Surface Tension for Engineering Physics.pptx

PPT on Surface Tension for Engineering Physics

1. Surface Tension for Basic Physics 1 st Semester Diploma Course Streams: DETCE, DEE (Lecture 2) Course Coordinator Mrs. Soumi Patra Assistant Professor of Physics Guru Nanak Institute of Technology(GNIT),Kolkata

2. Table of Contents Introduction Concept of Cohesive and Adhesive force Effect of Surface Tension Surface Tension Molecular theory of Surface tension

3. Introduction Surface tension is the elastic tendency of a fluid surface which makes it acquire the least surface area possible. Surface tension allows insects (e.g. water striders), usually denser than water, to float and stride on a water surface. At liquid–air interfaces, surface tension results from the greater attraction of liquid molecules to each other (due to cohesion) than to the molecules in the air (due to adhesion). The net effect is an inward force at its surface that causes the liquid to behave as if its surface were covered with a stretched elastic membrane. Thus, the surface becomes under tension from the imbalanced forces, which is probably where the term "surface tension" came from

4. Concept of Cohesive and Adhesive force Cohesion is the property of like molecules (of the same) to stick to each other due to substance mutual attraction. Adhesion is the property of different molecules or surfaces to cling to each other. When the cohesive force of the liquid is stronger than the adhesive force of the liquid to the wall, the liquid concaves down in order to reduce contact with the surface of the all. When the adhesive force of the liquid to the wall is stronger than the cohesive force of the liquid, the liquid is more attracted to the wall than its neighbors, causing the upward concavity. Concave up Meniscus Concave down Meniscus

5. Effect of Surface Tension Water strider walking on the water surface Water beading on a leaf Paper clip floats on water

6. Surface Tension The property of a liquid due to which its free surface tries to have minimum surface area and behaves as if it were under tension some what like a stretched elastic membrane is called surface tension Surface tension of a liquid is measured by the force acting per unit length, the direction of this force being perpendicular to the line and tangential to the free surface of liquid. So if F is the force acting on one side of imaginary line of length L , then T = (F/L) It depends only on the nature of liquid and is independent of the area of surface. It is a scalar as it has a unique direction which is not to be specified. Dimension : [MT – 2 ]. (Similar to force constant) Units : N/m (S.I.) and Dyne/cm [C.G.S.] It is a molecular phenomenon.

7. Molecular Theory of Surface Tension The maximum distance up to which the force of attraction between two molecules is appreciable is called molecular range . A sphere with a molecule as centre and radius equal to molecular range is called the sphere of influence. The liquid enclosed between free surface (PQ) of the liquid and an imaginary plane (RS) at a distance r (equal to molecular range) from the free surface of the liquid form a liquid film. To understand the tension acting on the free surface of a liquid, let us consider four liquid molecules like A, B, C and D. Their sphere of influence are shown in the figure.

8. Examples of Surface Tension (1) When mercury is split on a clean glass plate, it forms globules. Tiny globules are spherical on the account of surface tension because force of gravity is negligible. The bigger globules get flattened from the middle but have round shape near the edges, figure (2) When a greased iron needle is placed gently on the surface of water at rest, so that it does not prick the water surface, the needle floats on the surface of water despite it being heavier because the weight of needle is balanced by the vertical components of the forces of surface tension. If the water surface is pricked by one end of the needle, the needle sinks down .

9. Surface Tension for Basic Physics 1 st Semester Diploma Course Streams: DETCE, DEE (Lecture 2) Course Coordinator Mrs. Soumi Patra Assistant Professor of Physics Guru Nanak Institute of Technology(GNIT),Kolkata

10. Table of Contents Factors affecting Surface Tension Applications of Surface Tension Surface Energy Angle of Contact Capillarity and Capillary Action Shape of Liquid Meniscus Question and Answer

11. Factors affecting Surface Tension Impurities: The presence of impurities in a liquid appreciably affect surface tension. Temperature: The surface tension decreases with rise in temperature. The temperature at which the surface tension of a liquid becomes zero is called critical temperature of the liquid. The variation in surface tension with temperature is linear and is given by the relation T t = T (1- α t) where T t , T are the surface tensions at t C and 0 C respectively and α is the temperature coefficient of surface tension. Examples: ( i ) Hot soup tastes better than the cold soup. (ii) Machinery parts get jammed in winter.

12. Applications of Surface Tension The oil, grease and dirt particles get mixed with soap solution easily. Hence clothes are washed easily. Due to low surface tension, the antiseptics spreads properly over wound. Surface tension of all lubricating oils and paints is kept low so that they spread over a large area. Oil spreads over the surface of water because the surface tension of oil is less than the surface tension of cold water. A rough sea can be calmed by pouring oil on its surface. In soldering, addition of ‘flux’ reduces the surface tension of molten tin, hence, it spreads.

