I need to calculate the surface tension of a liquid with the ring-method of du Nouy and the correct formula to use is:
γ=F4πR
I understand that the surface tension is the force that acts on a certain length which, in this case, is the circumference of the ring. Since the circumference of the ring is 2πR where does this extra ×2 come from?
Thanks!
Answer
If you look at a cross section of the ring the geometry looks like:
Note that the ring raises a cylinder of the fluid with an inside and ouside surface, and both surfaces pull on the ring. The force from each surface is γ2πr, so the total force is γ4πr.
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