 So, we will go to the next topic now, the entire thermal physics we have studied because we want to create heat engines out of it and convert heat energy into the useful work. That is what is the final topic of this chapter which is heat engine. So all this knowledge that we have acquired will be utilized to create the heat engine. Please write down heat engine, please write down the definition first. Heat engine is a device by which a system undergo a cyclic process, cyclic process to convert heat to work. So, we are going to study how heat can be converted into work. So, before we get into all of this, we need to understand few basic terminologies. So, every heat engine will have a working fluid, please write down working fluid or a substance you can say. In case of the internal combustion engine, the working fluid is the air and fuel mixture which burns or explodes, then you have a hot reservoir, this is nothing but source of heat energy. Third is the cold reservoir, it is nothing but the sink heat energy. So, we have already learned that there will always be one portion of a cycle which will absorb heat and another portion it will release heat if there is some work coming out of that. So, that is the reason why there has to be at least one hot reservoir from which you are absorbing heat and one cold reservoir through which you are giving away the heat energy. So, formally you can say that heat engine between two temperatures. Now, you might be arguing maybe if something is coming in your mind that why it should be between two temperatures only, why cannot it be multiple temperatures. The reason why it is between the just two temperatures is because this is going to optimize the efficiency or you are going to get maximum efficiency. So, this is the primary requirement. So, let's say hot reservoir has a temperature T1, this is your system, this is the colder reservoir, let's say temperature is T2. So, from hot reservoir it absorbs Q1 and it releases Q2 to the colder reservoir and does some work. Okay, so this is the block diagram of any heat engine and this is your system or the working fluid. Can you tell me the relation between W, Q1 and Q2? W is equal to Q1 minus Q2. How it comes? Sir, because internal energy will be zero for cyclic process. So, heat absorbed minus heat released will be the work, sir. Correct. For cyclic process, we have delta Q is equal to delta U plus W. Okay, so delta Q is heat absorbed minus heat released. This should be equal to delta U which is zero plus W. Okay, so W is equal to Q1 minus Q2. So, this is the basic relation for the heat engine. Now, if you talk about efficiency of heat engine, efficiency of heat engine should be what? What do you think is a useful thing? W, W divided by what? Q1, Q2. W divided by Q2. Q1. Q1, you are giving Q1, but Q2 is happening on its own. You don't want to waste the heat energy, but that is what the second law of thermodynamics is. You have to give some heat energy as a tax to the universe, otherwise you can't function. You cannot convert entire heat into work. So, that is what Q2 is. You have to give away the heat, but ideally you don't want to minimize Q2, but you have to burn the fuel for Q1. So, what you are getting, which is W divided by what you are giving, which is Q1. Okay, and W is equal to Q1 minus Q2. So, I can write it like this. This can be further simplified as 1 minus Q2 by Q1. So, this is the efficiency of any heat engine, 1 minus Q2 by Q1. Okay. Should we take a small break? No. Yes, sir. Yes, sir. Yes, sir. Yes, sir. Let's take a poll. This is an anonymous poll. Please answer. Should we take a break? Oh my God. So many are saying we should not take a break. All right. So, this is what you have said. Second law of thermodynamics is so basic that you can't even put it in words properly. Forget about defining it. So, second law of thermodynamics is that basic that people have attempted to just to explain what does it imply. Okay. So, that is the reason why there are multiple explanations available to the second law of thermodynamics. All right. So, we are going to just discuss the explanation of second law of thermodynamics. There are two statements of second law of thermodynamics that are given by some great scientists. So, the first one is Kelvin Planck statement. Please write down. So, Kelvin Planck statement, according to that, there cannot be, please write down, very important, no process is possible whose sole result is absorption of heat from a reservoir and converting to work and converting completely. That is what the statement is about. You cannot have 100% efficiency. You cannot convert. In a layman term, you can say that Kelvin Planck is trying to say that you can never convert entire heat into work. There has to be some amount of heat which should be released. This is the Kelvin Planck statement. This is according to the second law of thermodynamics. In chemistry, you might have learned that the entropy of an isolated system should always increase in any process. So, the universe is an isolated system. So, entropy of the universe should increase in any process. So, whenever there is something to do with heat, a portion of heat goes into the increase in the entropy of the universe. That's like tax you give to the universe. This is Kelvin Planck statement and the second statement is Clausius statement. According to Clausius statement, no process is possible whose sole result is to transfer heat from cold object to a hot object. So, that is why if you leave the hot object spontaneously, it will cool down. But spontaneously, a colder object cannot become hot. This is what Clausius statement is. If you want to make sure that the colder object, if you want to make sure the heat from the cold object goes to the hot object, you must do some work on the system. Sole result is not possible that solely only this is happening. That is not going to happen. So, if you find any exception to these two statements, then you are wrong. These statements will still be valid. They are very basic. Fine. So, the first statement says you cannot have 100% efficiency. The second statement says you must do the work in order to extract heat from the cold object. So, that is why if there is a refrigerator, there has to be a compressor along with it. Without compressor, refrigerator is not possible. Now, let's talk about one basic thing. Then we will complete the chapter with the Carnot cycle efficiency derivation and that's all for the chapter. So, it will take a little bit more time. So, let us finish the chapter unnecessarily. Let's not leave 15-20 minutes of the chapter for the next class. So, we will start the fresh chapter the next class. Okay.