SURFACE

TENSION

Course: Diploma

Subject: Applied Science

Physics

Unit: II

Chapter: II

Cohesive & Adhesive forces The force which Bound the molecules together is known

as molecular force.

Molecular forces are of 2 types

1. Cohesive

2. Adhesive

Cohesive forces are forces of attraction between

molecules of same substances

Adhesive forces are forces of attraction between

molecules of different substances

The example of adhesive force-gum stick to paper

Cohesive force is responsible for shape of body.

So solid has large cohesive force & definite shape ,& not

in liquids & gases.

The force range called molecular range is 10-9m.

The sphere of the range of force is called sphere of

influence & molecule attracts another molecule when it

lies in sphere of influence.

Cohesive forces are responsible for surface tension.

Angle of contact The angle made by the tangent to the liquid surface ,at the

point of contact with the wall of container measured from

inside the liquid is called …………..of lqd to solid.

The angle between Hg & glass=140º

The angle between Water & glass~0º

If the angle is acute (<90º)=shape is concave, liquid wets

wall of container.

If the angle is obtuse (>90º)=shape is convex, liquid does

not wet wall of container.

For turpentine 17º & for paraffin it is 107º

Surface tension All molecules contains some energy in the form of heat,

kinetic ,vibrations ,etc.

Inter molecular force also exist.

Many phenomena are working on surface tension :

Small insects on liquid surface

Shape of drops

Capillary reactions

Surface of liquid in capillary

Capillary Action Capillary tube is dipped

in liquid of density ρ

Radius of tube is r

H is height of liquid to be rise in tube

Angle of contact is θ

Surface tension is T

Total upward force

T cosθ2πr

The force acts at angle θwith wall of container.

Total torque T is in two

direction up & down so,

Components are Tcosθ

for upward & Tsinθ for

downward.

Ttotal=Tsinθ+Tcosθ

Horizontal component is

zero so T=Tcosθ

The total torque in

capillary is Tcosθx2πr

Upward force supports

the weight of liquid in

capillary tube.

So total weight(force) of

liquid in capillary tube

=ρπr2hg

Up force = down force

Tcosθx2πr= ρπr2hg

For water θ=0

So,

2cos

rh gT

2

rh gT

factors affecting surface tension Temperature ↑surface tension ↓

Critical temperature Surface tension. zero

Critical temperature of water 374ºK

Increase with impurity.

Problems based on surface tension Hg drops are spherical on glass=surface tension is

higher

Greased needle is floating over the water

Shaving brush is deepen in water hair spreads in diff.

directions

Ink to be drawn continuously due to narrow capillary

from nib of pen

Ink soak by paper due to capillaries on paper

Rain water soaked by earth

Plants, Trees

In hot water adding detergent surface area of cloths is

increase & cleaned easily

For making balls of metal fall from height in cold

water during fall they splits up into spherical shape.

Capillary tube of r=0.5mm is dipped in water.

How far will water rise in tube? T=7x10-3N/m

2cos

rh gT

Find radius of capillary tube in which

water rises up to 3 cm. T= 72x10-3 N/m.

2cos

rh gT

hwater=3.6cm at dia 0.4mm capillary tube.

Find hwater in 0.3 mm capillary tube

1 1

2cos

rh gT

2 2

2cos

r h gT

How much the mercury will fall down in a

capillary tube?

D=1mm,θ=138º,T=0.54 N/m,ρ=13.6x103

kg/m3

2cos

rh gT

REFERENCE BOOKS AUTHOR/PUBLICATION

ENGINEERING PHYSICS S S PATEL (ATUL PRAKASHAN)

MODERN ENGINEERING

PHYSICSA S VASUDEVA

ENGINEERING PHYSICS K. RAJGOPALAN

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