L. R. & S. M. VISSANJI ACADEMY
SECONDARY SECTION - 2016-17
CHEMISTRY - GRADE: VIII
PRESSURE IN FLUIDS AND ATMOSPHERIC PRESSURE
 THRUST: It is the total force acting normally on a given surface. The S.I. unit is
Newton.
 Thrust exerted by a body on a surface = Weight of the body.
 Thrust is a vector quantity. Its S.I. unit is newton (N) and C.G.S unit is dyne,
where
 1N = 105 dyne
 The gravitational unit of thrust in M.K.S system is kgf and C.G.S system is gf
1kgf = 9.8 N and 1 gf = 980 dyne
As 1 N = 105 dyne,
1 kgf = 9.8 x 105 dyne ( dividing both sides by 1000)
=
We get,
1gf
= 9.8 x 102 dyne
1gf
= 980 dyne
The effect of thrust is expressed in terms of thrust per unit area of surface. This
quantity is called pressure.
Pressure is a scalar quantity. Since pressure is inversely proportional to area, larger
the thrust exerted on a given area, greater is the pressure exerted on it.
Thus for a given thrust,


The pressure on a surface can be increased by reducing the area of surface.
The pressure on a surface can be decreased by increasing the area of
surface.
UNITS OF PRESSURE
The S.I. unit of pressure is
Its C.G.S unit is
i.e. dyne cm-2
i.e. N m-2 which is named as Pascal.
1N m-2 = 10 dyne cm-2
Other units of pressure:






1 bar = 105 N m -2
1 millibar = 10-3 bar = 102 N m-2
1 atmosphere (atm) = 0.76 m of Hg ( or 76 cm of Hg or 760 mm of Hg)
1 atmosphere = 1.013 x 105 Pa
1 torr = 1 mm of Hg
1 atm = 760 torr
PRESSURE IN FLUIDS:




Fluids is the name given to a substance which begins to flow under the effect of
the external force applied. Liquids and gases are fluids.
A solid exerts pressure only on the surface on which it is placed.
A fluid contained in a vessel exerts pressure at all points and in all directions.
Pressure exerted by a liquid at point is given by the relation:
From the above expression, it is clear that the pressure at a point inside the liquid
depends directly on the following three factors:



Depth of the point below the free surface ( h ),
Density of the liquid ( ),
Acceleration due to gravity ( g )
Total pressure in a liquid at a depth h
= atmospheric pressure + pressure due to the liquid column
+
=
LAWS OF LIQUID PRESSURE





Inside the liquid, pressure increases with the increase in depth from its free
surface.
In a stationary liquid, pressure is same at all points on a horizontal plane.
Pressure is same in all directions about a point in liquid.
Pressure at the same depth is different in different liquids. It increases with
the increase in density of the liquid.
A liquid seeks its own level.
PASCAL’S LAW
Pascal’s law states that the pressure exerted anywhere in a confined liquid is
transmitted equally and undiminished in all directions throughout the liquid.
The principle of hydraulic machines is that a small force applied on a smaller piston
is transmitted to produce a large force on the bigger piston.
Principle used in hydraulic machine
Pascal’s law is applied in the following practical devices:
Hydraulic press ( or Brahman press)
Hydraulic brakes.
Hydraulic jack ( or hydraulic lift)
ATMOSPHERIC PRESSURE



The envelope of air around the earth is called atmosphere which is bound to the
earth by the gravitational attraction of the earth. Air has weight which provides the
thrust. The thrust due to a column of air on a unit surface area of the earth is
known as atmospheric pressure.
The atmospheric pressure on the sea level is 1.013 x 10 5 N m-2. If the average
area of human body is nearly 2m2, force experienced by the human body will be
2.026 x 105 N.
But the human body is not adversely affected by this pressure because in the
veins, blood pressure is slightly greater than atmospheric pressure which counter
balances the atmospheric pressure.
MEASUREMNT OF ATMOSPHERIC PRESSURE
 To measure atmospheric pressure at a place, barometer is used.
 In the barometer, if liquid is used as barometric liquid, it is known as a liquid
barometer. If a gas is used, then it is known as aneroid barometer.
Simple barometer
Fortin’s barometer
Aneroid barometer

If the aneroid barometer directly indicates the altitude ( in metres) instead of
atmospheric pressure, it is called an altimeter.