Pressure

 Pressure: It is defined as force acting unit per area.

atmospheric pressure:  1 atm=1.01 bar=1.01*10^5 N/m^2=760 torr.

Some important point:

Atm pressure decrease with altitude, due to this it is difficult to cook on the mountain.

It is measure by barometer.

sudden fall in barometer indication of storm.

Slow rise in barometer indication of clear weather.

slow fall in barometer indication of rain.

Pressure in liquid:

• The pressure exerted by liquid at depth h below the surface of liquid is given as hdg.
• Pressure at a point in a static liquid has same value in all direction.
• Pressure at a point in a liquid is proportional to the depth of the point from free surface and density of liquid.

Floatation:

Buoyant Force: A force acts on a body by the liquid in upward direction. It is equal to the weight of liquid displaced by the body.

Its study was first made by Archimedes.

Law of Floatation:

• Density of material of body is less than or equal to the density of liquid.
• The Centre of gravity of the body and Centre of gravity of displaced liquid should be in one vertical line.

when ice floating in liquid melts completely , the level of liquid + water

• Fall if density of liquid < density of water
• Rise if density of liquid >density of water.

Some important point:

• Relative density is measured by hydrometer
• purity of milk is measure by lactometer.
• when ice floats in water, its 1/10 part remains outside water.
• if floating ice in water in a vessel melts, the level of water in vessel doesn't change.

Surface Tension: The tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area.

A liquid drops attains spherical shape due to surface tension. St= F/L

** Surface Tension of liquid decreases with the increase of temperature and becomes zero at critical temperature.

Capillary Tube: A tube having very narrow and uniform bore is called capillary tube.

• The height by which liquid ascends and descends in a capillary tube depends upon the radius of tube.
• The liquid which wets the wall of tube rise in the tube(water).
• The liquid which doesn't wet the wall of tube descends in the tube(mercury).

Illustration of capillarity:

• The oil in the wick of lamp rise due to capillary action of thread.
• The root hair of plant draw water from the soil through capillary action.
• To prevent loss of water due to capillary action, the soil is loosened and split into pieces by the farmers.
• Action of towel in soaking up water from the body is due to capillary action of cotton cloth.
• Needle float due to surface tension.
• The addition of detergent decrease the surface tension of water and thus increase the cleaning ability.
• Bubble of soap solution are big because addition of soap decreases the surface tension of water.
• When kerosine oil is sprinkled on water, its surface tension decreases.
• Excess pressure in liquid drop:  2T/R
• Excess pressure in a soap bubble= 4T/R