Non-equilibrium thermodynamics is the branch of thermodynamics that studies systems that are not in thermal, mechanical, or chemical equilibrium. Unlike classical (equilibrium) thermodynamics, which only describes systems in stable, balanced states, non-equilibrium thermodynamics focuses on how systems evolve over time, how gradients (in temperature, pressure, concentration, etc.) drive flows, and how irreversibility arises.
It deals with dynamic processes, such as heat conduction, diffusion, chemical reactions, and viscous flow, where systems are continuously changing and exchanging energy or matter with their surroundings.
Key Concepts:
- Entropy production is always positive in non-equilibrium systems, reflecting irreversible processes.
- Fluxes and forces: Flow (of heat, mass, charge) is driven by thermodynamic forces (like temperature or concentration gradients).
- Described by phenomenological laws such as Fourier’s law (heat flow), Fick’s law (diffusion), and Ohm’s law (electric current).
Examples:
- Heat transfer from a hot object to a cold one.
- Diffusion of salt in water or gases mixing in the air.
- Chemical reactions moving toward equilibrium in living cells or industrial processes.
- Biological systems, which constantly operate far from equilibrium (e.g., metabolism, neural activity).
Non-equilibrium thermodynamics is essential for understanding real-world systems, which are rarely in perfect equilibrium, and it is crucial in fields like engineering, biology, geophysics, and materials science.