13. Surface Energy The molecules on the liquid surface experience net downward force. So to bring a molecule from the interior of the liquid to the free surface, some work is required to be done against the intermolecular force of attraction, which will be stored as potential energy of the molecule on the surface. The potential energy of surface molecules per unit area of the surface is called surface energy. Unit : Joule/m2 (S.I.) erg/cm2 (C.G.S.) Dimension : [MT –2 ] Work Done in Blowing a Liquid Drop or Soap Bubble. If the initial radius of liquid drop is r 1 and final radius of liquid drop is r 2 then W = T x Increment in surface area W = T x 4 π [ r 2 2 – r 1 2 ] [drop has only one free surface] (2) In case of soap bubble W = T x 8 π [ r 2 2 – r 1 2 ] [Bubble has two free surfaces]

14. Angle of Contact When a liquid is in contact with a solid, the angle between tangent drawn to the free surface of the liquid and the surface of solid at the point of contact measured inside the liquid is called angle of contact. Characteristics: The angle of contact is constant for a given liquid-solid pair. The value of angle of contact depends upon nature of liquid and solid in contact. It depends upon the medium which exists above the free liquid surface. The angle of contact changes due to impurity or temperature.

15. Capillarity and capillary action The phenomenon of rise or fall of liquid level inside a capillary tube when it is dipped in the liquid is called capillary action or capillarity. Example: Blood capillaries, vascular bundles in plants tissue etc. If a capillary tube is dipped in a liquid which partially or completely wets the solid then the liquid level rises inside the tube. If a capillary tube is dipped in a liquid which does not wet the solid, then the liquid level falls inside the capillary tube. Applications of capillarity: i ) Blotting paper contains small pores. These pores act as capillaries and hence quickly absorb ink. ii) Wick in oil lamp contains threads, which act as capillaries and oil rises up in the wick of the lamp. iii) Sap and water rise upto the topmost leaves in the tree by capillary action. iv) Towel used in every day life is made up of cotton. It absorbs water or moisture from the body due to capillarity of towel. v) Piece of sponge retains water, due to capillarity.

16. Shape of Liquid Meniscus When a capillary tube is dipped in a liquid, the liquid surface becomes curved near the point of contact. This curved surface is due to the resultant of two forces i.e. the force of cohesion and the force of adhesion. The curved surface of the liquid is called meniscus of the liquid. If liquid molecule A is in contact with solid (i.e. wall of capillary tube) then forces acting on molecule A are ( i ) Force of adhesion F a (acts outwards at right angle to the wall of the tube). (ii) Force of cohesion F c (acts at an angle 45degree to the vertical). Resultant force F N depends upon the value of F a and F c . Water in silver coated capillary tube Water in glass capillary tube Mercury in glass capillary tube

17. Surface Tension for Basic Physics 1 st Semester Diploma Course Streams: DETCE, DEE (Lecture 2) Course Coordinator Mrs. Soumi Patra Assistant Professor of Physics Guru Nanak Institute of Technology(GNIT),Kolkata

18. Question and Answer 1. Explain why oil spreads over the surface of water while water does not do so? i . The surface tension of oil is less than the surface tension of water ii. Thus, when oil is dropped on the surface of water, it spreads in all directions due to the higher force of surface tension of water. 2. How are small insects able to walk on the water surface ? i . The free surface of water behaves as stretched membrane due to surface tension. ii. Due to weight of insect, the membrane is stretched. Thus surface tension acts in an inclined manner iii. The vertical component of surface tension balances the weight of the insect.

19. Question and Answer 3. What is the effect of temperature on angle of contact? As temperature increases, surface tension decreases thus angle of contact decreases. 4. The surface tension of water at 0 C is 70 dyne/cm. Find surface tension of water at 25 C. [ α for water = 0.0027/ C] Given : T = 70 dyne/cm, t 1 = 0 C, t 2 = 25 C, α = 0.0027 / C To find : Surface tension (T) Formula: T = T [1 – α ( t 2 - t 1 )] =70[1-0.0027(25-0)] = 65.275 dyne/cm 5. Why Rain drops are spherical? Due to surface tension, the liquid tries to acquire minimum surface area. For a given volume, sphere has minimum surface area, therefore the liquid assumes the shape of a drop.

20. Question and Answer 6 . Oil spreads on cold water but remains as a drop on hot water ? It is due to the fact that surface tension of oil is less than that of the cold water but is greater than that of the hot water. 7. Calculate the force required to take away a flat circular plate of radius 0.01 m from the surface of water. The surface tension of water is 0.075 N/m. Solution : Given: r = 0.01 m, T = 0.075 N/m Formula : F = T x l The plate is flat Force due to surface tension acts on only one face l = 2r , F = 0.075 (2 x 3.142 x 0.01) F = 0.004717 N

21. Question and Answer 8.A horizontal circular loop of wire of radius 0.02 m is lowered into crude oil and forms a film. The force due to surface tension of the liquid is 0.0113 N. Calculate the surface tension of crude oil . Solution : Given: r = 0.02 m, F = 0.0113 N Formula: F = Tl As there are two sides of a circular loop acted upon by the force, l = 2 x 2 π r . From formula, F = 4 π rT => T = F / 4 π r => T = 0.0113 / 4 x 3.142 x 0.02 = 0.04495 N/m

22. Question and Answer 9. A soap bubble of radius 12 cm is blown. Surface tension of soap solution is 30 dyne/cm. Calculate the work done in blowing the soap bubble. Solution : Given: r = 12 cm, T = 30 dyne/cm Formula : W = T x ∆A Initial surface area of soap bubble = 0 Final surface area A = 2 x 4 π r 2 Increase in surface area ∆A = 2 x 4 π r 2 From formula, W = T x 8 π r 2 = 30 x 8 x 3.14 x (12) 2 = 108518.4 erg = 108518.4 x 10 -7 J

23. Thank You