[GS] Physics [Science] Capillarity, Viscous Force, Viscocity & Critical Velocity

[GS] Physics

[Science]

Capillarity, Viscous Force, Viscocity & Critical Velocity

• Capillarity: If a capillary tube is dipped in a liquid, liquid ascends or descends in this tube.This phenomenon is called capillarity.

The height by which liquid ascends or descends in a capillary tube depends upon the radius of the tube.

The capillarity depends on the nature of liquid and solid both.The liquid which wets the wall of tube rises in the tube and the liquid which does not wet the wall of the tube descends in the tube.

For example, when a glass capillary tube is dipped in water, water rises in the tube and shape of water is concave,similarly when a glass capillary tube is dipped in mercury,mercury descends in the tube and shape is convex.

• A piece of blotting paper soaks ink because the pores of the blotting paper serve as capillary tubes.

• The oil in the wick of a lamp rises due to capillary action of threads in the wick.

• The root hairs of plants draws water from the soil through capillary action.

• Action of towel in soaking up water from the body is due to capillary action of cotton in the towel.

• Viscous force: The force which opposes the relative motion between different layers of liquid or gases is called viscous force.

• Viscosity: It is the property of a liquid by virtue of which it opposes the relative motion between its different layers.

• Viscosity is the property of liquids and gases both.

• The viscosity of a liquid is due to cohesive force between its molecules.

• The viscosity of a gas is due to diffusion of its molecules from one layer to other layer.

• Viscosity of gases is much less than that of liquids.There is no viscosity in solids.

• Viscosity of an ideal fluid is zero.

• With rise in temperature,viscosity of liquids decreases and that for gases increases.

• Viscosity of fluid is measured by its coefficient of viscosity.

• Critical velocity: The maximum velocity up to which fluid motion is streamline is called critical velocity.Clearly,if the velocity of flow is below critical velocity,flow is streamline and of the velocity is above the critical velocity,flow is turbulent.

If the velocity of flow is less than critical velocity,the rate of flow of fluid depends on viscosity of fluid.If the velocity of fluid is more than critical velocity,the rate of flow depends on the density of fluid.Due to this reason,on eruption of the volcano the lava coming out of its flow very swiftly although it is very dense having large viscosity.