Thermodynamics is the branch of physical science dedicated to the study of heat, work, temperature, and the transfer of energy. A concept central to this study is thermodynamic equilibrium, which describes a state where a system experiences no net macroscopic change over time. When a system reaches this state, its measurable properties, such as temperature and pressure, become uniform and remain constant indefinitely without any external influence. This condition represents the final, stable state toward which any isolated system naturally progresses.
The Three Required Conditions for Equilibrium
For a system to be in a true state of overall thermodynamic equilibrium, three distinct criteria must be met simultaneously across all parts of the system. If any one of these conditions is violated, the system is not considered to be in complete equilibrium. These conditions ensure that no remaining internal drivers or forces could cause the system to spontaneously change its state.
The first criterion is Thermal Equilibrium, achieved when the temperature is uniform throughout the entire system. This means there is no net flow of heat energy within the system or between the system and its surroundings.
This concept is formalized by the Zeroth Law of Thermodynamics, which establishes that if two separate systems are each in thermal equilibrium with a third system, they are also in thermal equilibrium with each other. Essentially, all components have reached the same temperature.
The second condition is Mechanical Equilibrium, which requires that the pressure be uniform everywhere within the system and that there are no unbalanced forces acting upon it. If a pressure difference existed, a net force would cause the system to expand or contract until the pressure equalized. In this state, the system is stationary, and there is no net work being done.
Finally, the system must satisfy Chemical Equilibrium, which dictates that the chemical composition is not undergoing any net change over time. Any possible chemical reactions are proceeding at equal rates in both the forward and reverse directions, resulting in a constant concentration of reactants and products. Additionally, there can be no net transfer of matter within the system, such as through diffusion.
Distinguishing Equilibrium from a Steady State
A common point of confusion is differentiating between a system in thermodynamic equilibrium and one in a steady state, as both appear unchanging at a macroscopic level. The fundamental distinction lies in the system’s boundaries and the flow of energy or matter. A system in thermodynamic equilibrium is isolated or closed, meaning there is no net flow of energy or mass across its boundaries.
Conversely, a steady state is a dynamic condition where the system’s properties remain constant over time, maintained by a continuous flow of energy or matter through the system. A steady state system is typically an open system, constantly interacting with its environment. For example, a lit light bulb maintains a constant temperature, requiring a continuous input of electrical energy and a continuous output of heat and light energy to sustain that state.
Internal driving forces, such as temperature or pressure gradients, are absent in thermodynamic equilibrium but often present in a steady state. In a steady state, the input is perfectly balanced by the output, whereas in equilibrium, there is simply no net input or output to begin with. All systems in thermodynamic equilibrium are also in a steady state, but a steady state system is not necessarily in thermodynamic equilibrium due to the required energy and matter fluxes.
Real-World Examples of Equilibrium in Action
Observing thermodynamic equilibrium often involves waiting for a process to fully complete in a contained environment. When a hot cup of tea is left in a closed room, it will eventually reach thermal equilibrium with the surrounding air. Heat flows from the warmer tea to the cooler air until their temperatures are identical, and no further net heat transfer occurs.
Another example involves a sealed container holding water and its vapor. At a constant temperature, the rate of evaporation becomes equal to the rate of condensation. This establishes both mechanical equilibrium (constant vapor pressure) and chemical equilibrium (constant amounts of liquid and gas), provided the temperature is uniform.
A sealed thermos flask, if left undisturbed for a very long time, will approach equilibrium as its contents’ temperature and pressure equalize with the external environment.
Most complex natural systems, like a living human body or an operating household refrigerator, are not in thermodynamic equilibrium. These systems are open and maintain a steady state by constantly taking in nutrients or electrical energy and expelling waste or heat. They continuously exchange energy and matter with their surroundings to maintain their internal, unchanging conditions.