Fluid flow: There are unbalanced forces in the system, there is no mechanical equilibrium, and parts of the system move.

Newton's Law of viscosity relates the force of friction between the layers of the fluid and the velocity gradient. In this Law a appears a constant of proportionality called viscosity.

\begin{equation} F_x=-\eta A \frac{dv_x}{dz} \end{equation}

Electrical conduction: When applying an electric field, charged particles (electrons, ions) move, producing an electric current.

Thermal conduction: There are temperature differences between the system and the surroundings or within the system, producing a flow of heat energy until the temperatures equalize.

The heat flow between two sources is described by Fourier's Law and is proportional to the temperature gradient. The constant of proportionality is called thermal conductivity.

\begin{equation}\frac{dq}{dt}=-kA\frac{dT}{dz}\end{equation}

Transport of matter: There are differences in concentrations of substances between different regions of the solution. A flow of matter occurs until the concentrations equalize.

The flow of matter is given by Fick's Law and is proportional to the concentration gradient. The constant of proportionality is called the diffusion coefficient.

\begin{equation}\frac{dn}{dt}=-DA\frac{dc}{dz}\end{equation}