Constantin Carathéodory formulated thermodynamics on a purely mathematical axiomatic basis. His statement on the second law is known as Carathéodory`s principle, which can be formulated as follows:[47] The laws of thermodynamics describe the relationships between thermal energy, or heat, and other forms of energy, and how energy affects matter. The first law of thermodynamics states that energy cannot be created or destroyed; The total amount of energy in the universe remains the same. The second law of thermodynamics deals with the quality of energy. It states that when energy is transferred or converted, it is increasingly wasted. The second law also states that there is a natural tendency of each isolated system to degenerate into a more disordered state. The two statements are indeed equivalent, because if the first were possible, the work obtained could, for example, be used to produce electricity, which could then be discharged by an electric heater installed in a body at a higher temperature. The net effect would be a heat flow from a lower temperature to a higher temperature, thus violating the second form (Clausius) of the second law. Conversely, if the second form were possible, the heat transferred to the higher temperature could be used to run a heat engine that would convert some of the heat into work.
The end result would be a conversion of heat into work at constant temperature – a violation of the first form (Kelvin) of the second law. Now consider a device that uses heat transfer to get work done. As mentioned in the previous section, such a device is called a heat engine, and such a device is shown schematically in Figure 15.16(b). Gasoline and diesel engines, jet engines, and steam turbines are all heat engines that use some of the heat transfer from a source. The heat transfer from the hot object (or hot tank) is called QhQh, while the heat transfer to the cold object (or cold tank) is QcQc and the engine work is WW. The temperatures of the hot and cold tanks are ThTh and TcTc, respectively. So what exactly is the connection between entropy and the second law? Recall that at the molecular level, heat is the random kinetic kinetic energy of molecules, and collisions between molecules provide the microscopic mechanism for transporting thermal energy from one place to another. Since individual collisions remain unchanged by reversing the direction of time, heat can flow in either direction. Thus, from the point of view of fundamental interactions, nothing stands in the way of a random event in which several slow (cold) molecules gather in the same place and ice forms while the surrounding water becomes warmer. Such random events can be expected from time to time in a container containing only a few molecules of water. However, the same random events are never observed in a large glass of water, not because they are impossible, but because they are extremely unlikely. Indeed, even a small glass of water contains a huge number of interacting molecules (about 1024), making it highly unlikely that a significant portion of cold molecules will gather in the same place during their random thermal motion.
Although such a spontaneous violation of the second law of thermodynamics is not impossible, an extremely patient physicist would have to wait several times for the age of the universe to see it. When a hot body and a cold body are brought into contact with each other, heat energy flows from the hot body to the cold body until they reach thermal equilibrium, that is, the same temperature. However, the heat will never move the other way again; The temperature difference between the two bodies will never increase spontaneously. To transport heat from a cold body to a hot body, the work must be done from an external energy source such as a heat pump. The second law of thermodynamics postulates: it is impossible for a cyclically operated device to completely convert heat into work. If the heat flows spontaneously from a higher temperature to a lower temperature, the reverse process requires a heat engine. Carnot showed that the maximum work of a heat engine is given by a cycle composed of two adiabates and two isotherms, whose efficiency The first law of thermodynamics provides the definition of the internal energy of a thermodynamic system and expresses its change to a closed system in terms of work and heat. [8] It can be associated with the law of conservation of energy.
[9] The second law deals with the direction of natural processes. [10] It is claimed that a natural process occurs in only one direction and is not reversible. For example, when a path is provided for conduction and radiation, heat always flows spontaneously from a warmer body to a cooler body. Such phenomena are considered in relation to the change in entropy. [11] [12] If an isolated system containing different subsystems is first maintained in internal thermodynamic equilibrium by internal partitioning by impermeable walls between the subsystems, then by operation the walls become more permeable, then the system spontaneously evolves to reach a new final internal thermodynamic equilibrium, and its total entropy, S {displaystyle S}, increases. where QQ is the net heat transfer during the cycle (Q = Qh −QcQ = Qh −Qc) and WW is the mesh work performed by the system. Since ΔU=0ΔU=0 is valid for a full cycle, we have We can use W=Qh−QcW=Qh−Qc to find the working power WW, provided that a cyclic process is used in the plant. In this process, water is boiled under pressure into high-temperature steam, which is used to power steam turbine generators, and then condensed into water to restart the cycle.