Post on 20-Jul-2015
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
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
Image reference Links1. http://ej.iop.org/images/1464-
4258/7/8/L02/Full/9737801.jpg
2 -4
http://www.mtarr.co.uk/courses/topics/0205_els/images/ch_
els_imgg.gif