Q#11
The contact angle between water and glass is 0°. When water is poured in a glass to the maximum of the capacity, the water surface is convex upward. The angle of contact on such a situation is more than 90°. Explain.Answer:
Due to the surface tension, the surface of the liquid acts as a stretched membrane. So even if some more water than the rim level of the glass is poured into it, the stretched water surface which holds the glass rim due to adhesion becomes convex upward and the contact angle between the water and the glass becomes more than 90° in this situation.
The zero degrees angle of contact for water and the glass is for the situation when the water level is below the rim level of the glass. In some special situations, the angle of contact may change.
Q#12
A uniform vertical tube of circular cross-section contains a liquid. The contact angle is 90°. Consider a diameter of the tube lying in the surface of the liquid. The surface to the right of this diameter pulls the surface on the left of it. What keeps the surface on the left in equilibrium?
Answer:
Let the diameter in consideration be AB as in the above figure. The surface to the right of AB pulls the surface on the left of AB (say by a total force = F) due to surface tension. The semicircular walls of the tube in contact with the surface to the left of AB also pulls it towards left with a net force F. Thus the semicircular surface ABD is pulled by two equal and opposite forces, making the net force on it zero. So it remains in equilibrium.
Q#13
When a glass capillary tube is dipped at one end in water, water rise in the tube. The gravitational potential energy is thus increased. Is it a violation of conservation of energy?
Answer:
No. It is not a violation of the conservation of energy. It can be explained due to two reasons:-
1. The gravitational potential energy of the part of the water that rises in the tube increases but simultaneously the level of the rest of the water goes down depending on the open surface area of the water. So some of the increased P.E. is compensated by the decrease in this water level.
2. The work done by the vertical component of the net force due to surface tension around the meniscus is stored as the net increase in the gravitational potential energy of the water in the tube.
Q#14
If a mosquito is dipped in water and released, it is not able to fly till it dry again. Explain.
Answer:
The wings are wet and stick together. To free the wings the mosquito needs to overcome the surface tension of the water which it finds unable to do. Without free wings, it is unable to fly. So it waits till the water evaporates and the wings are dry again.
Q#15
The force of surface tension acts tangentially to the surface whereas the force due to air pressure acts perpendicularly on the surface. How is then the force due to excess pressure inside a bubble balanced by the force due to the surface tension?
Answer:
The surface of the bubble is curved. The forces perpendicular to the surface has different directions from point to point. The effect of the pressure inside (which is perpendicular to the surface) over an area is to produce a net force tangential to the surface that tries to increase the surface area of the bubble. While the force of the surface tension (acting also tangentially) tries to decrease the area of the bubble. Thus the pressure and the surface tension forces balance each other.
Q#16
When the size of a soap bubble is increased by pushing more air in it, the surface area increases. Does it mean that the average separation between the surface molecules is increased?
Answer:
By pushing more air in the bubble the work is done to bring more molecules from the liquid contained between the outer and inner surface of the bubble to these two surfaces. So the more surface area is due to the inclusion of more molecules not due to an increase in average separation between the surface molecules.
Q#17
The frictional force between solids operates even when they do not move with respect to each other. Do we have a viscous force acting between two layers even if there is no relative motion?
Answer:
No. Though both frictional force and the viscous force oppose the relative motions, it is the difference between them that viscous force only comes into play when there is a relative motion between two layers.
Q#18
Q#13
When a glass capillary tube is dipped at one end in water, water rise in the tube. The gravitational potential energy is thus increased. Is it a violation of conservation of energy?
Answer:
No. It is not a violation of the conservation of energy. It can be explained due to two reasons:-
1. The gravitational potential energy of the part of the water that rises in the tube increases but simultaneously the level of the rest of the water goes down depending on the open surface area of the water. So some of the increased P.E. is compensated by the decrease in this water level.
2. The work done by the vertical component of the net force due to surface tension around the meniscus is stored as the net increase in the gravitational potential energy of the water in the tube.
Q#14
If a mosquito is dipped in water and released, it is not able to fly till it dry again. Explain.
Answer:
The wings are wet and stick together. To free the wings the mosquito needs to overcome the surface tension of the water which it finds unable to do. Without free wings, it is unable to fly. So it waits till the water evaporates and the wings are dry again.
Q#15
The force of surface tension acts tangentially to the surface whereas the force due to air pressure acts perpendicularly on the surface. How is then the force due to excess pressure inside a bubble balanced by the force due to the surface tension?
Answer:
The surface of the bubble is curved. The forces perpendicular to the surface has different directions from point to point. The effect of the pressure inside (which is perpendicular to the surface) over an area is to produce a net force tangential to the surface that tries to increase the surface area of the bubble. While the force of the surface tension (acting also tangentially) tries to decrease the area of the bubble. Thus the pressure and the surface tension forces balance each other.
Q#16
When the size of a soap bubble is increased by pushing more air in it, the surface area increases. Does it mean that the average separation between the surface molecules is increased?
Answer:
By pushing more air in the bubble the work is done to bring more molecules from the liquid contained between the outer and inner surface of the bubble to these two surfaces. So the more surface area is due to the inclusion of more molecules not due to an increase in average separation between the surface molecules.
Q#17
The frictional force between solids operates even when they do not move with respect to each other. Do we have a viscous force acting between two layers even if there is no relative motion?
Answer:
No. Though both frictional force and the viscous force oppose the relative motions, it is the difference between them that viscous force only comes into play when there is a relative motion between two layers.
Q#18
Water near the bed of a deep river is quiet while that near the surface flows. Give reasons.
Answer:
Answer:
It is due to the property of a liquid which is called viscosity. The viscosity of a liquid opposes the relative motion between layers of the liquid. The layer of the water in contact with the river bed is still because the river bed has no movement. The layer above it moves with some speed due to the resistance of the lower layer. And each layer moves with some relative speed than its lower layer. Thus a velocity gradient is formed with zero at the bottom and maximum near the surface.
Q#19
Q#19
If the water in one flask and castor oil in other are violently shaken and kept on a table, which will come to rest earlier?
Answer:
The castor oil has more viscosity than the water. It means the layers of castor oil will offer more resistance in the relative movement of the layers. So the castor oil will come to rest earlier.
The castor oil has more viscosity than the water. It means the layers of castor oil will offer more resistance in the relative movement of the layers. So the castor oil will come to rest earlier.
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