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Atmospheric n gas pressure

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Atmospheric n gas pressure

  1. 1. Atmospheric PressureAtmospheric Pressure
  2. 2. Atmospheric PressureAtmospheric Pressure Earth Atmosphere Surface of the Earth Atmospheric pressure exerted on the surface of the Earth (or at the sea level) as well as objects on Earth.
  3. 3. Atmospheric PressureAtmospheric Pressure  The gasThe gas molecules havemolecules have weight.weight.  AtmosphericAtmospheric Pressure isPressure is caused by thecaused by the weight (force) ofweight (force) of the thick layer ofthe thick layer of air above theair above the earth’s surface.earth’s surface.
  4. 4. HOW does a human body withstand the pressure on its surface? Living cells maintain an internal pressure that is about the same as 1 atm, which is similar to the atmospheric pressure. So, we don’t feel high atmospheric pressure exerted to us.
  5. 5. Surface of bottle Internal pressure External pressure External pressure Internal pressure Internal Pressure = External pressure
  6. 6. Atmospheric Pressure acts equally in allAtmospheric Pressure acts equally in all direction.direction.
  7. 7. Characteristics of AtmosphericCharacteristics of Atmospheric PressurePressure  It is not affected by the surface area.It is not affected by the surface area.  The pressure exerted by the air molecules at sea level is 1.013 × 105 Pa.  At sea level, PAt sea level, Patmatm == 1.013 × 105 Pa (Nm-2 ) = 76 cmHg= 76 cmHg = 10mH= 10mH22OO
  8. 8. Atmospheric Pressure decreases with altitudeAtmospheric Pressure decreases with altitude because air is gets thinner as altitude increases.because air is gets thinner as altitude increases.
  9. 9. Peter goes to the top of mountain and feels breathing difficulty. The number of air molecules _________. The collision rate of air molecules _________ and he encounters ______ atmospheric pressure. So, he has to breath _____ when he goes up to mountain ( _____ altitude). John enters to the underground mines and feels breathing difficulty. The number of air molecules _________. The collision rate of air molecules _________ and he encounters ______ atmospheric pressure. So, he has to breath _____ when he goes down mines ( ____ altitude). decreases decreases lower more high increases increases higher less low
  10. 10. Existence of AtmosphericExistence of Atmospheric PressurePressure
  11. 11. At normal condition, the rate of collision on inner wall is same with on outer wall. So, the air pressure inside and outside of the bottle is the same. After the air has been pumped out, there are almost no more air molecule inside the bottle to balance the force of collision of air molecule outside the bottle. The external force due to atmospheric pressure crashes the bottle.
  12. 12. Existence of AtmosphericExistence of Atmospheric PressurePressure
  13. 13. Glass filled by water Atmospheric pressure Cardboard Why the cardboard does not fall from the mouth of the inverted glass filled with water? The atmospheric pressure that presses the cardboard against the glass produce a force that is strong enough to support the weight of the water in the glass.
  14. 14. Existence of AtmosphericExistence of Atmospheric PressurePressure syringesyringe BarometerBarometer
  15. 15. Application of atmospheric pressure Pouring the condensed milk from its can A rubber suction cap on a smooth surface Drinking with a straw Removing dust with a vacuum cleaner Putting plastic sticker onto the inner surface windscreen Measuring blood pressure
  16. 16. The presence of a second hole is to enable air flow into the can. So, the air pressure inside the can same as atmospheric pressure. It will force the milk out of the lower hole. Pouring the condensed milk from its can Milk Air flows into the can Atmospheric pressure
  17. 17. A rubber suction cap on a smooth surface When air is forced out of the suction cup, a partial vacuum is created in the space between the cup and the smooth surface. The surrounding atmospheric pressure forces the cup tightly against the smooth surface.
  18. 18. Removing dust with a vacuum cleaner When the vacuum cleaner switches on, the motor will work and the fan blades turn, force the air forward towards the exhaust port. The pressure level in the area behind the fan drops below the pressure level outside the vacuum cleaner. This creates partial vacuum inside the vacuum cleaner. The dust will flows into the vacuum cleaner through the intake port because the air pressure inside the vacuum cleaner is lower than the pressure outside.
  19. 19. Drinking with a straw Straw Atmospheric pressure forces the drink into the straw Air being sucked up and creating a partial vacuum in the straw When the air is being sucked from the straw, a partial vacuum is created in the straw. The surrounding atmospheric pressure will force the drink into the straw and enable it to be moved into the mouth.
  20. 20. Putting plastic sticker onto the inner surface windscreen When the plastic is placed on the glass surface, air is being forced out of the space between the sticker and the glass surface, creating a partial vacuum. The surrounding atmospheric pressure will hold the sticker tightly on the windscreen. Windscreen Sticker Pressure Air is being force out
  21. 21. Instruments for measuringInstruments for measuring Atmospheric PressureAtmospheric Pressure Mercury barometerMercury barometer Fortin barometerFortin barometer Aneroid barometerAneroid barometer
