find all the kinetic energy of the molecules, all of it and then say that okay, just forget about temperature, this is the kinetic energy but it becomes a very tedious process and inaccurate process, fine. So there is this term called temperature which is defined, coin which tells you you know relatively what is the kinetic energy of the molecules, right. So this is what the temperature is and there has been, there had been lot of research as in past to just quantify what it is because during those days people were trying to quantify each and everything defined. Like for example length, length was quantified, okay this much is 1 meter, this much is probably 1 feet, like that they have quantified, okay and similarly they have quantified mass, time and everything they have quantified but when it comes to something like temperature, they were not able to quantify it very well, the reason is that when I say 0 length, what does it mean, 0 meters means nothing is there, less than 0 length is not possible, get it, similarly less than 0 kg is not possible, so there is absolute 0 there, you cannot go below that, right, similarly you need to define the temperature in such a way that 0 means 0, are you getting it but nobody was able to find during that time what is 0, what is absolute 0, fine, so that is the reason why they could not identify an absolute scale, so they took the help of the most common thing available to everyone which is what, water, okay and with respect to water they have come up with a scale to measure the temperature, is that thing understood, right, so but then when they have defined they will assume something to be 0, they will say okay fine where the water freezes at 0 degree Celsius but is less than 0 possible, yes it is possible, so that 0 is not absolute 0, are you getting it, that is just a relative 0, it is 0 with respect to the water at that level, getting it, so degree Celsius scale is defined right down, this temperature scale is defined, see what happens is when you know that absolute scale for example for length we know 0 is 0, so all you have to do is define what is 1 meter, then 2 meter is double of that, 3 meter is triple of that like that, it becomes straight forward thing but when it comes to relative scale you need to first assume what is 0 and then you have to assume what is 1, get it, so the assumption here is that write down the temperature of ice, temperature of ice is assumed to be 0 degree Celsius, just like you assume gravitation potential to be 0 at some horizontal level even though it is not getting it, like that we are assuming something to be 0 degree Celsius which is temperature of ice and then we are assuming temperature of boiling water to be 100 degree Celsius, all this is happening at 1 atmosphere fine and between ice and this boiling point entire scale is divided into 100 equal parts, so it is a linear scale fine, equally spaced intervals from 0 to 100, any doubts? Now if I have defined a scale like this, can you get an idea of what is 150, if you just move one step whatever you have defined you know exactly what is 1, so after 100 you just add 50 of 1s, look at 150, so that is how Celsius scale is defined and then during the same time in England there was this Fahrenheit scale that was developed, degree Fahrenheit I call it Fahrenheit scale, here they have taken some other basis, they have not taken water, I am not actually very sure what basis they have taken but you know they have taken the basis in such a way that the temperature of melting ice, this is melting ice, because ice can be at less than 0 degree Celsius also but melting ice has to be at 0, so temperature of melting ice is 32 degree Fahrenheit, temperature of boiling water, any idea what it is? Remember no, 212 degree Fahrenheit, 8 out of 10 now both are linear scale, as in between 32 and 212, what is the difference? 212 and 32, 180 right, so 180 equal divisions are there, here 100 equal divisions are there, okay, now if I tell you a temperature in degree Celsius, can you convert into Fahrenheit or vice versa, can you do that? Try to see whether you can derive it, assume that temperature in degree Celsius scale is C, what is F? Anyone how will you go about it? Any idea? Tell me this, if I plot this graph, suppose on y axis you have Fahrenheit and on x axis you have degree Celsius, okay, will it be a straight line, it has to be, both are linear scales, okay, both are linear scale, so there will be a straight line equation between F and C, okay, can you find out the line, can you plot a line, do that, you have two points, right, yeah yeah, that is the way you have to do it, since it is a straight line, the slope should be same, try to do this, what is the first point, when degree Celsius is 0, Fahrenheit is 32, so this is the first point, 0, 32, okay, second point, 212, 100, 200, so this point is 100, 200, fine, you have this as your straight line, fine, now since it is a straight line, the slope should be constant, right, so you just assume a random point somewhere here, whose coordinates are C, F, okay, take these two points initially and then these two points, equate the slope from two points, this is A, B and C, so if you take A and C, the slope should be 212 minus 32 divided by 100 minus 0, this should be equal to F minus 32 divided by C minus 0, any