 Do not, you know, find excuse to miss the classes just because some school test is happening because ultimately, you know, nobody is going to ask you about your school marks, unless you're trying to plan to, if you're planning to go abroad, that is a different thing. But otherwise, you know, it is sad to say that nobody is going to check how much you got in school and then give you admissions in colleges. And I have never seen someone who does well in exam like Jay and Neat doesn't do well in boards. I have never seen it. It will never happen. So if you focus on comedy exams, everything else will be taken care of. But if you focus on school exams, comedy exams will never get taken care of. Okay. Anyways, so I guess those who are missing the class, they are not hearing what I'm saying. The message is for them Tuesday, Tuesday, Akash Tuesday. All right, guys, I hope all of you have seen your seniors results. Have you all seen it? Those results were out of only 50 kids we had 50 kids who clear class 12, Alan, Fidji and Akash. They have lakhs and lakhs of kids all over India who pass out every year. So of course, absolute results if you see they will have better ranks. But if you see number of students what we have out of that what we have got, so your seniors have done really well. Okay. With school and all, they have, you know, the best part is what they have done it with the school. Their results was better than Alan integrated results. Okay. So it can be done with these schools. Do not keep that in your mind that those who have done integrated course. They have more chance and all that. That's all nonsense. It doesn't matter. If you keep the quality and focus on the proper way of preparing, you are going to do better. Okay. Anyways, disturbance in the voice. Someone is saying there's disturbance in the voice. Is my voice clear? Now it is fine. Some buzzing sound. Okay. Okay. Okay. Now it is fine. All right. See what happens is when you, when you keep many electronics equipments together, they start resonating so the disturbance comes. Anyways. All right, guys, so let us start this topic. Long pending topic, which I guess was done in, is it done thermal physics. Thermodynamics I think is done in HSR and in Viper also they have started. Am I correct? Not in Viper. Okay. Okay. All right. Anyways, so the best part about thermal physics is what? What is the best part? It's easy. Okay. After learning mechanics, once you see thermodynamics, thermo properties of matter, you will embrace it from both hands. You'll be like, Oh, here is my thing, which I can rely on. Okay. In fact, when I was a student, and I had to prepare for the comedy exams. Then, you know, I have spent a lot of time preparing for the, for mechanics and other mechanical mechanics related topics. But I have scored the most in topics like thermal physics, electrostatics, magnetism. Okay. These sections of physics, they will save you in the tough times. Okay. So this is a very, very important section and a very good section for you to score. All right. It's a very scoring section and out. And then it has a good weightage. It has three chapters. It has thermal properties of matter. It has thermodynamics and kinetic theory. So good amount of weightage is there. So I would say that take this section very, very seriously because in the exam, you will see everything new. You might be feeling that you are very comfortable with mechanics, most of you, and you think that mechanics can be done easily. But the moment I give you a some weird kind of scenario, you know, it can be the, the, the newness of the problem will be lot more in mechanics than in thermodynamics. In thermodynamics, thermal properties, every question which will be there in comedy exam, it'll feel as if you're seen before. So you'll be at ease with. Okay. So do not, do not take this lightly. But then having said all that, you may find mechanics more interesting than thermodynamics because mechanics theories very little. Okay. Okay. Mechanics is like solving a puzzle. Right. Thermodynamics is not exactly like that. There is good amount of theory also and numericals as well. So you need to understand the story behind it. Not I mean, you have to understand it like a story, not like a, you know, bulleted points as such getting it. All right. So let me summarize to you this section. This section name is by the way, thermal physics. Thermal physics is a part of physics that deals with heat, temperature, and its phenomenon. Okay. Very simple. We have never considered anything happening because of temperature till now. We have never considered heat energy. We always considered potential energy and kinetic energy because they were mechanical energies. Now is the time to get into the aspects of heat. Okay. So thermal physics divided into three parts. The first part of the thermal physics deals with thermal properties of matter. Write down thermal properties of matter and probably the biggest chapter of all three. Okay. So here, thermal properties of matter, our focus is to understand how the how the materials are different from each other with respect to their thermal properties. The first chapter, you remember mechanical properties of solids, wherein we have discussed the properties which are mechanical in nature. Now we are going to discuss properties which are thermal in nature. Okay. And