PRESENTED BY ANKIT SHARMACLASS:11 C

PRESENTATION ON PHYSICS

CONTENTS

SURFACE TENSION.VISCOSITY.

I. INTRODUCTION

Fluid Mechanics, physical science dealing with the action of fluids at rest or in motion, and with applications and devices in engineering using fluids. Fluid mechanics is basic to such diverse fields as aeronautics chemical, civil, and mechanical engineering meteorology, naval architecture and oceanography

SURFACE TENSION

PROPERTIES OF LIQUID:*In this presentation we will Study about various properties of liquids.

FORMATION OF BUBBLE

The surface tension of water provides the necessary wall tension for the formation of bubbles with water. The tendency to minimize that wall tension pulls the bubbles into spherical shapes

SURFACE TENSION

FORCE OF ADHESION:Adhesion, attraction between the surfaces of two

bodies. The term is sometimes used to denote the tendency of two adjacent surfaces, which may be of different chemical compositions, to cling to each other, whereas cohesion is used to refer to the attraction between portions of a single body. For example, if a sheet of glass is lowered into water and withdrawn, some water will cling to the glass (adhesion) but the rest will be pulled back into the main body of water (cohesion).

EXAMPLE OF SURFACE TENSION

Surface Tension in Water Droplets Surface tension causes these water droplets on leaves to bead up and form the smallest surface possible. The water molecules at the surface are pulled in by the cohesive force between themselves and molecules inside the droplet. The water keeps its droplet shape because there are no water molecules outside the surface to balance this inward pull

SURFACE TENSION

FORCE OF COHESION:

Cohesion, phenomenon of intermolecular forces holding particles of a

substance together. Cohesion differs from adhesion in being the force of

attraction between adjacent particles within the same body; adhesion is the

interaction between the surfaces of different bodies. The force of cohesion in

gases can be observed in the liquefaction of a gas, which is the result of a

number of molecules being pressed together to produce forces of attraction

high enough to give a liquid structure.

Cohesion in liquids is reflected in the surface tension caused by the

unbalanced inward pull on the surface molecules, and also in the

transformation of a liquid into a solid state when the molecules are brought

sufficiently close together. Cohesion in solids depends on the pattern of

distribution of atoms, molecules, and ions, which in turn depends on the state

of equilibrium (or lack of it) of the atomic particles. In many organic

compounds, which form molecular crystals, for example, the atoms are bound

strongly into molecules, but the molecules are bound weakly to each other.

EXAMPLE OF SURFACE TENSION Capillary Action, elevation or depression of the surface of a liquid where it is in contact with a solid, such as the sides of a tube. This phenomenon is an exception to the hydrostatic law that a liquid seeks its own level. It is most marked in capillary tubes, that is, tubes of very small diameter. Capillary action depends on the forces created by surface tension and by wetting of the sides of the tube. If the forces of adhesion of the liquid to the solid (wetting) exceed the forces of cohesion within the liquid (surface tension), the surface of the liquid will be concave, and the liquid will rise up the tube, that is, it will rise above the hydrostatic level .

EXAMPLE OF SURFACE TENSION

Osmosis The experiment shown above demonstrates the process of osmosis. Water flows through a semipermeable membrane into a sugar solution, diluting the solution. The sugar molecules cannot pass through the membrane, so the water outside remains pure.

EXAMPLES OF SURFACE TENSION Soap This diagram represents the action of soap

on dirt in a fabric. Once soap has dissolved in water, its molecules will surround any patch of dirt on the fabric, forming a ring around it called a micelle. This occurs because soap molecules have “ends” that differ in their properties. One end is attracted to water (hydrophilic), the other is attracted to nonsoluble substances such as oil and grease (hydrophobic). When soap molecules attach themselves to grease stains, they form a new surface that is soluble in water. Cleaning action is the absorption of dirt and grease into the center of soap micelles, which transforms a stain into a soluble substance that can be rinsed away.

EXAMPLES OF SURFACE TENSION

Surface Tension in Fluids Surface tension is caused by forces between the molecules within a fluid pulling on each other. Within a fluid, molecules are generally surrounded by neighbouring molecules, as seen here in this illustration with Molecule B. However, it is different at the fluid surface, as suggested with Molecule A. At the surface Molecule A is not pulled equally in all directions, because there are no fluid molecules above it. The result is that the surface molecules are pulled tightly into the

Surface Tension and Droplets

Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. The spherical shape minimizes then necessary "wall tension" of the surface layer according to LaPlace's law. At left is a single early morning dewdrop in an emerging dogwood blossom.