doubt, so we have F minus 32 divided by 118 is equal to C by numerator, okay, C is equal to 5 by 9 F minus 32, right, so this is how you can derive a relation between degree Celsius and Fahrenheit, any doubts, so if I give you a temperature in degree Celsius, you should be able to convert in degree Celsius, now people did stop there, they were always trying to find out is there any absolute scale, okay, so in that particular quest, there was one experiment that was conducted with gases, okay, so let's see what is that experiment, this is the temperature and this one is the pressure, okay, now they have taken a gas at a particular pressure in temperature, so this is the initial pressure in temperature, okay, now they are cooling it down, what will happen to its temperature, down, pressure, down, okay, so next point is observed, okay, then again they are cooling it down, another point is observed like that, when they are connecting it, they are getting like this, a straight line and after this point, the gas liquefies, it changes the state, fine, but if you extend the line, you know that pressure cannot be less than zero, okay, so when pressure is zero, they are trying to find what could have been the temperature, so this gas liquefied because it's a real gas, it's not an ideal gas, had it been the ideal gas, it will not liquefy and it will just come and hit here and this temperature is minus of 273.15 degrees Celsius, okay, now this is for only one gas, let's say gas one, similarly they have taken another gas, all right, now when they have taken another gas, they are also they are trying to liquefy it, as in sorry, they are trying to decrease the temperature, which temperature decreased and then to their surprise, it came and meet the same point, when pressure is zero, the temperature of both the gases appear to be at same point, okay and no matter which gas you take, approximately all the gases, they are converging at the same temperature, as if there is something unique for that temperature, something unique about this temperature, okay, what is that unique thing? Temperature cannot go below this because negative pressure is not defined, it's not possible, fine, so if temperature cannot go below this, what does it mean? This is the absolute temperature, this is the absolute zero, okay, so there comes a new scale, which is an absolute scale, so since it is an absolute scale, we don't call it degree, degree is a relative term, like degree Celsius, degree Fahrenheit, it's a relative comparison with some substance, Kelvin is not a comparison, it's an absolute scale, okay, so a Kelvin scale is defined right down, Kelvin temperature scale is defined in such a way that zero Kelvin is actually minus of 273.15 degrees Celsius, sorry, yeah, zero Kelvin is this and change in temperature in Kelvin, Delta as in one Kelvin is change of one Kelvin is equal to change of one degree Celsius, getting it, so Delta T in Kelvin is equal to Delta T in degree Celsius, they are equal, fine, so you will see lot of formulas in this chapter where Delta T will come, okay, so you don't need to convert degree Celsius into Kelvin or Kelvin to degree Celsius because both Delta T's are same, okay, but if suppose there is a formula in which only T is coming, like sigma E 80 raised to power 4, there is no Delta T, there you need to convert temperature into Kelvin, which is an absolute scale, okay, this happens to be the SI unit also for temperature Kelvin, okay, any doubts? No, now can you tell me if the temperature is C degree Celsius or let's say X degree Celsius, what is its temperature in Kelvin, how much that should be? X plus 273.15, okay, so this much temperature in degree Celsius is equivalent to X plus 273.15, okay, and just so that our calculation is simpler, we many times ignore 0.15, we just say 273, we just add 273 degree Celsius to arrive at the SI unit of the temperature, which is Kelvin, fine, any doubt with introduction to temperature, right, so this is how we measure the temperature, once you know how you can measure the temperature, now you can talk about the thermal properties one by one, okay, now like what we have discussed, you know you can devise your own way of measuring temperature, you can have a gas trapped inside a container and you can define in such a way temperature that pressure when the temperature, pressure when ice is melting and that temperature, you know that pressure corresponds to some temperature and that boiling water, what is the pressure inside that container correspond to some other temperature, you can develop and scale yourself, okay, so don't feel, I mean afraid of solving such questions because if you go and solve some high level questions, they will twist the question in such a way that you know you get, I mean many times you assume that, okay, fine, this is Kelvin scale, this is degree temperature scale, this is Fahrenheit scale and that's it, but what happens is that a good question will test you on the basics, whether you have understood how the scales are defined, they will give a scenario in such a way that they will say, okay, assume there is a new temperature scale that has developed, so this is our condition, so you need to be very good at analyzing the situation, but then that will come through problem practice, we will do it a little bit later.