we are going to deal with the macro factors over here. What does it mean? I do not care what is happening at the molecular level. When I'm giving heat, I'm not going to check what happens to the molecule starts vibrating more all that I'm not going to check. I'm going to only check that what happens to the material as a whole. Okay. What will happen at the boundaries of the material through the boundaries heat will come from the surrounding or heat will go to the surrounding through the boundaries work is done on the system or by the system all that you know basically I want to highlight here that we are not getting into the molecular level only the macro factors fine. Then the next chapter is thermodynamics. What do you think this is about anybody who has done this chapter in school? What do you think thermodynamics is about anyone from NPSHSR? I think NPSSR has made name for himself couple of for couple of years now NPSR has been generating great results. So good guys tell me thermodynamics is done in your school right what does it summarize it what we do in this chapter. What we do in this chapter relation between heat and the other form of the energy. Okay. What else what do you think Nikhil Karthik Kaushik what do you think Aditya Swaraj how temperature will flow. Okay. Okay. All right thermodynamics guys the thermo properties of matter. Listen here the first chapter will tell you what are the properties of the matter. What are the properties of all the matters which are thermal in nature. Second chapter will tell you how to use this property how to use this property and get some work done first chapter tells you property second chapter tells you how to use this. Okay. So thermodynamics is about converting heat into work. I mean it's the biggest discovery guys because generating heat energy is so easy isn't it you don't have to care about efficiency at all. I mean unless until you are burning the petrol and diesel which is costly otherwise in primitive time generating heat energy is so easy just light up the fire and heat energy is there. So there was a discovery of converting heat into useful work. Okay. And this discovery has come before even thermodynamics laws came in. So when Napoleon went for a war and he lost to Russians Russians or some other European country he came back and analyzed what made Napoleon lose the war and he pinpointed the reason that what made him lose the war was that he did not have efficient engine heat engine. So he had asked people to formulate it get the most efficient heat engine he has declared I mean he has in a way told everyone that all the future wars will be won by that country which has most efficient heat engine. So car not came in with the car not cycle you know all those things happened during those times. So converting heat into work was happening before even people knew about the laws of thermodynamics but then laws of thermodynamics came in and from that point onwards you can actually you know do it in a more aware manner. See flow of temperature you learn in thermo property the matter itself that is I mean flow of temperature doesn't mean anything flow of heat means something. Okay. Anyways we'll discuss it the the third chapter which is there in this sequence is kinetic theory. Who's kinetic theory. Theory means some sort of reasoning right some sort of reason as in there is some concept that is what the theory means. Okay. So third chapter tells you what is the reason actual reason that there is a temperature. What is the actual reason there is a pressure. What do you mean by pressure at the molecular level what the molecules are doing because of which temperature is there what the molecules are doing because of which pressure is there. Okay. So according to kinetic theory according to kinetic theory all the properties. Okay. All the properties which are thermal in nature they arise due to movement or oscillation whatever you may want to call it of atoms. Molecules. Okay. So in third chapter we are going to understand what actually happens at the molecular level get deeper insights into it and understand pressure temperature and many other things. Okay. So this is what the thermal physics is a very simple topic and very very scoring do not do not I mean you know what I have seen why I'm telling you all of that again and again because the moment people see something simple. They start taking it for granted and they're like okay fine. This is so simple. I can do it at the end. Let me focus on mechanics. I will take a thermal physics at the end. But that is the biggest mistake. Okay. Because thermal physics although it is simple it is not so simple that you just completely ignored it. Fine. The best way to learn thermal physics is whenever I'm doing with you like for example today I'm doing this first chapter with you whatever I have done today. Today itself you solve all the not today maybe couple of days starting from today to complete everything with me by the time I'm done with the thermal physics. You should be done with the thermal physics from your side and you can you don't need to do it again ever for next one and a half years. Okay. So that is how it should be done. Anyways so I have told you enough about the importance of it now it is up to you. So write down the chapter name guys the first chapter name thermal properties of matter thermal