IB Physics The Centrifuge

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A tutorial sheet on the pressure difference in a rotating fluid causing centripetal force, a widely misunderstood concept.

  1. In a centrifuge a column of fluid of density ρ rotates in a horizontal plane at an angular speed ⍵. Show that the pressure in the fluid P at a distance r from the axis of rotation is given by P(r) = P0 +1/2 ρ ⍵2(r2 - r02), where P0 is the pressure at r = r0.
  2. Explain why a marble goes to the outside of the centrifuge and a cork to the inside. See The Physics Teacher, vol 34, October 1996, p 422, The Marble, Cork and Centrifuge by Thomas J Pickett.
  3. If the object is denser than the fluid the pressure in the fluid is not great enough to maintain the circular motion at that distance and so the object will drift outward through the fluid to a larger value of r.