How does the concept of flux relate to non-equilibrium thermodynamics?
The correct answer and explanation is:
Correct Answer:
In non-equilibrium thermodynamics, flux refers to the rate at which a physical quantity such as mass, energy, or charge flows through a system and is directly related to the presence of gradients such as temperature, concentration, or pressure. Fluxes are central to describing how systems evolve toward equilibrium.
Explanation (300 words):
In non-equilibrium thermodynamics, flux is a fundamental concept used to describe how materials and energy move through systems that are not in thermodynamic equilibrium. Unlike equilibrium systems, which are static and have no net flows, non-equilibrium systems experience continuous movement of particles, energy, or other quantities due to gradients in temperature, chemical potential, or pressure.
Flux can be defined as the amount of a physical quantity passing through a unit area per unit time. Examples include heat flux (transfer of thermal energy), mass flux (movement of molecules), and electric current (movement of charge). These fluxes arise in response to what are known as driving forces. For instance, a temperature difference across a material drives a heat flux, while a concentration difference drives a diffusion flux.
The relationship between fluxes and driving forces is often described using phenomenological laws, such as Fourier’s law for heat conduction, Fick’s law for diffusion, and Ohm’s law for electrical conduction. In each case, the flux is proportional to the gradient of the driving force. The proportionality constants in these laws (such as thermal conductivity or diffusion coefficient) are material-specific properties.
In the framework of non-equilibrium thermodynamics, especially as developed by Onsager, fluxes and forces are connected through linear relations. This approach also introduces the concept of entropy production, which is always positive in non-equilibrium processes. The rate of entropy production is given by the sum of products of fluxes and their corresponding forces. This reflects the second law of thermodynamics and quantifies how systems dissipate energy as they move toward equilibrium.
Overall, fluxes in non-equilibrium thermodynamics are essential for describing how systems change over time and how irreversible processes occur.