  22. 22. Measuring atmospheric pressure Simple Barometer Made of a glass tube of about 100 cm filled with liquid (normally mercury is used). For mercury, the liquid level will drop as it flows into a bowl to until a vertical height of above 76 cm from the surface of mercury in the bowl. ρHg = 1.36 × 104 kg m-3 g = 9.8 N kg-1 h = 76 cm = 0.76 m P = 1.36 × 104 × 9.8 × 0.76 = 1.103 × 105 Pa
  23. 23. Question Liquid A has been filled into the barometer to measure the atmospheric pressure at sea level. According to the figure, find the density of the liquid A. Solution As we know, atmospheric pressure, P is equal to 1.013 × 105 Pa at sea level. So, P = 1.013 × 105 = hρg 1.013 × 105 = 1.10 × ρA × 10 ρA = 9209 kg m-3
  24. 24. Fortin BarometerFortin Barometer  More accurateMore accurate  unit mmHgunit mmHg
  25. 25. Fortin barometers have to be set before each reading is taken. Using the screw at the bottom adjust the mercury level, seen in the glass reservoir, until the surface just touches the tip of the pointer. The barometer is then ready for a reading to be taken.
  26. 26. Aneroid BarometerAneroid Barometer  More handy and mobileMore handy and mobile
  27. 27. Aneroid BarometerAneroid Barometer • the partially evacuated chamber expands and contracts in response to changes in atmospheric pressure
  28. 28. AneroidAneroid BarometerBarometer  Also an altimeterAlso an altimeter
  29. 29. Normally used by pilot to determine the atmospheric pressure in sky. Sometimes, it is also used to measure the height above the sea level (altitude). So, it called altimeter. Aneroid BarometerAneroid Barometer
  30. 30. Gas PressureGas Pressure
  31. 31. Kinetic MolecularKinetic Molecular TheoryTheory Basic assumptions:Basic assumptions:  A gas consists of a collection of small particles which are moving rapidly and randomly in straight-line motion and obeying Newton's Laws.  Gas molecules are constantly colliding with one another and the collisions is perfectly elastic (that is, no energy is gained or lost during the collision).  Average kinetic energy is equal to the temperature.Average kinetic energy is equal to the temperature.
  32. 32. Gas Molecule Pressure in gases due to collision of molecules with the wall of a container.
  33. 33. Gas PressureGas Pressure  Gas molecules collide withGas molecules collide with the wall of the container andthe wall of the container and change velocity andchange velocity and momentum,momentum,  the rate of change ofthe rate of change of momentum = Forcemomentum = Force  The force on the wall of theThe force on the wall of the container creates gascontainer creates gas pressure.pressure.
  34. 34. Pressure Rate of collisions Number of particles Speed of particles Volume of container
  35. 35. When number of particleWhen number of particle INCREASESINCREASES Distance between particles __________, ______ collisions occurs, pressure __________. DECREASE MORE INCREASE
  36. 36. When volume of containerWhen volume of container INCREASEINCREASE Particles have ______ space to move around, _____ collisions occurs, pressure __________. MORE LESS DECREASE
  37. 37. When temperature inside of containerWhen temperature inside of container INCREASEINCREASE P P P P P P Particles gain ______ energy and move ________, average speed __________, ______ collisions occurs, pressure __________. MORE MORE INCREASE INCREASE FASTER
  38. 38. Instruments for measuringInstruments for measuring Gas pressureGas pressure  ManometerManometer  Bourdon gaugeBourdon gauge
  39. 39. Measuring gas pressureMeasuring gas pressure Manometer Manometer consists of a U-tube that is filled with a liquid, oil or mercury. The figure shows a manometer is not connect of gas supply. The atmospheric pressure acts on both surfaces of the liquid at points A and B. P0 – Atmospheric pressure
  40. 40. The figure shows one end of manometer is connected to the gas supply. The gas would exert a pressure on the liquid at point A. If the pressure greater than atmospheric pressure, liquid level at point A will be forced down. Liquid in another end (point B) will be forced up in equilibrium. P = Patm + hρg
  41. 41. Question A mercury manometer with one end attached to a gas supply measures a difference in the level of mercury of 32 cm as in figure. Calculate the pressure of the gas supply in (a) cmHg (b) Pascal [ Patm = 76 cmHg; g = 10 Nkg-1 ; ρmercury = 1.36 × 104 kgm-3 ] Solution (a) Pressure = Atmospheric pressure + pressure due to mercury column = 76 cmHg + 32 cmHg = 108 cmHg (b) Pressure of gas supply = hρg= 108 × 10-2 × 1.36 × 104 × 10 = 1.46 × 105 Pa
  42. 42. Question A mercury manometer with one end attached to a gas supply measures a difference in the level of mercury of 10 cm as in figure. Calculate the pressure of the gas supply in (a) cmHg (b) Pascal [ Patm = 76 cmHg; g = 10 Nkg-1 ; ρmercury = 1.36 × 104 kgm-3 ] Solution (a) Pgas = PHg + Patm = 10 cmHg + 76 cmHg = 86 cmHg (b) Pressure of gas supply = hρg = 86 × 10-2 × 1.36 × 104 × 10 = 1.1696 × 105 Pa
  43. 43. Bourdon gaugeBourdon gauge • More accurate • Measures in unit Pascal
  44. 44. Bourdon gaugeBourdon gauge  When gas supply is connected the pressureWhen gas supply is connected the pressure in the gas acts to straighten the copper coilin the gas acts to straighten the copper coil The movementThe movement of the coil isof the coil is transferred to thetransferred to the lever systemlever system which actuates awhich actuates a pointer to movepointer to move across a scaleacross a scale which has beenwhich has been calibrated to givecalibrated to give readings ofreadings of pressurpressur

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