properties of matter. Can you tell me with respect to the mechanical properties you have two chapters mechanical properties of solids and mechanical properties of fluids. Why you have a single chapter for all the matters when it comes to thermal properties. Quickly tell me. So so phenomena is more or less same at macro level. Okay at macro level the concept behind the properties which are shown they can't the concept is same. Okay. So that is the reason why you have a single chapter. Okay. Now tell me some of the thermal properties which I mean the property. An object has which is different from I mean let's say let's talk about mechanical properties of solid in mechanical properties you have strength strength of each solid is different. So that is a mechanical property. Similarly thermal property what do you think will vary from one object to the other which is related to heat and temperature. Tell me I mean whatever comes in your mind it doesn't matter that you have to be correct all the time. Okay. Do you know the J advance rank we the second rank we have 2400 rank in J advance. In class 11th he was not even visible class 12th he was he started working hard. He used to wake up and but then the one thing which I have all always admired about him that he kept on asking doubts without feeling ashamed about it. And now you see he is in IIT you should not be ashamed of telling what you think how much heat can be used to change the temperature. Okay that is different for one object to the other. What else. Conductivity. Okay. Let me write down somebody said boiling point boiling point of course different for different this thing then somebody said what is that conductivity. Thermal conductivity isn't it makes sense as in how fast copper conducts heat will be different from how fast a wood conducts heat. Okay now the amount of heat required to raise the temperature this will be different for different objects that is also a property. Okay that is this this thing is basically heat capacity how much capacity you have to take the heat without increasing the temperature. Okay radiation also somebody said radiation. Okay. Okay what else do you have you learned about the heat and temperature in class 9 that when you heat up something it expands have you learned about it. Right so expansion so thermal expansion what else anything else do you think I'm missing here. No no not that thermal stress it will get developed because of the expansion property. No no that's it melting point. Okay melting point is different for different objects then you have have you heard of convection so guys these are the properties that we are going to learn in this chapter. Getting it this is these are the set of properties which we are there might be other thermal properties also you can say entropy and all that but then this is the these are the properties that we are going to study in this chapter isn't convection only in fluid. Yeah fluid is required for convection different fluid will have different this thing convecting properties. Okay that'll come under radiation mother. All right so the thing is that this conductivity radiation and convection. Okay these are related to these are related to heat transfer heat transfer rate how fast something is transferring. Okay so this is what the focus is in these three white marked topics. Okay then you have this boiling point. Boiling point melting point. Okay these are basically related to change of phase or state as in solid becoming liquid liquid becoming gas. Okay that has to do with heat so you keep on supplying heat solid will become liquid and liquid will become gas also ultimately fine. Then of course you have these basic properties which are thermal expansion and heat required to raise the temperature fine. So basically what we have to study thermal expansion heat required to raise the temperature then change of state and heat transfer. So broadly four things you have to study fine so the first and foremost since we have to study this much and I am not going to assume that you know anything related to thermal physics. In fact this book itself the NCRT book doesn't even assume that you know temperature itself they assume you don't even know what is temperature. So that's how the chapter starts from that what is temperature once you know what is temperature then you can use temperature to define everything else. Okay do you have you heard of this term called heat is flowing heat flows from one point to the other something like that this kind of phrase have you heard heat is flowing right heat flows from hot body to cold body that you have heard about. In fact when thermodynamics was discovered the branch of science when it was formally getting you know formally it was taking shape people were correlating it with fluids people thought that the heat is an invisible fluid hidden inside objects. And it flows from higher temperature to lower temperature this is how they started studying thermodynamics and thermal physics and that is that the language still continues people still say heat is flowing. But it is not an invisible fluid earlier days people have you know correlated with fluid so much that you know they used to say that from higher pressure to lower pressure the water flows similarly high temperature to lower temperature heat flows. So temperature is like a pressure of