Revision Notes: Pressure In Fluids And Atmospheric Pressure | Physics Class 9 ICSE PDF Download

Thrust and Pressure

• Thrust is the force acting perpendicularly on an object.
• It is a vector quantity. Its SI unit is newton (N).
• Pressure is the force or thrust acting perpendicularly on a unit area of the object.
  Pressure = (Thrust/Area) = F/A
• The same force acting on a smaller area exerts a larger pressure and on a larger area exerts a smaller pressure.
• The SI unit of pressure is N/m². This unit is named as pascal (Pa).
• One pascal is the pressure exerted on a surface of area 1 m² by a force of 1 N acting normally on it.

Pressure in fluids

• A substance which flows is called a fluid.
• Fluids exert pressure on the base and walls of the container in which they are enclosed.
• Pressure exerted in any confined mass of fluid is transmitted uniformly in all directions.

Pressure Exerted by a Liquid Column

• The pressure on a surface at a depth ‘h’ is
  
• The total pressure inside a liquid column at a depth is the sum of the atmospheric pressure on the liquid surface and the pressure due to the liquid column.
  Total pressure = P₀ + hρg

Laws of Liquid Pressure

• Pressure is the same in all directions about a point in the liquid.
• In a stationary liquid, the pressure is the same at all points on a horizontal plane.
• Pressure at a point inside the liquid increases with depth from the free surface.
• Pressure at the same depth is different in different liquids. It increases with increase in density.
• A liquid seeks its own level.

Some Consequences of Liquid Pressure

Pascal’s law

• Pascal's law: The pressure exerted anywhere in a confined liquid is transmitted equally and undiminished in all directions throughout the liquid.

Application of Pascal’s law

• The hydraulic press, hydraulic jack and hydraulic brakes work on Pascal’s law of transmission of pressure.

Examples of Hydraulic Machines

Uses:

• For engraving monograms on books.
  
• For extracting juice from sugarcane.
• For squeezing oil from linseed and cotton seeds.
• For pressing cotton bales and goods, quilts, books etc.

Hydraulic jack:
A hydraulic jack is used to lift heavy vehicles at service stations for repair works.

Hydraulic brakes:

Atmospheric Pressure

• The envelope of air surrounding the Earth is called the atmosphere.
• The thrust exerted per unit area of the Earth’s surface due to a column of air is called the atmospheric pressure on the Earth’s surface.
• Its value is about 1 kgf/cm² = 10⁵ N m^-2 = 10⁵ Pa.
• We do not feel this large amount of thrust as our blood pressure balances this thrust.
  1 atm = 0.76 m of Hg = 76 cm of Hg =1.013 x 10⁵ Pa

Consequences of Atmospheric Pressure

• We can suck a drink with a straw
• We can fill a syringe with the liquid
• We can fill ink into a fountain pen
• Rubber suckers work due to atmospheric pressure
• Action of a siphon system is based on atmospheric pressure
  

Measurement of Atmospheric Pressure

• Atmospheric pressure is measured using a barometer.

Simple barometer: Torricelli first designed a simple barometer using mercury as the barometric liquid.Advantages of using mercury as a barometric liquid

• The density of mercury is greater than that of all other liquids, so only 0·76 m height of the mercury column is needed to balance the normal atmospheric pressure.
• The vapour pressure of mercury is negligible, so it does not affect the barometric height.
• The mercury neither wets nor sticks to the glass tube. Therefore, it gives the correct reading.
• The surface of mercury is shining and opaque. Therefore, it is easily seen while taking the observation.
• It can easily be obtained in the pure state.

Disadvantages of using water as a barometric liquid

• The density of water is low, so nearly 10·4 m height of the water column is needed to balance the normal atmospheric pressure. It will be very inconvenient to use a tube of height 10·4 m for a barometer.
• The vapour pressure of water is high, so the vapours in the vacuum space will make the reading inaccurate.
• Water sticks to the glass tube and wets it, so the reading becomes inaccurate.
• Because water is transparent, its surface is not easily seen while taking the observation.

Defects of a simple barometer

• There is no protection for the glass tube.
• The surface of mercury in the trough is open; therefore, there are chances that the impurities may fall in and get mixed with the mercury of the trough.
• It is inconvenient to move the barometer from one place to another.
• A scale cannot be fixed with the tube (or it cannot be marked on the tube) to measure the atmospheric pressure.

Fortin’s barometer: It is a modified form of a simple barometer. It is used in the laboratory to measure the atmospheric pressure. It also uses mercury as barometric liquid.

Aneroid barometer: This barometer does not have a liquid. It is light and portable. Hence, it can easily be carried from one place to another. It is calibrated in such a way that it reads the atmospheric pressure directly.

Uses of a barometer

• To measure the atmospheric pressure at a place
• For weather forecast
• As an altimeter

Variation of atmospheric pressure with altitude

• The pressure of air decreases with increase in altitude.
• The decrease in atmospheric pressure is due to
  • Decrease in height of the air column which causes a linear decrease in atmospheric pressure.
  • Decrease in density of air which makes the decrease in atmospheric pressure less rapid with increase in altitude.
• The following graph shows the variation of atmospheric pressure with height above sea level.
  
• Consequences of travelling to high altitudes
  • The air pressure reduces and hence breathing problems occur. Also, the nose may bleed.
  • A fountain pen will start leaking due to the pressure difference at a high altitude compared to the air pressure on the surface.

Weather Forecast by a Barometer

A weather forecast is done as follows:

• If the barometric height at a place falls, then it means that the pressure at that place has fallen, indicating the coming of a storm or cyclone.
• If the barometric height gradually falls, then it indicates that the moisture is increasing, i.e. there is a possibility of rain.
• A gradual increase in the barometric height means that the moisture in the air is decreasing. This indicates dry weather.
• A sudden rise in the barometric height means a flow of air from that place to surrounding low pressure areas. This indicates extremely dry weather.
• If there is no abrupt change in the barometric height, then it indicates that the atmospheric pressure is normal, i.e. the weather is fair.

Altimeter

• An altimeter is a device used in an aircraft to measure its altitude. Because the atmospheric pressure decreases with the increase in height above the sea level, a barometer measuring the atmospheric pressure can be used to determine the altitude of a place above the sea level.
• An aneroid barometer which has the scale calibrated in terms of height of ascent can be used as an altimeter.
• The scale of an altimeter is graduated with height increasing towards the left because the atmospheric pressure decreases with increase of height above the sea level.