Thermodynamics is a branch of physics that studies a system’s energy and work. It was invented, when scientists were first learning how to build and operate steam engines. Thermodynamics only deals with a system’s large-scale response, which we can observe and measure in experiments. The kinetic theory of gases describes small-scale gas interactions. The methods complement each other; some principles are easier to understand in terms of thermodynamics, while others are easier to explain in terms of kinetic theory. Here, we are going to learn about the three laws of thermodynamics, focusing second law of thermodynamics.
What are heat engines?
A heat engine typically uses heat energy to do work and then exhausts the heat that cannot be used for work. The study of the relationships between heat and work is known as thermodynamics. The operation of a heat engine is constrained by the first and second law of thermodynamics. The first law applies energy conservation to the system and the second limits the machine’s potential efficiency and determines the direction of energy flow.
PV diagrams are a primary visualization tool for heat engine studies. Because engines typically use a gas as a working substance, ideal gas law connects the PV diagram to temperature, allowing the three essential state variables for the gas to be tracked throughout the engine cycle. Because work is only done when the volume of the gas changes, the diagram provides a visual representation of the work done. Because the internal energy of an ideal gas varies with temperature, the PV diagram, together with the temperatures calculated from the ideal gas law, determine the changes in the internal energy of the gas, allowing the amount of heat added to be calculated using the first law of thermodynamics.
The PV diagram for a cyclic heat engine process will be a closed loop. The amount of work done during that cycle is represented by the area covered by the loop. By comparing an engine cycle’s PV diagram to that of a Carnot cycle (the most efficient type of heat engine), one can get an idea of its relative efficiency.
Laws of thermodynamics
There are three main laws of thermodynamics. Each law leads to the definition of thermodynamic properties, which aid in understanding and predicting the behavior of a physical system. We will show some simple examples of these laws and properties for a variety of physical systems, with a focus on thermodynamics in the study of propulsion systems and high-speed flows. Fortunately, many classical thermodynamic examples involve gas dynamics. Regrettably, the numbering system for the three laws of thermodynamics is a little perplexing. Let’s start with the Zeroth law.
Zeroth law of thermodynamics
The zeroth law of thermodynamics entails some straightforward definitions of thermodynamic equilibrium. Thermodynamic equilibrium results in a large scale definition of temperature, as opposed to a small scale definition based on kinetic energy of molecules. It states that:
If a thermodynamic system A is in equilibrium with a system B, which is in equilibrium with a third system C then system A and system C are also in equilibrium.
First law of thermodynamics
The first law of thermodynamics relates the various forms of kinetic and potential energy in a system to the work that a system can do and heat transfer. This law is sometimes used to define internal energy, and it introduces a new state variable, enthalpy. The first law of thermodynamics allows for a large number of possible states of a system. However, past experience shows that only certain states occur. It states that:
Energy can never be created or destroyed, however it converts its form from one to another. As its equation is
Q = U + W
The heat Q added to a system is used by the system to increase its internal energy U and to do work W.
State second law of thermodynamics
First law of thermodynamics only tell us the usage of heat but nothing about the conditions and direction of heat transfer, second law is concerned with it. The second law states that the total entropy of a system plus its environment cannot decrease; it can be constant for reversible processes but must always increase for irreversible processes.
According to Lord Kelvin’s statement:
It is impossible to devise a process which can convert all the heat, extracted from a single reservoir, completely into the work, without leaving any change in the working system.
In other words, no heat engine can exist, which do not dissipates heat energy. This statement means that a single heat reservoir, no matter how much energy it contains, cannot be made to perform any work. This is absolutely true for our atmosphere and oceans which contain a large amount of heat energy but this energy cannot be converted into mechanical work.
As a consequence of second law of thermodynamics, two bodies which have different temperatures, are essential to concert heat into work. Hence, for the working of a heat engine, there must be a source of heat at a high temperature and a sink at low temperature, to which heat can be expelled. This is the reason due to which energy from oceans cannot be utilized as there is no reservoir at a temperature lower than any one of the two.