liquid so temperature difference is required for heat to flow pressure difference is required to water to flow. So that's how strictly they have correlated everything so many concept you'll see will be related to fluids still. Okay but later on of course people have understood that heat is not a fluid it is a form of energy you'll have done this experiment to find out mechanical equivalent of heat and then it was all clear. Okay in fact there is a big story let's not get into all that. Anyways so talking about the temperature we need to first discuss what it is and once you learn what it is in physics you need to also learn how to put a number to it how to quantify it. Just knowing what is mass is not enough you should know what what does it what does it mean 5 kg means what 10 kg means what you need to put a number right similarly temperature right down. Has it ever happened to you that when you when you are in a room and you touch let's say wood. And then you touch the floor or you touch a metal metal feels colder than the wood has it ever happened metal feels colder than the wood right do you think temperature of metal is lower or higher than the wood metal in the room. Do you think temperature what what do you're feeling you're feeling temperature or something else they're the same temperature you're feeling heat flow conductivity of metal is better conductivity of metal is better because your body temperature is more than the metal. Metal is able to absorb more heat from you. Okay but if suppose same metal becomes becomes you know hotter its temperature becomes much higher and same wood also becomes much wood temperature is higher and metal temperature is higher then also metal will feel the more hot. Because of the because of our senses our senses do not feel temperature our senses only feel the transfer of heat. Okay it's like we as humans we cannot feel the uniform motion we can only feel the acceleration similarly with the heat we can't feel the temperature but if we can feel the heat flow. Okay yes okay then what is the temperature temperature is what then temperature is a quantity which tells you how much hot an object is okay so it is the degree of hotness degree of hotness or coldness a body has. Okay so this will quantify that now you can debate on that why do we need to define something like this the reason is that if I tell you. If I tell you that there is an object here which is let's say which is minus 50 degree Celsius there is another object here which is let's say 100 degree Celsius. Okay then I put an object over here then I put another object here. Okay now over here the temperature is minus 100 degree Celsius let's say and there the temperature is 98 degree Celsius where do you think heat will flow more from here to there or from there to there. Case number one or case number two both the bodies are having the same material same type of contact case one right it is a case one case one will be faster right so you cannot you cannot just look at the body. You can't just look at one single object and say that you know this object will transmit the heat. Because it depends on the other object also isn't it you need to compare the two objects and then tell whether heat will flow and whether it will flow from one object to the other or other to the first one. So what are you comparing what are you comparing you're not comparing heat you're comparing how much hot or what is the degree of hotness of this object compared to that object and then you'll say okay heat will transfer from here to here or there to here. Okay so this is what the temperature is it will tell you the direction of the heat flow. Okay it will be like quantifying the state of an object what is the thermodynamic state. Okay it's like mass is kept at a height of H its potential energy is MGH right so you can't just say that the potential energy of this object is higher or lower there can be another mass at slightly higher height. Okay same way I'm just telling you the state the object has what is the state the temperature is the state right now. Okay right so this is what we need to quantify all right so now that we need to put a number of course I have used degree Celsius already but ignore that suppose you don't know anything which is very difficult to do actually. So we need to put we need to we need to devise a scale to measure the temperature. Okay now do you remember when we have to devise a scale to measure the length itself what we had to define we have to define what is one meter do you remember. Do you remember we had defined what is one meter then it was clear what is five meter what is ten meter what is six meters do you remember or not. Right so when we define the scale of anything we define one unit of it what is one kg what is one meter what is one second things like that. Okay but the problem with defining the temperature is something else for example if you have to define kg what is one you already know what is zero zero kg means what nothing nothing is there. Similarly when you have to define the length zero centimeter means what nothing absence absent length is absent. Similarly zero second means time hasn't moved at all right but then over here every time some scale would have been defined saying that this is the lowest temperature and people say well let's call it as zero. Then somebody will come