WBCHSE Class 11 Physics Viscosity And Surface Tension Long Answer Questions

Viscosity And Surface Tension Long Answer Type Questions And Answers

Question 1. What is velocity gradient? What is its dimension?
Answer:

In a horizontal streamline flow, the rate of change of velocity with distance \(\left(\frac{d u}{d x}\right)\) a direction perpendicular to the flow of the liquid is called the velocity gradient.

Dimension of velocity gradient = \(\left(\frac{d u}{d x}\right)=\frac{\mathrm{LT}^{-1}}{\mathrm{~L}}=\mathrm{T}^{-1} \text {. }\)

Question 2. How can you detect whether the motion of a liquid is streamlined or turbulent?
Answer:

Let a liquid flow through a narrow pipe of uniform cross-section. A coloured solution is injected along the axis of the pipe at the point of entry. If this coloured solution flows like a thread along the axis of the pipe, then it is in streamlined motion. The coloured solution spreads all over the liquid in the case of turbulent motion.

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Question 3. Write down the characteristics of streamline motion.
Answer:

Characteristics of streamline motion:

1. The velocity of a fluid particle at any point of a streamline remains the same in magnitude and direction it does not change with time.
2. The layer of liquid in contact with the solid surface remains at rest, i.e., the velocity of that layer is zero.
3. In streamline motion, a liquid is assumed to be arranged in parallel layers one over the other.
4. Two streamlines never intersect each other.
5. In the tube of flow, if the streamlines get crowded, then the velocity of fluid flow is greater there, but if the streamlines remain comparatively apart then the velocity of fluid flow is less there.

Question 4. Discuss the differences between viscosity and friction.
Answer:

WBBSE Class 11 Viscosity and Surface Tension Long Answer Questions

Question 5. Why do two streamlines never intersect each other?
Answer:

If two streamlines intersect each other, then, at the point of intersection, we can draw two tangents to the two streamlines, which would imply two different directions of the velocity of the particle at that point. But in a streamline any particle can move only in one direction and hence two streamlines never intersect each other.

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Question 6. Why does the end of a glass rod become round on heating?
Answer:

When glass starts melting on heat absorption, the liquid surface tries to attain minimum surface area due to the property of surface tension. So the end of a glass rod attains a hemispherical shape.

Question 7. Why does machine parts get jammed in winter?
Answer:

The temperature of the atmosphere decreases in winter. So the viscosity of the lubricating oil increases very rapidly. Due to this reason the machine parts which are directly in contact with the oil. do not move smoothly, that is why they get jammed.

Question 8. What is an ideal fluid?
Answer:

An ideal fluid is incompressible, i.e., the density or volume of that fluid does not change on pressing it. It is non-viscous too. There is no tangential force acting between two adjacent layers of that fluid.

Question 9. Between two lubricating oils A and B, the coefficient of viscosity of A is greater than that of B. For a machine, which one of them is suitable in summer?
Answer:

In summer, the viscosity of a liquid decreases due to increase in temperature. A is more suitable because its coefficient of viscosity is greater.

Question 10. During a cyclone, bits of paper, leaves from a tree, etc.. enter the twister and move upwards revolving continuously. Explain.
Answer:

During a cyclone, the velocity of air inside the twister is greater than that of air outside it and hence the air pressure within the twister becomes low.

So, bits of paper, leaves, etc., enter the centre of the twister from the outer high-pressure zone. Inside the twister, the speed of air in the upward direction is higher; so the pieces of paper, leaves, etc., keep moving upwards revolving continuously.

Question 11. Discuss the importance of the streamlined shape of a fish.
Answer:

A fish experiences viscous drag force while moving through water. The body of a fish is tapered at the head and at the tail, and is compressed at the sides. This is a type of streamlined shape.

For this reason, a fish experiences less viscous drag force while swimming through water and the water flowing by the fish follows streamlines. So they are able to control their direction of motion very easily. For the same reason, the shape of airplanes, fast moving trains, or racing cars, are made streamlined.

Surface Tension Measurement Techniques

Question 12. State whether critical velocity and terminal velocity are the same?
Answer:

Critical velocity and terminal velocity are not the same. The critical velocity of a liquid is the limiting velocity for streamlining flow of the liquid, but terminal velocity is the constant velocity acquired by a body moving through a fluid.

Question 13. A lead ball is allowed to fall through an elongated column filled with glycerine. What sort of graph would we get on plotting the velocity (v) and the distance traversed (s) by the lead ball?
Answer:

Glycerine is a viscous liquid. We know that the velocity of the lead ball will increase at first, and, after sometime, it will move with a uniform velocity (terminal velocity). The v-s graph or velocity-displacement graph obtained is as shown in Fig.

