Thermal Equilibrium vs. Thermodynamic Equilibrium: Defining the Key Concepts in Thermodynamics
Introduction
Thermal equilibrium and thermodynamic equilibrium are crucial concepts in thermodynamics, yet they serve different purposes and describe distinct states of a system. This article aims to clarify the differences between these two key concepts, making it easier for students, researchers, and professionals to understand the nuances within thermodynamics.
Thermal Equilibrium
Definition
A system is in thermal equilibrium when it has no net heat transfer between the components of the system, meaning they have reached the same temperature. This occurs when two or more parts of a system are at the same temperature and there is no net flow of heat between them.
Conditions
For thermal equilibrium to occur, the following conditions must be met:
The system must be in thermal contact. No heat should flow between the components of the system.Example
A common example of thermal equilibrium is a hot cup of coffee in a cold room. Over time, the coffee will cool down and the room will warm up, reaching a state where both are at the same temperature, thus achieving thermal equilibrium.
Note: Thermal equilibrium is specific to temperature and does not necessarily imply that the entire system is in complete balance. There could still be mechanical or chemical imbalances present.
Thermodynamic Equilibrium
Definition
Thermodynamic equilibrium is a broader concept that encompasses not only thermal equilibrium but also mechanical and chemical equilibrium. It is achieved when a system reaches a state where all macroscopic properties, such as pressure, temperature, and chemical potential, do not change over time. In other words, it is a state of maximum entropy.
Conditions
A system must satisfy the following three conditions to be in thermodynamic equilibrium:
Thermal Equilibrium: No temperature gradient exists. Mechanical Equilibrium: No pressure gradient exists, and forces are balanced. Chemical Equilibrium: No chemical reactions are occurring, and concentrations of species are constant.Example
A sealed container of gas at a uniform temperature and pressure, with no chemical reactions taking place, is an example of thermodynamic equilibrium.
Thermal vs. Thermodynamic Equilibrium: Key Differences
Thermal equilibrium is particularly concerned with temperature and the kinetic energy (KE) of molecules. In gases, temperatures are proportional to the mean kinetic energy of the molecules. Conversely, thermodynamic equilibrium takes a broader view, considering entropy, which is influenced by all forms of internal energy, including phase changes, potential energy (PE), and chemical and potential energy.
Entropy is a measure of the distribution of energy in a system. In a thermodynamically closed system, entropy will increase until it reaches a maximum, corresponding to a state of equilibrium. This maximum entropy state is crucial because it indicates that all energy potentials within the system are balanced. If we assume there are no phase changes in a column of the troposphere and no chemical reactions, the critical factor to consider is the sum of potential energy (PE) and kinetic energy (KE).
At this maximum entropy state, the sum of PE and KE should be equal throughout the system, including the top and bottom of a column. While the potential energy (PE) is higher at the top of the column, the kinetic energy (KE) is correspondingly lower, leading to a temperature gradient. This gradient was first explained by Josef Loschmidt in the 1870s.
A thermodynamically closed system, therefore, does not necessarily have an isothermal (constant temperature) state. It allows for a temperature gradient, as long as the total internal energy balances out.
Conclusion
In summary, thermal equilibrium concerns temperature and kinetic energy, while thermodynamic equilibrium encompasses a broader range of properties, including mechanical and chemical states. Understanding these differences is essential for grasping the complexities of thermodynamics and applying them in various scientific and engineering contexts.
For more in-depth exploration into these concepts, please refer to my 2013 paper titled “Planetary Core and Surface Temperatures” available on ResearchGate, LinkedIn, and SSRN.