The core characteristic of thermal equilibrium is that the system's macroscopic properties are stable, the temperature is uniform, and there is no net heat flow. It is a dynamic, idealized stable state that can be described by the laws of thermodynamics. This state is not only the cornerstone of thermodynamic analysis but also provides the theoretical basis for defining temperature and measuring heat.
Main characteristics of thermal equilibrium:
Constant temperature (thermal equilibrium): The temperature is the same in all parts of the system and between the system and its surroundings (if in contact). There is no heat transfer driven by temperature differences. This is the most fundamental criterion for thermal equilibrium, derived from the zeroth law of thermodynamics: If two systems are in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other.
No macroscopic net heat transfer: Although microscopic particles are constantly colliding and exchanging energy, from a macroscopic perspective, there is no net heat flow within the system or between the system and its surroundings. That is, "heat absorption = heat release," achieving a heat balance.
No macroscopic net heat transfer: Although microscopic particles are still colliding and exchanging energy, from a macroscopic perspective, there is no net heat flow within the system or between the system and its surroundings. That is, "heat absorption = heat release," achieving a heat balance. Dynamic Equilibrium (Thermodynamic Equilibrium) From a microscopic perspective, molecules are constantly in random thermal motion, exchanging energy continuously, but the system as a whole exhibits a kind of "dynamic equilibrium within stillness." This equilibrium is called thermodynamic equilibrium, a result of statistical laws.
Maximum Entropy (In Isolated Systems) According to the second law of thermodynamics, isolated systems spontaneously tend towards increasing entropy. When thermal equilibrium is reached, the entropy reaches its maximum possible value under those conditions, and the system's disorder is at its highest. At this point, ΔS = 0, and the process is irreversible.
Minimum Gibbs Free Energy (Under Isothermal and Isobaric Conditions) In an open environment, the system tends towards the state of minimum Gibbs free energy (G). When ΔG = 0, the system reaches thermodynamic equilibrium, at which point ΔH = TΔS, indicating that enthalpy change and entropy change are in equilibrium.
Existence of Fluctuations Although macroscopically stable, due to the random motion of microscopic particles, the system state will fluctuate slightly around the equilibrium value, called fluctuations. In macroscopic systems, fluctuations are extremely small and usually negligible.
The basis of temperature measurement:
Because systems in thermal equilibrium have the same temperature, we can use a thermometer to measure the temperature of an unknown object through thermal contact. This principle is the physical basis of all temperature measurements.
Classification and conditions of thermal equilibrium:
|
Equilibrium type |
Condition |
Manifestation |
|
Thermal equilibrium |
Uniform temperature |
no heat transfer |
|
Mechanical equilibrium |
Uniform pressure |
no macroscopic force difference, no volume change or acceleration |
|
Chemical equilibrium |
Equal chemical potential |
no substance transformation or concentration change |
|
Phase equilibrium |
Equal chemical potential of each phase |
no phase change (e.g., coexistence of water and ice) |
Only when all the above equilibrium conditions are met simultaneously is the system truly in complete thermodynamic equilibrium.
Practical significance and idealized explanation:
Ideal model: Strictly speaking, thermal equilibrium requires the system to be completely isolated (no exchange of matter or energy), which is difficult to achieve in reality, but can be approximated when the process is slow (e.g., gas state changes in a cylinder).
Wide applications: From thermometer design and heat engine efficiency analysis to phase change research in materials science, the concept of thermal equilibrium is ubiquitous.
Non-equilibrium state comparison: If the system has a continuous energy input (such as living organisms or atmospheric circulation), it may maintain a "non-equilibrium steady state" (dissipative structure), but this does not fall under the category of classical thermal equilibrium.