and say listen here is a colder object heat will flow from heat will flow from whatever you are saying colder to this object then you can't say that is absence of temperature you can't say it is zero. Okay so basically absolute zero initially was absent nobody knew what is zero in temperature zero means what nobody knows. Okay so with respect to temperature you not only need to define what is one unit you also have to define what is zero it is like defining potential energy. You have to assume something is zero because nobody knows what is zero potential energy similarly here nobody knows right now nobody knows what is zero temperature. So you have to tell everyone that let us assume that is zero. Okay so the most common available substance which is accessible to the entire world easily is water. Okay so we have used water as a reference to create a scale. Okay and we have to define two things right unit and zero so water has a very well defined freezing point and a well defined boiling point. So freezing point I'll say zero you could have said boiling point is one but then you knew that between freezing point and boiling point there is a big difference it takes some time. So why not assume boiling point to be hundred and say that between freezing point and boiling point there are hundred equal steps so that is also fine. Right so we have used water at the atmospheric temperature we say that the freezing point right down this is a degree Celsius scale the freezing point was assumed to be zero degree Celsius guys. Okay and the boiling point was assumed to be hundred degree Celsius and there were hundred equal divisions hundred equal divisions fine. So this is with respect to the Celsius scale this is called Celsius scale and it is a relative scale because everything is measured with respect to water. So it is called relative scale and every relative scale has this degree degree Celsius degree Fahrenheit. So this is just this thing notion that if it is a relative scale will put a degree over there. Okay now people were not talking during those times there was no mobile phone there was no television nothing in fact there was no transport system like aircraft and everything not very easily accessible. So one part of the world people might be doing something the other part of the world people might be doing something else. So in London I think in UK somewhere there were a set of people set of prominent scientists who were developing the Fahrenheit scale. Okay and they had taken some some salt solution with ammonium chloride and some mixture along with it and they have used some reference points of that solution for some reason. Okay so we will not get into the details of what exactly that solution is but what I can do here I can tell you that with respect to the Fahrenheit scale write down this Fahrenheit scale Fahrenheit scale. I hope the spelling is correct with respect to Fahrenheit scale the what is zero degree Celsius. Zero degree Celsius how many Fahrenheit do you know this 3232 degree Fahrenheit leave the point and 100 degree Celsius is how many Fahrenheit 212 degree Fahrenheit. Okay fine 212 Jaren not that Fahrenheit has H F A is it anyways so you can see between 32 and 22 how many divisions are there how many divisions are there. Between them 180 180 divisions and between zero and 100 there are 100 division so one division of zero degree Celsius one division of Celsius is not same as one division of the Fahrenheit but both are linear scale 180 equal division that side 100 equal divisions that side. So if you plot a graph between Celsius and Fahrenheit what kind of graph you will get what kind of graph you will get Celsius and Fahrenheit straight line all of you agree. Okay take Fahrenheit on the Y axis that's how I guess in your textbook it is take the Fahrenheit on the Y axis Celsius on the X axis straight line you will get. So can you plot that straight line all of you plot that straight line everyone let me know once done done when Celsius is zero Fahrenheit will be 32 so this point is 0 comma 32 and there will be another point over here which will be 100 comma 212 isn't it and you will get a line like that. Can I extend this line like that can I do that everyone can extend this line till eternity right I can do that. Okay now what is the basic property of a straight line what is the basic property of a straight line in coordinate geometry you have learned the property of a straight line the slope is same slope is same everywhere. Okay so to find out the slope you need two points okay you can say this is sorry this is point one point a point B and take a random point over here which is you can say it is C comma F C comma F is like a variable point. Alright this is point number P okay so between A and B you find out the slope and between A and P you found out the slope and equate it if you see between A and B the slope is 212 y2 minus y1 divided by x2 minus x1. Okay this is equal to F minus 32 divided by C minus 0 okay getting it so I am getting C by 100 is equal to F minus 32 divided by 180 so this is how you can convert Celsius into Fahrenheit. Okay so don't worry nobody is going to ask you to convert it it can't be that simple in a complete exam but in school exam the derivation is very important you should know how to derive it for your school exams clear to everyone everyone is clear right