surface tension | viscosity | physics short question answers | NEB physics | Knowledge of Physics

Q.1.) How do you define surface tension ? Write down it's units and dimensions.

Ans:- The property due to which  any liquid behaves as an elastically stretched membrane with the tendency to contract in order to occupy minimum surface area is called surface tension. Surface tension of a liquid is measured by the force or tension acting on unit length of an imaginary line on the liquid surface. The direction of the force is perpendicular to the unit length imaginary line and tangential to the liquid surface.

`\therefore` Surface Tension = `\frac{F}{l}` .......(1)

Where, F = acting force and l = length of imaginary line on the liquid surface.

Units of Surface Tension:

In SI unit, force is measured in Newton(N) and length is measured in meter(m).  Therefore, ratio of this force and the length has the units of newton per meter. Hence, the units of surface tension in SI system is Newton per meter (`Nm^-1`).

Dimensions of Surface Tension:

Dimension of force, F = [`MLT^-2`]

Dimension of length, l = [`M^0LT^0`]

Where, M means Mass, L means Length and T means Time.

Dimension of Surface Tension = `\frac{F}{l}` = `\frac{[MLT^-2]}{[M^0LT^0]}`

                                                  = [`ML^0T^-2`]

Thus, dimensions of surface tension are [`ML^0T^-2`]. This dimensional formula suggests that only mass and time are involved in surface tension. This means mass per unit square of time is the surface tension for given liquid surface.  

Q.2.) What are viscosity and coefficient of viscosity? Derive dimensional formula for coefficient of viscosity.

Ans:- 

Viscosity:

In liquids, the velocity of the layer in contact to fixed surface, that is at the bottom of the container, is zero. However, the velocity of liquid layer on the surface is maximum. In such a situation, the liquid molecules on the upper layer try to accelerate the molecules of lower layers. Unlikely the molecules on the lower layer try to decelerate  the molecules on the upper layer. In this way, two consecutive layers tend to destroy their relative motion as there exists dragging force between these layers. To maintain the relative motion between these layers, an external force needs to be applied. Thus, the property of liquid due to which it's different layers oppose their relative motion is known as viscosity.  

Coefficient of Viscosity(`\eta`):

The coefficient of viscosity of a liquid is can simply be defined as the tangential force per unit area of the liquid layer needed to maintain unit velocity gradient normal to the direction of flow.

`therefore` Coefficient of Viscosity = `\frac{F}{A \frac{dv}{dx}}` .......(1)

Where, F = Force, A = surface area of layer and `\frac{dv}{dx}` = velocity gradient.

Note: Velocity gradient is the change in velocity per unit change in linear distance or length.

Dimensional formula for coefficient of Viscosity:

Dimension of force, F = [`MLT^-2`]

Dimension of area, A = [`M^0L^2T^0`]

Dimension of velocity gradient, `\frac{dv}{dx}` = [`M^0L^0T^-1`]

`\therefore` `\eta` = `\frac{[MLT^-2]}{[M^0L^2T^0] \times [M^0L^0T^-1] }` [`\therefore` Using (1)]

                            = `\frac{[MLT^-2]}{[M^0L^2T^-1]}`

                           = `[ML^-1T^-1]`

`\therefore` `\eta` = `[ML^-1T^-1]` which is required dimensional formula for coefficient of viscosity.

Q. 3.) Why do the clouds seem floating in the sky?

Ans:- The clouds consist of very small raindrops. For small body, the terminal velocity is also small because the terminal velocity is directly proportional to the square of radius of the body. Therefore, the small drops of water in the clouds in the sky acquire terminal velocity far before it react to the ground. Hence, the water drops seem to be floating in the sky in the form of cloud.

Q.4.) Why does the flag flutter on strong wind?

Ans:- When the wind blow over the flag, the velocity of wind at the top of the flag is greater than the velocity at the bottom of the flag. According to Bernoulli's theorem, the pressure of air at the top becomes less than that at the bottom of the flag. Due to this difference in pressure, at two ends, the flag flutters.

Note:

Bernoulli's Theorem:

According to Bernoulli's theorem, the sum of the potential energy, kinetic energy and the pressure energy per unit mass of a given ideal fluid or  the system of liquid in a streamline motion is always constant.

`\therefore` `E = gh + \frac{v^2}{2} + \frac{P}{\rho}` = constant ..........(1)

This relation is called Bernoulli's equation.

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Q.5.) What is the effect of temperature on coefficient of viscosity of a liquid?

