Force between two plates
If two flat plates are placed in contact with a thin
film of water between them it is found difficult to pull the two apart. This is due to the reduced
pressure within the water film, since the edges will be hemispherical (Figure 1).
Since the film is convex in one
direction and concave in the other the pressure difference (
Dp)
is given by:
Δp = T(1/r —
1/R)and the force (F) required to separate two plates of area A is given
by:
Force needed to separate the plates (F) = AT(1/r – 1/R)
Example problem
Calculate the force between two circular plates of radius 5 cm with a 0.01 mm thick film of water between them:
Using: F = AT(1/r – 1/R)
F = π(5x10-2)2x 72.7x10-3 x 199980 = 110 N
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