I'll move ahead then. Okay so for a very long time Celsius and Fahrenheit scales were used and then there was this great scientist named Lord Kelvin who has finally discovered the absolute zero of the temperature there cannot be any temperature lesser than zero Kelvin zero means zero. Okay this is what is the Kelvin scale let us understand the philosophy behind Kelvin scale and how it has evolved alright so write down Kelvin scale so like I said Kelvin scale doesn't uses anything as a reference okay it should be independent of any material as such because it is an absolute scale. So Kelvin did a very simple experiment actually he had plotted a graph between pressure and temperature can there be pressure less than zero everyone can there be pressure less than zero zero pressure is absolute or not. Zero means zero can there be less than zero pressure everyone that cannot be less than zero pressure okay fine okay so now what is seen is that suppose you take a gas gas is right now at this stage pressure and temperature. Okay so the gas pressure is reduced little bit so it's pressure and temperature becomes this much then it is further reduced it has become that much further reduce it has become that much so if you continuously reduce its pressure the temperature also reduces. Okay so a well defined line is getting developed and after sometime the gas becomes liquid but then although it you can't keep on plotting for liquid because you are measuring pressure of the gas so what you do is that you extend this line further extend this line and it will meet here somewhere. Okay and Kelvin said that this is absolute zero because if you decrease the pressure temperature decreases beyond this I can't decrease the pressure so temperature can also not be decreased. But then you can claim that okay this is true only for let's say one gas what about the other gas okay so Kelvin did the same experiment. So this is the point where gas becomes liquid okay Kelvin did the same experiment with the other gas also and he has found out this kind of nature okay he had. Found out the same the unique point where the other gas at the other state will have zero pressure if you extend this line backwards. Okay this is for gas to like this you can do the experiment for any gas and you will find out at unique point over here which is 273.15 minus of that degree Celsius at this temperature the pressure becomes zero fine. All right so everyone assume that this is the absolute zero okay temperature beyond this is not possible okay so basically in the Kelvin scale minus 273.15 degree Celsius is equal to zero Kelvin with Kelvin you don't put any degree because it is not a relative scale fine. And then everybody has accepted the SI unit of the temperature is K Kelvin okay yes Ashwat okay so tell me if you have a temperature in degree Celsius what you should do to convert it into Kelvin. Suppose you have 27 degree Celsius I think you already know all of these very very basic things if you have 27 degree Celsius what you do is that you need to add 273.15 to it but we ignore 0.15 okay because you know we don't like fractions. So what you do is that you add 273 to it and you can say this is 300 Kelvin okay yes yes that is true temperature cannot be more than less than minus 273.15 okay. And you know it also sometimes do you know that temperature of the same location if you measured in two different ways you get two different things the reality is that the way you measure temperature or the way you define the temperature. Temperature is what kind of theory of gases if you learn temperature represents kinetic energy of the gas kinetic energy of the gas is the temperature. So if temperature is T kinetic energy of the gas molecule is 3 by 2 KT T is a temperature in Kelvin so can kinetic energy be less than zero. No so temperature in Kelvin cannot be less than zero because kinetic energy is directly related to temperature. Temperature is kinetic energy, kinetic energy is temperature okay. So once there is a vacuum if there is no gas molecule nothing then absence of the molecule is also zero kinetic energy so that is also minus 273 degree Celsius okay. Anyways now do you guys know that delta T change in temperature in degree Celsius is equal to change in temperature in Kelvin delta T in degree Celsius is equal to delta T in Kelvin okay. So if you have a formula in which delta T is there okay so in some equation if there is delta T okay and in the question delta T is given in degree Celsius you don't need to convert it into Kelvin. Because delta T of degree Celsius the step the one unit of Kelvin is same as one unit of the degree Celsius okay. So you don't need to convert it but if in a formula only T is there delta T is not there T is there then you have to convert degree Celsius into Kelvin. Everyone is clear about whatever I have said do you all understand whatever I have said yeah I said that delta T of degree Celsius is same as delta T of the Kelvin delta T. So from freezing point to boiling point change in temperature is 100 degree Celsius change in temperature is also 100 Kelvin from 273 it has gone to 373 so change is 100 that's what I said. Alright so now we have defined the temperature what is the temperature now we can use a temperature to understand the properties of the material or the matter okay. So the first property wait a second.