Q:

# How Does the Principal of Centrifugation Enable a Centrifuge to Work?

A:

A centrifuge separates the components of a mixture, such as blood, by using centrifugal force to subject the sample to a force of gravity hundreds or thousands of times greater than normal Earth gravity. The density of a substance determines the degree by which it will be affected by gravity. When the force of gravity is greatly multiplied through centrifugal force, denser substances are physically separated, or forced out of, the portion of a sample which is of a lesser density.

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Centrifugal force is the force the pulls a rotating object or substance away from its center of rotation. This force is a result of the inertia of the spinning body and is relative to the mass of the spinning object, its distance from the center of rotation and the rotational speed.

If all factors are controlled and held equal except for the spinning object's density, then the centrifugal force exerted upon that object is proportional only to that object's density. In the case of a mixture spinning within a centrifuge, two or more unequal densities are involved. The result is that the component with the greatest density is forced into occupying a position which is the furthest from the center of rotation. When this occurs within a sealed container, such as a test tube, the components within a mixture separate based upon their individual densities.

The centrifugal force generated by a laboratory centrifuge enables substances to be separated selectively out of a sample mixture though the choice of different speed settings. By following a repeated process of centrifuge spins, or through an appropriately chosen speed setting, a sample can be brought to a state where a pure solution or liquid substance can be poured off, while anything of a greater density is left behind in the container.