pressure in fluids

• Thrust is the force acting normally on a surface
• Thrust is vector quantity
• 1kgf=9.8 Newton
• 1Newton =10^5 dyne
• Pressure is the thrust per unit area of surface
• Pressure is scalar quantity
• Pressure SI unit is pascal or N/m^2
• laws of liquid pressure:

  1. Pressure increases with increase in depth
  2. In a stationary liquid pressure is same at all points in a horizontal plane.
  3. pressure is same in all directions about a point inside the liquid
  4. Pressure at same depth is different in different liquids. It increases with the increase in density.
  5. A liquid seeks its own level
• Pressure at a point depends inside the liquid depends on :

  1. depth of the point below the free surface
  2. density of liquid
  3. acceleration due to gravity
• 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 machine is that a small force applied on a smaller piston is transmitted to produce a large force on the bigger piston
• define pressure

  pressure is the thrust per unit area of surface
• the thrust exerted per unit area on the earth's surface due to a column of air, is called atmospheric pressure on the surface of the earth
• A barometer is an instrument which is used to measure the atmospheric pressure.
• Three types of barometer:

  • simple barometer
  • Fortin's barometer
  • Aneroid barometer
• 1 bar = 10^5 Nm^-2.
• The pressure P1 at a certain depth in river water and P2 at the same depth in sea water are related as: P1 < P2
• Advantages of using mercury as barometric liquid:

  1. The vapour pressure of mercury is negligible, so the vapours in the torricellian vacuum does not affect the barometric height.
  2. Mercury neither wets nor sticks to the glass tube, therefore it gives accurate reading.
  3. it can easily obtained in a pure state
  4. Mercury has a high density which means the height of the column can be a reasonable size to measure atmospheric pressure.
• Disadvantages of simple barometer:

  • There is no protection for the glass tube.
  • it is not portable
  • A scale cannot be fixed with the tube to measure the atmospheric pressure
  • The surface of mercury in the trough is open, therefore there are chances that the impurities may fall in and get mixed with mercury of the trough
• Disadvantages of using water as a barometric liquid:

  • The density of water is low which is equal to 10^3 kgm^−3, so nearly 10.4 m height of water column is needed to balance the atmospheric pressure
  • The vapor pressure of water is high so its vapors in the vacuum space will make the reading inaccurate.
  • Water sticks with the glass tube and wets it, so the reading becomes inaccurate
  • water is transparent, so its surface is not easily seen while taking the observation.
• Uses of barometer:

  • To measure the atmospheric pressure at a place.
  • For weather forecasting.
  • as an altimeter to measure the height.
• • If the barometric height at a place suddenly falls it indicates the coming of a storm or a cyclone
  • If the barometric height at a place gradually falls it indicates that there is a possibility of rain.
  • If the barometric height at a place gradual increase it indicates the coming of dry weather
  • If the barometric height at a place sudden rise it indicates that the coming of extremely dry weather.
  • If the barometric height at a place has no abrupt change it indicates the weather will remains unchanged.