Name: Specific Heat Capacity of Water
Uploaded: 2019-07-19
Duration: 6 min 44 s
Description: Due to a lack of electricity, sometimes, we keep our cold drinks in water to maintain their coldness. Why? Because it takes time to draw heat out of water. W...

Question

Two glass discs, of radius

R = 5.0 cm

, were wetted with water and put together so that, the thickness of the water layer, between them, was

h = 1.9 μm

. Assuming the wetting to be complete, find the force that has to be applied at right angles to the plates, in order to pull them apart.

Accepted Answer

We know that, angle of contact for water glass interface is, $θ = 0 °$ . The situation is as shown in the figure.

Since angle of contact is, $θ = 0 °$ , so the force due to surface tension $F_{α}$ on glass plate at every point is horizontal in inward direction as shown in the figure.

For a circular cross-section of drop, resultant of force due to surface tension is zero.

Now curved surface of water is nearly cylindrical with radius, $R = \frac{h}{2}$ .

$∴$ Pressure difference $P_{0} - P_{1}$ at the meniscus is given by,

$P_{0} - P_{1} = \frac{α}{R}$
where $α$ , is the surface tension.

$\Rightarrow P_{0} - P_{1} = \frac{2 α}{h} . . . . . . . . (1)$

Now, force on the plate due to pressure difference on two side of plate is,

$F = P_{0} A - P_{1} A = (P_{0} - P_{1}) A \Rightarrow F = \frac{2 α}{h} A \Rightarrow F = \frac{2 α π R^{2}}{h} . . . . . . . (2)$

$[∵ A = π R^{2}]$

Hence, the force that has to be applied, at right angles to the plates in order to pull them apart is given by, $F = \frac{2 α π R^{2}}{h}$ .

Given, $R = 5.0 cm = 5 \times 10^{- 2} m$ and $h = 1.9 μm = 1.9 \times 10^{- 6} m$

Also, $ρ = 1000 kg m^{- 3}$ and $α = 0.072 N m^{- 1}$

$∴$ Putting in equation $(3)$ , we get,

$F = \frac{2 \times 3.14 \times {(5 \times 10^{- 2})}^{2} \times 0.072}{1000} \Rightarrow F = 0.6 kN$

Two glass discs, of radius $R = 5.0 cm$ , were wetted with water and put together so that, the thickness of the water layer, between them, was $h = 1.9 μm$ . Assuming the wetting to be complete, find the force that has to be applied at right angles to the plates, in order to pull them apart.

Important Questions on THERMODYNAMICS AND MOLECULAR PHYSICS

Learn and Prepare for any exam you want !

Two glass discs, of radius R=5.0 cm, were wetted with water and put together so that, the thickness of the water layer, between them, was h=1.9 μm. Assuming the wetting to be complete, find the force that has to be applied at right angles to the plates, in order to pull them apart.

Important Questions on THERMODYNAMICS AND MOLECULAR PHYSICS

Learn and Prepare for any exam you want !

Two glass discs, of radius $R = 5.0 cm$ , were wetted with water and put together so that, the thickness of the water layer, between them, was $h = 1.9 μm$ . Assuming the wetting to be complete, find the force that has to be applied at right angles to the plates, in order to pull them apart.