Question 14. To make a piece of paper float horizontally in air, we allow air to flow over the upper surface of the paper, but not below its lower surface. Why?
Answer:

If air is allowed to flow horizontally over the upper surface of the paper, the velocity of air above the paper will be comparatively higher than that below it. According to Bernoulli’s theorem, the lower surface of the paper will experience a higher pressure than the upper surface. The resultant upward pressure keeps the piece of paper floating horizontally in air.

Question 15. Why flags flutter in a windy day?
Answer:

Wind generally flows in different velocities by the two sides of a flag. According to Bernoulli’s theorem, the air pressure becomes lower at the side where the velocity of wind is more. A flag flutters in a windy day due to this difference in air pressure and also due to the random push by the particles present in the atmosphere.

Question 16. A large drop of water breaks up into a large number of small droplets. Does the surface energy increase?
Answer:

• In this case, the surface energy will increase.
• It can be shown by calculation that the total surface area of the smaller droplets of water is greater than the surface area of the large drop.
• So, during the breaking up of a large drop of water into many small droplets, the surface area increases. So, increase in surface area x surface tension = increase in surface energy.

Question 17. If a large number of water droplets coalesce to form a single large drop, then state whether the total surface energy increases or decreases. Explain.
Answer:

In this case, the surface energy decreases.

• The total surface area of the small droplets is greater than the surface area of the large drop formed.
• So, when a large number of water droplets coalesce to form a single large drop, the surface area decreases.
• Hence, it indicates a decrease in surface energy, because decrease in surface energy = decrease in surface area x surface tension.

Real-Life Examples of Surface Tension Effects

Question 18. Why are small drops of water in air spherical in shape?
Answer:

Due to surface tension, the liquid surface always tries to contract itself to minimise its surface area. Among all objects of equal volume, the surface area of a sphere is the minimum and hence in air every small drop of water takes the shape of a sphere.

Question 19. If a few spherical drops of a liquid coalesce to form a larger drop, will its temperature rise or fall? Explain.
Answer:

When a few spherical drops of a liquid coalesce to form a larger drop, the surface area decreases. As a result, some surface energy is released. This surface energy is converted into heat energy, thereby the temperature of the large drop increases.

Question 20. Mention a pair of solid and liquid for each of the following cases where the angle of contact is

1. 90°
2. less than 90°
3. more than 90°.

Answer:

1. In the case of silver and water, the angle of contact is
   9°.
2. In the case of glass and water, the angle of contact is less than 90°.
3. In the case of glass and mercury, the angle of contact is more than 90°.

Question 21. Why does water stick to the fingers, but mercury does not?
Answer:

• Since the angle of contact of water with respect to our body is acute, it sticks to our fingers. It means that the adhesive force between water and our fingers is higher than the cohesive force between water molecules.
• But the angle of contact of mercury with respect to our body is obtuse and hence it does not stick to our fingers. In this case, the cohesive force between mercury molecules is higher.

Question 22. Why does water rise through a capillary tube whereas mercury goes down through it?
Answer:

We know that if h is the rise of a liquid in a capillary tube, then h = \(\frac{2 T \cos \theta}{r \rho g} .\)

Here, T = surface tension, r = radius of the tube, p = density of the liquid, θ = angle of contact.

• Now, in the case of water, the angle of contact θ < 90°. So cosθ is a positive quantity and hence h is positive. So, water rises in a capillary tube.
• In the case of mercury, the angle of contact θ > 90°. So, cosθ is a negative quantity and hence h is negative. So, mercury goes down in a capillary tube.

Comparative Analysis of Viscosity and Surface Tension

Question 23. Why does the nib of a fountain pen have a slit at its centre?
Answer:

The slit helps the ink to flow to the tip of the nib through capillary action; sometimes against the force of gravity.

Question 24. Why do we use a detergent to wash dirty clothes?
Answer:

Since water has a comparatively high surface ten¬sion, it cannot penetrate the minute pores of dirty clothes. On mixing a detergent with water, the surface tension decreases; so water enters the pores and washes out the dirt of the clothes.

Question 25. To what height will water rise in a capillary tube provided there is no gravity acting on it?
Answer:

In the absence of gravity, there is no resistance against the rise of a liquid in 3 capillary tube due to surface tension. So, if the length of the capillary tube is infinite, water will rise to that infinite height.

Again, if the tube is of finite length, water fills it up completely. Even if water spills out, due to capillary action, more water will be drawn into the tube to fill it completely.

Capillary rise, \(h=\frac{2 T \cos \theta}{r \rho g}\)

In absence of gravity, g = 0.

So, h → ∞.

Question 26. In an experiment on surface tension, water rises up to a height of 0.1 m in a capillary tube. If the same experiment is performed in a satellite moving around the earth, what will be the rise in the capillary tube?
Answer:

The weight of the water column in the capillary tube will be zero in an orbiting satellite. Hence, due to surface tension, water will rise up to the top of the tube, and the capillary tube will be completely filled with water.