Surface tension and adhesion determine the shape of this drop on a twig. It dropped a short time later, and took a more nearly spherical shape as it fell. Falling drops take a variety of shapes

due to oscillation and the effects of air friction.

CAPILLARY ACTION IS THE RESULT OF ADHESION AND

SURFACE TENSION. ADHESION OF WATER TO THE WALLS OF A

VESSEL WILL CAUSE AN UPWARD FORCE ON THE

L IQUID AT THE EDGES AND RESULT IN A MENISCUS WHICH TURNS UPWARD. THE SURFACE TENSION ACTS TO HOLD THE

SURFACE INTACT, SO INSTEAD OF JUST THE EDGES MOVING UPWARD, THE WHOLE L IQUID

SURFACE IS DRAGGED UPWARD.

Capillary Action

PASCAL’S LAW

Pascal’s Law Pascal’s law, developed by French mathematician Blaise Pascal, states that the pressure on a fluid is equal in all directions and in all parts of the container. As liquid flows into the large container at the bottom of this illustration, pressure pushes the liquid equally up into the tubes above the container. The liquid rises to the same level in all of the tubes, reguardless of the shape or angle of the tube.

LAMINAR AND TURBOLENT FLOW

Laminar and Turbulent Motion by equations derived by CAt low velocities, fluids flow in a streamlined pattern called laminar motion. Laminar motion can be described mathematically laude Navier and Sir George Stokes in the mid-1800s. At high velocities, fluids flow in a complex pattern called turbulent motion. For fluids flowing in pipes, the transition from laminar to turbulent motion depends on the diameter of the pipe and the velocity, density, and viscosity of the fluid.

Internal Energy Internal energy is defined as the energy associated with the random,

disordered motion of molecules. It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the

invisible microscopic energy on the atomic and molecular scale. For example, a room temperature glass of water sitting on a table has no

apparent energy, either potential or kinetic . But on the microscopic scale it is a seething mass of high speed molecules traveling at hundreds of meters

per second. If the water were tossed across the room, this microscopic energy would not necessarily be changed when we superimpose an ordered

large scale motion on the water as a whole.

Microscopic Energy Internal energy involves energy on the microscopic scale.

For an ideal monoatomic gas, this is just the translational kinetic energy of the linear motion of the "hard sphere" type atoms , and the behavior of the system is well described by kinetic theory. However, for polyatomic gases there is rotational and vibrational kinetic energy as well. Then in liquids and solids there is potential energy associated with the intermolecular attractive forces. A simplified visualization of the contributions to internal energy can be helpful in understanding phase transitions and other phenomena which involve internal energy.

Pressure of the bubble

The pressure difference between the inside and outside of a bubble depends upon the surface tension and the radius of the bubble. The relationship can be obtained by visualizing the bubble as two hemispheres and noting that the internal pressure which tends to push the hemispheres apart is counteracted by the surface tension acting around the cirumference of the circle.

For a bubble with two surfaces providing tension tension, the pressure relationship is:

Pressure relationship

The net upward force on the top hemisphere of the bubble is just the pressure difference times the area of the equatorial circle:The surface tension force downward around circle is twice the surface tension times the circumference, since two surfaces contribute to the force:

This gives

This latter case also applies to the case of a bubble surrounded by a liquid, such as the case of the alveoli of the lungs.

Hydraulic Lift The hydraulic lift works on the principle that the effort required to move something is the product of the force and the distance the object is moved. By using an incompressible fluid to transmit the force, the hydraulic lift allows a small force applied over a large distance to have the same effect as a large force applied over a small distance. In order to fill the large cylinder under the car with fluid, however, the small pump must be operated many, many times.

Applications of Fluid Mechanics

The laws of fluid mechanics are observable in many everyday situations. For example, the pressure exerted by water at the bottom of a pond will be the same as the pressure exerted by water at the bottom of a much narrower pipe, provided depth remains constant. If a longer pipe filled with water is tilted so that it reaches a maximum height of 15 m, its water will exert the same pressure as the other examples (left). Fluids can flow up as well as down in devices such as siphons (right). Hydrostatic force causes water in the siphon to flow up and over the edge until the bucket is empty or the suction is broken. A siphon is particularly useful for emptying containers that should not be tipped.

Archimedes's Principle

An object is subject to an upward force when it is immersed in liquid. The force is equal to the weight of the liquid displaced. The apparent weight of a block of aluminium (1) immersed in water is reduced by an amount equal to the weight of water displaced. If a block of wood (2) is completely immersed in water, the upward force is greater than the weight of the wood. (Wood is less dense than water, so the weight of the block of wood is less than that of the same volume of water.) So the block rises and partly emerges to displace less water until the upward force exactly equals the weight of the block.