Centrifuges:
A centrifuge is a device that is used to separate suspending particles and lighter liquid from a more dense liquid. It consists of tubes suspended from a common central shaft which can turn rapidly. As the centrifuge turns, the liquids get separated- the heavier liquid is collected farther from the axis of rotation and the lighter liquid is found nearer the axis.
In order to understand the underlying principle of working, consider a small element ΔV of the liquid at a distance R from the axis of rotation. If the density of the liquid is ρ, then the centripetal force F required to keep the element in circular motion is such that
| F = (ρΔV)ω2R |
Where ω (=ν/R) is the angular speed and ρΔV is the mass of the element.
If the same element ΔV is now filled with a lighter liquid of density ρ'(ρ'<ρ), then the centripetal force F’ required to keep this element in circular motion is-
| F’ = (ρ’ΔV)ω2R |
Since the rest of the liquid continues to exert a force F the net force acting on the lighter liquid is (F- F’) where
| (F – F’) = (ρ – ρ’) ΔVω2R |
It is on account of this force acting towards the centre that the lighter particles are urged towards the centre and thus they are separated. It is to be noted that no force pushes the denser material outward rather the entire material is pulled inward as the centrifuge turns, however as the denser material responds less readily than the lighter material owing to greater inertia, the denser liquid is left behind.
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