Questions - Answers
Long Answer Question (8 Marks)
1. Describe how do you determine the Surface tension of a liquid using capillary rise method with relevant theory.
A: Determination of Surface tension by capillary rise method:
a) A long wire bent twice at right angles is taken and is attached to a clean dry glass capillary tube.
b) This tube is vertically dipped in a glass beaker containing the liquid and the liquid rises into the tube upto a certain extent.
c) The height of the tube is adjusted, so that the tip of wire B, just touches the surface of the liquid in the beaker.
Experimental arrangement
d) Microscope is focussed to the lower meniscus 'A' of liquid in the tube and reading R1 is noted. Then, the liquid beaker is slowly removed without disturbing the tube and wire. Then microscope is focussed against tip of 'B' and reading R2 is noted.
e) The difference between the two readings of microscope gives the height of raised liquid (h).
f) The diameter of the capillary tube is measured using microscope by keeping the tube horizontal and 'r' is determined.
g) The surface tension 'T' of the given liquid can be determined on substituting the values of h, r, d and g in the equation, where d is density of the liquid, g - acceleration due to gravity.
Theory:
i) Let a capillary tube of inner radius 'r' is dipped into a liquid of angle of contact θ, which is less than 90º.
ii) Liquid rises up in the capillary tube to a height 'h' with a Surface tension 'T' having a clear lower meniscus.
iii) Surface tension is resolved into two mutually perpendicular components; T cos θ parallel to glass wall and T sin θ perpendicular to glass wall of capillary tube.
Capillary rise Volume of the liquid in meniscus
iv) Horizontal components of 'T' at the diametrically opposite points cancel each other. But the vertical components all around the circumference of the circle of contact remain and will be added.
vii) For the equilibrium condition of the liquid meniscus,
The net upward force = Weight of the raised liquid
viii) For a narrow tube 
is negligible and for water θ = 0º, [cos 0º = 1]
Hence there is no resultant force on the molecule A, that is much inside the liquid.
iii) It is clear that the upward cohesive forces are lesser than that of downward cohesive forces acting on the molecule B, which is just below the surface. Hence there lies a resultant downward force on B.
iv) As the upward cohesive forces are absent on molecule C, lies on liquid surface, it experiences a maximum downward force.
v) Due to this downward force, the liquid surface behaves like a stretched elastic membrane. Liquid then tend to acquire minimum surface area in order to have minimum potential energy and greater stability. This tendency is called surface tension.
2. Obtain an expression for the excess pressure inside a liquid drop.
A: i) The condition for the shape of the liquid drop in equilibrium is that the potential energy is to be minimum. If surface tension alone, is considered the potential energy will be minimum for minimum surface area and hence the drop must be of spherical shape.
ii) The net force on the molecule present near the surface of a spherical liquid drop are subjected to resultant inward pull due to surface tension. Hence pressure inside the drop will greater than the pressure outside.
iii) Let us consider the equilibrium along any diametrical plane of spherical drop of radius r. The surface tension acts along the rim of length 2Πr and force due to excess pressure P acts on the area Πr2.
Excess pressure in liquid drop
iv) Under the equilibrium, the upward force acting due excess pressure 'P' on the hemisphere is equal to the downward force acting along circumference due to surface tension of liquid.
i.e., P (Πr2) = T (2Πr2)
3. What is the significance of capillarity in daily life?
A: Phenomena of capillarity help us many ways in daily life. A few examples among them are:
1) Action of towel in soaking up moisture from the body is due to capillary action of cotton in the towel.
2) The walls gets dampened in rainy season due to absorption of water by bricks by capillary action.
3) The flow of ink through a nib is due to capillary action.
4) The rise of oil in the wick of lamp is due to capillary action.
4. Distinguish between Streamline and turbulent flow of liquids?
A: Differences between streamline flow and turbulent flow:
5. A spinning ball through air takes a curved path then a ball that doesnot have spin what force come in to play for a spinning ball?
A: i) If a moving ball spins in air, it drags some air with it. So, on the top of the ball, the translational velocity and the linear velocity due to rotation of the ball act in the same direction (V +
VR)
ii) At the bottom of the ball, the two velocities act in opposite directions (V - VR)
iii) As (V - VR) < (V + VR), from Bernoulli's principle, due to difference in velocities, the pressure on the top of the ball is less and is more at the bottom. Due to this pressure difference an upward force acts on the ball. This is called dynamic lift.
iv) Force of buoyancy come into play for a spinning ball.
6. Define Bernoulli's theorem?
A: "When a non viscous fluid flows between two points, the sum of the pressure energy, kinetic energy and potential energy of the fluid per unit mass is constant at all points in the path offlow".
Very Short Answer Questions (2 Marks)
1. Define Surface tension and give its dimensional formula.
A: The tangential force acting per unit length of an imaginary line drawn on the surface and perpendicular to the length is called Surface tension.
Dimensional formula: MLoT-2.
2. Explain why rain drops are spherical in nature?
A: Because, for a given volume sphere has minimum surface area. Surface area is minimum due to Surface tension. So rain drops are spherical in nature.
3. What are cohesive, adhesive forces?
A: The force of attraction between the molecules of same substance is called cohesive force
The force of attraction between the molecules of different substance is called adhesive force