Ans:- Rise in temperature causes the kinetic energy of the molecules of liquid to increase. Increase kinetic energy decreases the force of cohesion between the molecules. The force of cohesion determines the viscosity of the liquid. The greater the cohesive force between the molecules, greater is the viscosity of the liquid. Hence, increase in temperature decreases the viscosity or vice-versa for the liquid.

    Note:

Cohesion: the state of sticking together tightly, molecular attraction by which molecules are attached together by mass.

Cohesive force: the force that causes molecules to attach together. It is produced by the molecular attraction in liquid.

Q.6.) Why do bubbles of air rise up through a liquid? 

Ans:- Since the density of air is less than the density of liquid through which the air blows, the terminal velocity of air is negative. Hence, the air bubbles moves up through a liquid. Smaller bubbles move slower than larger bubbles. It is because the terminal velocity of the bubble is directly proportional to square of it's radius. 

Q.7.) Why does small pin when placed gently over the surface of water floats?

Ans: Due to surface tension the free surface of water acts as a stretching membrane. So, when a small pin is placed gently in the surface of water, the surface tries to shrink and it supports the weight of the pin by providing upward force. Due to this fact, a small pin placed gently over water surface floats.

Q.8.) Why is it easier to wash clothes with hot water-soap solution?

 Ans: The surface tension of the liquid surface decrease with rise in temperature. Thus, the dirt particles get mixed easily with the soap solution if hot water is used. Hence, it is easier to wash clothes with hot water-soap solution.

Q.9.)  Explain why the rain drops are spherical in shape?

Ans: Rain drops are very small in shape. Therefore, the effect of gravity due to earth is negligible compared to the potential energy due to surface tension. The liquid tends to contract so that its surface area is minimum. Due to the least surface area, the surface of the liquid drop takes spherical shape. Therefore, the small liquid drops are spherical in shape.

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Q.10.) The large liquid drops are flat in shape, explain.

Ans: Due to high weight of the liquid drop, the effect of gravity on the drop is dominant over the potential energy due to surface tension. So, the drop tends to have as less potential energy as possible. To make the gravitational potential energy minimum, the drops flatten and lower the center of gravity. Due to this reason, the large liquid drops are spherical in shape.

Q.11.) What are effects of impurities on surface tension of water?

Ans: If the added impurities are soluble in liquid, it increases the surface tension due to attractive force of solute molecules. For example, impurities such as common salt and zinc sulfate enhance the surface tension of water. On the other hand, if the added impurities are insoluble in liquid, it reduces surface tension because of reduction of intermolecular force. For example, mixing of soap in water, phenol in water or kerosene oil in water reduces the surface tension of water.

Q.12.) Why does water remain cool in earthen pot?

Ans: An earthen pot consists of large number pores in it. The pores act as capillaries. The water inside the pot comes outside by capillary action through these capillaries. The water reaching the outer surface of the pot evaporates. The heat needed for water at `100^0`C to evaporate or convert to steam at `100^0`C is called latent heat of evaporation. This amount of heat needed to water of surface outside the pot is provided by the water inside the pot. This causes loss of heat for water inside the pot. Due to this loss of heat, the water inside the pot remains cool. 

Q.13.) Why does the wick of a lantern dipped in oil keep on burning?

Ans: Cotton wicks have a number of pores. These pores serve as fine capillaries. The oil in the capillaries of wick rises continuously and  it keeps burning, shown in above figure. 

Q.14.) The hairs of painting brush spread out when it remains dipped in water, explain why is that so?

Ans: The reason for this amazing phenomenon is surface tension of water. When the hairs of painting brush are dipped in water, there does not exists any free water surfaces on tip of the hairs inside water. No free surface means no surface tension. That is why the hairs of painting brush spread when dipped in water.

Additionally,

When the brush is taken out of the water, free surfaces of water exist on tip of hairs. The free surface of the water due to it's surface tension tend to contract in order to occupy minimum area. As a result, the hairs get clung together when taken out of the water.

Q.15.) An oil spread on cold water. However, it may remain a drop in hot water. Explain.

Ans: Surface tension is property that causes the liquid molecules stick together, and prevent the liquid from spreading. If the surface of the oil is less than that of water, it spreads on water. Otherwise it does not. The surface tension of cold water is higher than that of hot water, because increasing temperature decreases the surface tension. Since, the surface tension of  oil lies between surface tensions of cold and hot water. The oil spreads on cold water but remains as drop in hot water.