Viscosity and Surface Tension: Important part-A 2 marks Short Questions and Answers

ANNA UNIVERSITY PART 'A' QUESTIONS & ANSWERS

1) Define critical velocity of a fluid.

The velocity at which the steady, streamline flow changes into turbulent flow is called critical velocity.

2) Define streamline flow.

Streamline Flow

In a streamline flow of a fluid, the velocity of the fluid does not change with time, either in magnitude or in direction.

3) Define Coefficient of viscosity.

The Coefficient of viscosity is defined as the tangential force per unit area required to maintain a unit velocity gradient.

Tangential Force F ∝ surface area of the layer (A)

F ∝ Velocity gradient

 F ∝ A dv/dx

(or) F = ‒ηA dv/dx pascal second

where η is the Coefficient of viscosity. The negative sign indicates that force is acting opposite to the direction of viscosity.

4) Differentiate between streamline flow and turbulent flow.

Streamline flow

1. A stream line flow is the flow in which each particle in the liquid moves in the same path and has same velocity as that of that of its preceding particle.

2. The process is regular.

3. The velocity at every point within the liquid remains constant both in magnitude and direction and it does not exceed the critical velocity.

4. It does not cause eddies or whirlpools

5. Example: Flow of liquid through a capillary tube.

Turbulent flow

1. A turbulent flow is the flow in which the particles moves in different directions and with different velocities.

2. The process is irregular.

3. The velocity is not constant and it exceeds the critical velocity.

4. It causes eddies and whirlpools.

5. Example: Flow of liquid from a tap.

ADDITIONAL PART 'A' QUESTIONS

1) Define viscosity.

Viscosity or internal friction of the liquid is the property of a liquid by virtue of which the liquid opposes the relative motion between its different layers.

2) Give the examples for high viscous and low viscous liquids.

High viscous liquids: Thick liquids such as castor oil, honey, coal tar, glycine etc.,

Low viscous liquids: Thin liquids such as water, alcohol, spirit, kerosine oil etc.

3) Mention some practical applications of the knowledge of viscosity.

(i) Lubrication: The phenomenon of viscosity and its variation with temperature helps to use a suitable lubricant for a certain machine. Liquids with moderate viscosities are good lubricants for light machinery. High viscous oils are used as lubricants for heavy machinery.

(ii) Damping of instruments: The viscosity of air and some liquids are used for damping the motion of some instruments. Paddles or vanes of such instruments move in the liquid or air. The viscous force opposes the motion. Thus the moving parts of the instruments attain the final position quickly.

4) Which rain drops fall faster, bigger one or smaller one? why?

Velocity v = P/4ηl (r²‒x²)

∴ v ∝ r²

Since the velocity is directly proportional to the radius of the droplet, bigger rain drops fall faster than the smaller one.

5) Why do we find fine cloud particles floating in the sky?

Cloud is an assembly of a large number of small droplets which are formed by the condensation of water vapour on dust or small particles. Under the action of gravity, upthrust of air and viscous drag of air takes place, the downward terminal velocity 'v' of the droplet is given by

v = P/4ηl (r²‒x²)

∴ v ∝ r²

As the radius of the rain droplets present in the cloud is very small, the terminal velocity is also very small. Hence, cloud particles appear floating.

6) What will happen if the density of the medium is greater than that of the body?

If the density of the medium is greater than the density of the body, the terminal velocity becomes negative and the sphere moves upwards with a constant velocity. Slow bubbling out of CO₂ from a soda water bottle is an example of this phenomenon.

7) Define critical velocity.

The velocity at which the streamline flow is changed into turbulent flow is called critical velocity.

8) Is it possible to determine the viscosity of water by Stokes' method?

Stokes' method is applicable for determining the coefficient of viscosity of highly viscous liquids like castor oil. In the case of water (medium), there is slip between the body and medium. Hence this method is not suitable for liquids like water (or) less denser liquids.

9) Mention some practical applications of the knowledge of viscosity.

(i) Lubrication: The knowledge of viscosity and its variation with temperature helps to identity a suitable lubricant for a certain machine. Liquids with moderate viscosities are good lubrications for light machinery. High viscous oils are used as lubrications for heavy machinery.

(ii) Damping of instruments: The viscosity of air and some liquids are used for dampling the motion of some instruments. The viscous force opposes the motion. Thus the moving parts of the instruments attain the final position quickly.

10) Which rain drops fall faster, big ones or small ones? Why?

Terminal velocity v = 2/9 . r²g/η . (ρ‒ρ')

∴ i.e, v ∝ r²

Since the terminal velocity is directly proportional to the square of the radius of the rain drop, bigger rain drops fall faster than smaller ones.

11) What is the principle used in stokes' method to determine the viscosity of a high viscous liquid.

When a spherical object, like steel ball moves through a high viscous liquid, when the viscous force is equal to the buoyout force it attains the terminal velocity. By finding the termial velocity, the co‒efficient of viscosity shall be determined.

12. List any four applications of Stokes' law.

(i) Stokes' law/principle is used in understanding the phenomenon of settling of particles in liquids.

(ii) This method is used to find the viscocity of high viscous liquids.

(iii) Stokes' law is used in various fields such as sedimentation, Aerosol science etc.

(iv) This principle is also used in fluid dynamics, parachute designing etc.

13. Define Surface tension.

Definition

Surface tension (σ) is defined as the force per unit length acting along the surface of a liquid. It is measured in units of force per unit length (N/m).

The formula to calculate surface tension is

σ = F/L

Where,

• σ is the surface tension,

• F is the force acting along the surface, and

• L is the length over which the force acts.

14. Brief the principle used in drop weight method for determing the surface tension of the liquid.

This method is based on the principle that the weight of a drop falling slowly out of a capillary held vertically is directly proportional to its surface tension. The drop falls when its weight just exceeds surface tension.

15. It is easier to spray water when soap is added to it than when is is pure. Why?

Yes, when soap is added to water, the surface tension decreases. Therefore less work is required to spray water. Hence, it is easier to spray soap water than pure water.

16. In the case of mercury, there is capillary depression. Why?

In mercury there exist a capillary depression, because the cohesive forces in the liquid molecules are greater than the forces of attraction between the glass and liquid molecules.

17. Water wets the glass surface while mercury does not. why?

Water wets the glass surface, because the forces of adhesion between water and glass are greater than the forces of cohesion between water molecules.

18. If two bubbles of different size are blown at opposite ends of a pipe, what will happen?

If two bubbles of different sizes are blown at opposite ends of a pipe, then, the smaller bubble will contract and the larger bubble will expand, showing that the air flows from smaller bubble which has less pressure.

19. List any four applications of drop weight method.

(i) Drop weight method is used to determine the purity of liquids in pharmaceutical and chemical industries.

(ii) This method is used to check the quality of paints, lubricants, inks, beverages, etc.,

(iii) It is mainly used in formulation of detergents and surfactants.

(iv) This method is also used to study the effects of temperature and additives.