text: In some cases, the average velocity field  exists because of a force; for example, the equation might describe the flow of ions dissolved in a liquid, with an electric field pulling the ions in some direction (as in gel electrophoresis). In this situation, it is usually called the drift–diffusion equation or the Smoluchowski equation, after Marian Smoluchowski who described it in 1915 (not to be confused with the Einstein–Smoluchowski relation or Smoluchowski coagulation equation).
  Typically, the average velocity is directly proportional to the applied force, giving the equation:
  where  is the force, and  characterizes the friction or viscous drag. (The inverse  is called mobility.)