In thermodynamics, a thermodynamic force is systems-based quantification of the energy tendencies. [1] In a sense, the thermodynamic force is the energetic description of the driving force of any process.

Conjugate variables
See main: Conjugate variables
In a generalized sense, a thermodynamic force is a tension whose conjugate extensity is length. [2] The central concept of thermodynamics is that of energy, the ability to do work. As stipulated by the first law, the total energy of the system and its surroundings is conserved. It may be transferred into a body by heating, compression, or addition of matter, and extracted from a body either by cooling, expansion, or extraction of matter. In mechanics, for comparison, energy transfer results from a force which causes displacement, the product of the two being the amount of energy transferred. In a similar way, thermodynamic systems can be thought of as transferring energy as the result of a “generalized force” causing a “generalized displacement”, with the product of the two being the amount of energy transferred. These thermodynamic force-displacement pairs are known as conjugate variables. The most common conjugate thermodynamic variables are pressure-volume (mechanical parameters), temperature-entropy (thermal parameters), and chemical potential-particle number (material parameters).

CHNOPSological systems
In biological systems, the much discussed “energy flow” through such systems is “Gibbs energy flow” and the thermodynamic force is the change in the Gibbs free energy ΔG during the movement, evolution, or change. [3] Said another way, in the phrasing of pre-1882 thermodynamics and chemistry, the thermodynamic forces that drive most of the “flows” in biological systems are affinities. When affinity is the difference in chemical potential between reactants and products, the corresponding flow is a chemical reaction; when it is the difference in chemical potential from one location to another, the flow is transport of matter. [3] In respect to the transport of species between two compartments or phases, the thermodynamic force for the transport is the difference in electrochemical potential between the compartments. [4]

Positive psychology
In the sense of the conception of mental "flow" states as defined by Croatian-born American psychologist Mihály Csíkszentmihályi, it should be note that the thermodynamic force is still the same as in the biological case, only this type of theory has yet to be published.

References
1. Dill, Ken A. and Bromberg, Sarina. (2003). Molecular Driving Forces: Statistical Thermodynamics in Chemistry and Biology, (ch. 7: “Thermodynamic Driving Forces”, pgs. 105-28). Taylor & Francis.
2. Perrot, Pierre. (1998). A to Z of Thermodynamics, (section: "force", pg 116). Oxford: Oxford University Press.
3. Kondepudi, Dilip. (2008). Introduction to Modern Thermodynamics, (section: Biological Systems, pg. 379). John Wiley and Sons.
4. Gräber, Peter and Milazzo, Giulio. (1997). Bioenergetics, (pg. 9). Birkhäuser.