The second law of thermodynamics is fundamental law of nature, unarguably one of the most valuable discoveries of mankind. However, this law is slightly confusing for most engineers or students. The main reason for this is because it has so many complex terms in it, and that there are many ways that this second law can be stated, but most importantly, the majority do not understand what are the applications of this law. In this video we will create a real, physical insight into this law, with a minimum use of mathematics. The million dollar question is, for what purpose is this law used. One of the main uses of the second law of thermodynamics Is to determine whether a process is spontaneous or not. Let’s consider a few examples. Here two gases are mixing together, air is being leaked from a balloon, and the mass is falling down, and the hot tea is losing its heat. You can see that the process of moving from state 1 to state 2, will happen spontaneously, which means without any external aid. But what about the opposite process? Will that happen spontaneously? Would this mixed gas become unmixed spontaneously without any external aid? From your own experience, you know this will not happen. So this process is not spontaneous. However, according to the rules of energy conservation, or the first law of thermodynamics, even the reverse process is possible. because in both states, the energy is the same. So what is missing here? There must be one more law which governs the direction of a process, and that law is second law of thermodynamics. Now, probably you have a question in your mind. Do I really require a law just to predict the direction of a process? I can predict the direction of all these processes just with my intuition. If you do have such questions in your mind, let us analyze one more example. This is a chemical reaction. Here I am putting two chemicals together in a chamber, and I am waiting for a reaction to happen. I want to check if two blue atoms react with one yellow atom to form a new molecule. Do you have an answer for this? This cannot be predicted through intuition. This is exactly what the second law is used for. In this video, we will review the second law. We will learn it well, and we will come back to this same problem, to this chemical reaction. Here are the two standard definitions of the second law You may have already heard of them Both these statements mean the same but they are not in the state directly state directly useful to engineers. Here is a useful form of the second law, that is useful for engineers. The Clausius Inequality. This is a small integrated equation, but with deep inner meanings please don’t caryatid about this equation We will conduct a physical experiment to understand it. the Clausius inequality means that if you take a cyclic process, such as in a refrigerator, and that all the heating action happening at the boundary divided by the temperature of the boundary the resulting value will be less than, or equal to zero This is an interesting inequality, which is true for all cyclic processes. To do this equation more application oriented, let us introduce a new term: the famous term of entropy.