- : Particle radius (in the case of non-spherical bodies, half of a suitable equivalent diameter is used as an approximation instead of the particle radius .)
- : dynamic viscosity of the fluid in which the particle is located
- : Particle speed (the frictional force acts opposite to the speed).
Stokes' law is needed for the Millikan experiment .
If the spheres sinking in a gas are so small that they are in the same order of magnitude as the mean free path of the gas molecules, the normal formula becomes imprecise. This can be remedied by the Cunningham correction derived by the British mathematician Ebenezer Cunningham in 1910 :
: experimentally determined constants, where for air ( = 68 nm under standard conditions ) applies:
The following relationship can also be used as an approximation for air:
- G. G. Stokes: On the effect of the internal friction of fluids on the motion of pendulums. In: Transactions of the Cambridge Philosophical Society , Volume 9, 1851, pp. 8-106. ( Online )
- Physics and chemistry of atmospheric aerosol , including Stokes' law and Cunningham's correction.
- E. Cunningham: On the velocity of steady fall of spherical particles through fluid medium . In: Proc. Roy. Soc. A. Band 83 , 1910, pp. 357-365 .
- CN Davies: Definitive equations for the fluid resistance of spheres . In: Proc. Phys. Soc. tape 57 , 1945, p. 259-270 .