Priyanka Das Tithi(Phy-111)-Surface Tention

Department of PhysicsIUBAT

May 21, 2012

WELCOME TO THE PHYSICS CLASS

Conducted by :

Priyanka Das Tithi

OBJECTIVES: TO KNOW THE ANSWERS OF THE FOLLOWING QUESTIONS. Why does a liquid drop become spherical in shape?

Why does a needle or paper clip float on water even though it is several times as dense as water?

Why does water wet an object but mercury does not though both are liquid?

Why do we use hot water to clean dirty cloths which are not easily cleaned by cold water?

WHAT IS FLUID?Fluid is something which can flow.

Fluid

Incompressible fluid

(Liquid)

Compressible fluid(Gas)

Surface tension is the special property of liquid which is the main point of today's discussion.

The surface of a liquid behaves somewhat like a stretched elastic membrane as if it has an invisible skin.

Surface Tension is an upwards force that enables insects such as water strider “to walk on water”.

SURFACE TENSION Definition:

The force of attraction per unit length of the boundary surface of the liquid is called surface tension.

Unit:Newton/meter (N/m).

Dimension:

SOME TERMS RELATING TO SURFACE TENSION Cohesive force:

The force of attraction between molecules of the same substance is called the cohesive force.

Adhesive force:The force of attraction between molecules of different substance is called the adhesive force.

Molecular Range:The maximum distance at which the molecules can attract each other is called molecular range.

Sphere of influence:A sphere having a radius equal to the molecular range with the molecules as the centre is called the sphere of influence.

Molecular theory of surface tension:

SURFACE ENERGY Definition:

The amount of work done to increase the surface area by unity is called surface energy.

Unit:

Dimension:

RELATION BETWEEN T AND E :The net force of pull on the wire due to the surface tension of both surfaces is F = 2TLThe work done against the surface tension by the external force is, W= Fx =2LTxThe increased area of the frame = 2LxSurface energy,

So, Surface Tension=Surface Energy

Angle of Contact : The angle between the tangent to theliquid surface and the solidsurface at the contact insidethe liquid is called the angleof contact. Capillarity : The rise or fall of a

liquid in the capillary tube is called

The capillarity.

THEORY OF CAPILLARITY :

Total upward force due to the surface tension is,

This force supports the weight of the liquid column.

So,

Some practical examples of surface tension.

The cause of becoming spherical shape of a liquid drop is “surface tension”.

If carefully placed on the surface, a small needle or paper clip can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink.