= Svedberg
{title2=$1\,\mathrm{S}=10^{-13}\,\mathrm{s}$}
A \b[svedberg], symbol $\mathrm{S}$, is a non-<SI> unit of <sedimentation coefficient>, with \b[$1\,\mathrm{S}=10^{-13}\,\mathrm{s}$]. A <sedimentation coefficient> is a settling <velocity> divided by the applied <acceleration>, so its physical dimension is <time>. It is a transport property under specified conditions, rather than the time required for a particle to cross a sample.<Sedimentation, creaming, centrifugation and diffusion in the IUPAC colloid terminology>[Table: sedimentation coefficient and Svedberg unit]{ref}
For example, let a particle have <sedimentation coefficient> $s=20\,\mathrm{S}=2\times10^{-12}\,\mathrm{s}$ in a centrifugal <acceleration> $a=5\times10^5\,\mathrm{m\,s^{-2}}$. Its local settling <speed> is $v=sa=10^{-6}\,\mathrm{m\,s^{-1}}$, or one micrometre per second. The <acceleration> in a rotating sample varies with radius, so this local calculation is not a whole-tube travel time.
Two joined particles do not generally have the sum of their separate <sedimentation coefficients>: joining changes both the net driving force and the resistance to motion. A label such as <16S ribosomal RNA> therefore describes sedimentation behavior, not a <molecular mass> of16 units or a count of16 components.
Back to article page
