An air bubble is rising up in a liquid will its potential energy increase or decreasre why?
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Surface tension is the property of any liquid by virtue of which tries to minimize its free surface area.
Surface tension of a liquid is measured as the force acting per length on an imaginary line drawn tangentially on the free surface the liquid.
Surface tension S = Force/Length = F/l = Work done/Change in area
Its SI unit is Nm-1 or Jm-2 and its dimensional formula is [MT-2].
It is a scalar quantity. Surface tension is a molecular phenomenon which is due to cohesive force and root cause of the force is electrical in nature.
Surface tension of a liquid depends only on the nature of liquid and independent of the surface area of film or length of the line .
Small liquid drops are spherical due to the property of surface tension.
Adhesive Force
The force of attraction acting between the molecules of different substances is called adhesive force, e.g., the force of attracts acting between the molecules of paper and ink, water and glass, etc.
wax in a candle, rise in the capillaries formed in the cotton wick and burns.Coffee powder is easily soluble in water because water immediately wets the fine granules of coffee by the action of capillarity.The water given to the fields rises in the innumerable capillaries formed in the stems of plants and trees and reaches the leaves.
Zurin’s Law
If a capillary tube of insufficient length is placed vertically in a then liquid never come out from the tube its own, as
Rh = constant ⇒ R1h1 = R2h2
where, R = radius of curvature of liquid meniscus and
h = height of liquid column.
When a tube is kept in inclined position in a liquid the vertical height remains unchanged then length of liquid column.

Liquid rises (water in glass capillary) or falls (mercury in capillary) due to property of surface tension
T = Rρgh / 2 cos θ
where, R = radius of capillary tube, h = height of liquid, p = density of liquid, e = angle of contact,
T = surface tension of liquid and 9 = acceleration due to gravity.
Excess Pressure due to Surface Tension
(i) Excess pressure inside a liquid drop = 2S / R
(ii) Excess pressure inside an air bubble in a liquid = 2S / R
(iii) Excess pressure inside a soap bubble = 4S / R
where, S = surface tension and R = radius of drop/bubble.
(iv) Work done in spraying a liquid drop of radius R into n droplets of radius r = T x increase in surface area
= 4πTR3 (1/r – 1/R)
Fall in temperature
Δθ = 3T/J (1/r – 1/R)
where. J = 4.2 J/cal.
(v) When n small drops are combined into a bigger drop, then work done is given by
W = 4πR2T (n 1/3 – 1)
Temperature increase
Δθ = 3T/J (1/r – 1/R)
(vi) When two bubbles of radii r1 and r2coalesce into a bubble of radius r isothermally, then
r2 = r12 + r22
(vii) When two soap bubbles of radii ‘1 and ‘2 are in contact with each other, then radius (r) of common interface.

Factors Affecting Surface Tension
Surface tension of a liquid decreases with increase temperature and becomes zero at critical temperature.At boiling point, surface tension of a liquid becomes zero becomes maximum at freezing point.Surface tension decreases when partially soluble impurities such as soap, detergent, dettol, phenol, etc are added in water.Surface tension increases when highly soluble impurities such as salt is added in water.When dust particles or oil spreads over the surface of water, its surface tension decreases.
When charge is given to a soap bubble, its size increases surface tension of the liquid decreases due to electrification.
In weightlessness condition liquid does not rise in a capillary tube.
Some Phenomena Based on Surface Tension
Medicines used for washing wounds, as detol, have a surface tension lower than water.Hot soup is more tasteful than the cold one because the surface tension of the hot soup is less than that of the cold and so spreads over a larger area of the tongue.Insects and mosquitoes swim on the surface of water in ponds and lakes due to surface tension. If kerosence oil is sprayed on the water surface, the surface tension of water is lowered and the insects and mosquitoes sink in water and are dead.If we deform a liquid drop by pushing it slightly, then due to surface tension it again becomes spherical
Surface tension of a liquid is measured as the force acting per length on an imaginary line drawn tangentially on the free surface the liquid.
Surface tension S = Force/Length = F/l = Work done/Change in area
Its SI unit is Nm-1 or Jm-2 and its dimensional formula is [MT-2].
It is a scalar quantity. Surface tension is a molecular phenomenon which is due to cohesive force and root cause of the force is electrical in nature.
Surface tension of a liquid depends only on the nature of liquid and independent of the surface area of film or length of the line .
Small liquid drops are spherical due to the property of surface tension.
Adhesive Force
The force of attraction acting between the molecules of different substances is called adhesive force, e.g., the force of attracts acting between the molecules of paper and ink, water and glass, etc.
wax in a candle, rise in the capillaries formed in the cotton wick and burns.Coffee powder is easily soluble in water because water immediately wets the fine granules of coffee by the action of capillarity.The water given to the fields rises in the innumerable capillaries formed in the stems of plants and trees and reaches the leaves.
Zurin’s Law
If a capillary tube of insufficient length is placed vertically in a then liquid never come out from the tube its own, as
Rh = constant ⇒ R1h1 = R2h2
where, R = radius of curvature of liquid meniscus and
h = height of liquid column.
When a tube is kept in inclined position in a liquid the vertical height remains unchanged then length of liquid column.

Liquid rises (water in glass capillary) or falls (mercury in capillary) due to property of surface tension
T = Rρgh / 2 cos θ
where, R = radius of capillary tube, h = height of liquid, p = density of liquid, e = angle of contact,
T = surface tension of liquid and 9 = acceleration due to gravity.
Excess Pressure due to Surface Tension
(i) Excess pressure inside a liquid drop = 2S / R
(ii) Excess pressure inside an air bubble in a liquid = 2S / R
(iii) Excess pressure inside a soap bubble = 4S / R
where, S = surface tension and R = radius of drop/bubble.
(iv) Work done in spraying a liquid drop of radius R into n droplets of radius r = T x increase in surface area
= 4πTR3 (1/r – 1/R)
Fall in temperature
Δθ = 3T/J (1/r – 1/R)
where. J = 4.2 J/cal.
(v) When n small drops are combined into a bigger drop, then work done is given by
W = 4πR2T (n 1/3 – 1)
Temperature increase
Δθ = 3T/J (1/r – 1/R)
(vi) When two bubbles of radii r1 and r2coalesce into a bubble of radius r isothermally, then
r2 = r12 + r22
(vii) When two soap bubbles of radii ‘1 and ‘2 are in contact with each other, then radius (r) of common interface.

Factors Affecting Surface Tension
Surface tension of a liquid decreases with increase temperature and becomes zero at critical temperature.At boiling point, surface tension of a liquid becomes zero becomes maximum at freezing point.Surface tension decreases when partially soluble impurities such as soap, detergent, dettol, phenol, etc are added in water.Surface tension increases when highly soluble impurities such as salt is added in water.When dust particles or oil spreads over the surface of water, its surface tension decreases.
When charge is given to a soap bubble, its size increases surface tension of the liquid decreases due to electrification.
In weightlessness condition liquid does not rise in a capillary tube.
Some Phenomena Based on Surface Tension
Medicines used for washing wounds, as detol, have a surface tension lower than water.Hot soup is more tasteful than the cold one because the surface tension of the hot soup is less than that of the cold and so spreads over a larger area of the tongue.Insects and mosquitoes swim on the surface of water in ponds and lakes due to surface tension. If kerosence oil is sprayed on the water surface, the surface tension of water is lowered and the insects and mosquitoes sink in water and are dead.If we deform a liquid drop by pushing it slightly, then due to surface tension it again becomes spherical
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