Pressure
Pressure: Pressure is defined as force acting
normally on unit area of the surface.
Pressure
(P) = F/A = Normal force on the surface/ Area
of the surface
SI unit of pressure is N/m2 also called pascal (Pa).
Pressure is a scalar quantity.
Atmospheric pressure:
Atmospheric pressure is that pressure which is exerted by mercury column of 76
cm length at 0o C at 45o latitude at sea-level. It is
equal to 76 cm column of mercury of cross-sectional area 1 cm2.
Generally it is measured in bar.
1 bar = 105
N/m2
Atmospheric pressure 1 atm = 1.01x105 N/m2
= 760 torr
► Atmospheric pressure decreases with altitude (Height from
earth surface). This is why (i) it is difficult to cook on mountains (ii)
The fountain pen of a passenger leaks in aeroplane at height.
► Atmospheric pressure is measured by barometer. With the
help of barometer, weather forecast can be made.
► Sudden fall in barometer reading is indication of storm.
► Slow fall in barometer reading is indication of rain.
Slow rise in
barometer reading is the indication of clear weather.
Pressure in liquid: Force
exerted on unit area of wall or base of the container by the molecules of
liquid is the pressure of liquid.
The
pressure exerted by liquid at depth h below the surface of liquid is given as
P=hdg where d is the density of liquid.
► Regarding pressure the following points are worth nothing:
(i)
In static liquid at same horizontal level, pressure is
same at all points.
(ii)
Pressure at a point in a static liquid has same
value in all direction.
(iii)
Pressure at a point in a liquid is proportional to
the depth of the point from the free surface.
(iv)
Pressure at a point in a liquid is proportional to
the density of the liquid.
Pascal law for
pressure of liquid:
(i)
If gravitational attraction is negligible, in
equilibrium condition, pressure is same at all points in liquid.
(ii)
If an external pressure is applied in an exclosed
fluid, it is transmitted undiminished to every direction.
► Hydraulic lift, Hydraulic press, Hydraulic break works on
Pascal law.
Effect of pressure on
Melting point and Boiling Point:
(i)
The melting point of a substance which expands on
fusion increases with the increases in pressure, for example – Wax.
(ii)
The melting point of a substance which contracts on
fusion decreases with the increases in temperature, for example - ice.
(iii)
Boiling point of all substances increases with the
increases in pressure.
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