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Mechanical Properties Of Fluids

Question
CBSEENPH11026211

Two glass plates are separated by water. If surface ension of water is 75 dyne/cm and area of each plate wetted by water is 8 cm2 and the distance between the plates is 0.12 mm, then the force applied to separate the two plates is

  • 102 dyne

  • 104 dyne

  • 105 dyne

  • 106 dyne

Solution

C.

105 dyne

The shape of water layer between the two plates is shown in figure

                

Thickness d of the film = 0.12 mm
                                  = 0.012 cm 

Radius R of the cylindrical face = d2 

Pressure difference across the surface 

                      = TR = 2Td

Area of each plate wetted by water = A 

Force F required to separate the two plates is given by

F =  pressure difference × area

    =  2Td A

Putting the given values, we get

F = 2 × 75 × 80.012

F = 105 dyne

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