TLS Online TPP Program

#Id: 2910

#Unit 13. Methods in Biology

The rate of sedimentation is dependent upon the applied centrifugal field (cm s-2), G,

that is determined by the radial distance, r, of the particle from the axis of rotation (in cm)

and the square of the angular velocity ω, of the rotor (in radians per second):

TLS Online TPP Program

#Id: 2911

#Unit 13. Methods in Biology

Since 3600 equals 2 radians, one revolution of the rotor can be expressed as 2 rad.

TLS Online TPP Program

#Id: 2919

#Unit 13. Methods in Biology

The relative centrifugal field (g), RCF, which is the ratio of the centrifugal
acceleration at a specified radius and the speed to the standard acceleration of gravity,
can be calculated from the following equation:

TLS Online TPP Program

#Id: 2921

#Unit 13. Methods in Biology

RCF units are therefore dimensionless (denoting multiples of g) and revolutions per
minute are usually abbreviated as r.p.m.

TLS Online TPP Program

#Id: 2922

#Unit 13. Methods in Biology

Nomograph for the determination of the relative centrifugal fi eld for a given rotor
speed and radius.

TLS Online TPP Program

#Id: 2927

#Unit 13. Methods in Biology

The rate of sedimentation is dependent not only upon the applied centrifugal field, but
also on the nature of the particle, i.e., its density and radius, and also the viscosity of the
surrounding medium. Stokes’ Law describes these relationships for the sedimentation of a
rigid spherical particle: