Laws Relate Forces to Flows

Pick s Law, Ohm s Law, and Fourier s Law Are Linear Laws that Relate Forces to Flows 

A single-phase system that is in thermodynamic equilibrium and not subject to external forces will have particles uniformly distributed in space. But a system having concentration gradients may not be in equilibrium. The thermodynamic tendency toward equilibrium is a tendency toward uniform concentrations. 

316 Chapter 18. Physical Kinetics Diffusion, Permeation Flow 

Steep concentration gradients cause large fluxes of particles. Small gradients cause small fluxes. Also, the larger the diffusion constant, the higher the flux, for a given gradient. Table 18.1 lists some molecular diffusion constants. 

Two other relationships have the same form as Pick s first law Fourier s law sa s that heat flow is proportional to the gradient of the temperature, and Ohm s law says that the flow of electrical current is proportional to the gradient of the voltage (see Chapter 22). 

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