Unit 3


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Unit 3

  1. 1. IES Auringis. Jaén. José Cayetano Bautista Expósito 4º ESO. Physics and Chemistry. Unit 3 <ul>Atmospheric pressure: We live at the bottom of the atmosphere of the earth. And, just as the creatures living at the bottom of the ocean are subjected to the pressure exerted on them by the water, we are subjected to the pressure exerted on us by the atmosphere. Air is heavy. It is a lot heavier than most people realize. Truth is, our atmosphere is rather thin; it is not very deep at all compared to the overall diameter of the earth. An analogy that is not too far off is that the atmosphere is about as thick as the skin of an onion, relatively speaking. However, the exact thickness of the atmosphere is hard to specify because the air just gradually thins out as we go higher. This confuses our onion-skin analogy just a bit. Measuring air pressure with a column of liquid; the Barometer One of the first methods used to measure pressure utilized a glass tube in the shape of a U that was partially filled with a liquid to form what we now know as a U-tube manometer. The pressure to be measured is connected to one arm of the U while the other is left open to the atmosphere. The pressure displaces the liquid by pushing down on the pressure side, forcing the liquid level to rise on the other side. The difference is heights of the liquid levels is proportional to the pressure being measured. The pressure is specified by simply giving the distance between the two liquid levels. If water is used as the liquid, the pressure is given as so many inches of water. As you can imagine from a practical standpoint, only very low pressures can be measured in this manner. Any convenient liquid can be used, but a U with shorter arms will result if the liquid is heavy. Accordingly, mercury is the liquid of choice because the density of mercury is something like 13.6 times as much as the density of water. This means that for arms of a given length, pressures 13.6 times as great can be measured if mercury is used. And if mercury is used, the pressure is given as so many inches of mercury. This is often written as in/Hg. (Chemical symbol for merc ury is Hg.) </ul>
  2. 2. IES Auringis. Jaén. José Cayetano Bautista Expósito 4º ESO. Physics and Chemistry. Unit 3. <ul>Standard Atmospheric Pressure (atm) is used as a reference for gas densities and volumes. The Standard Atmospheric Pressure is defined at sea-level at 273oK (0oC) and is 1.01325 bar or 101325 Pa (absolute). In imperial units the Standard Atmospheric Pressure is 14.696 psi. 1 atm = 1.01325 bar = 101.3 kPa = 14.696 psi (lbf/in2)= 760 mmHg =760 torr = 1013 mbar Pressure Units: S ince 1 Pa is a small pressure unit, the unit hectoPascal (hPa) is widely used, especially in meteorology. The unit kiloPascal (kPa) is commonly used design of technical applications like piping systems and similar. 1 hectoPascal = 100 Pascal = 1 millibar 1 kiloPascal = 1000 Pascal Some Alternative Units of Pressure 1 bar - 100,000 Pa 1 millibar - 100 Pa 1 atmosphere - 101,325 Pa A torr (torr) is named after Torricelli and is the pressure produced by a column of mercury 1 mm high - equals to 1 / 760th of an atmosphere. 1 atm = 760 torr = 14.696 psi </ul>
  3. 3. IES Auringis. Jaén. José Cayetano Bautista Expósito 4º ESO. Physics and Chemistry. Unit 3. <ul>Aneroid barometers. Most barometers we encounter use a mechanical mechanism of some sort rather than columns of liquid to sense atmospheric pressure. These are called aneroid barometers. The word &quot;aneroid&quot; somehow means &quot;without liquid.&quot; A typical aneroid barometer utilizes a bellows made of thin metal that expands or contracts as the air pressure changes. The movement of the bellows is communicated to the indicator needle by an intricate linkage that also provides for adjustment and calibration. In fact, the altimeters on our planes are aneroid barometers with a few modifications. Digital barometers change pressure into an analog electrical signal by the physical deformation of strain gages. Pressure produces a deflection of the diaphragm which introduces strain to the gages. The strain will produce an electrical resistance change proportional to the pressure. This electrical signal is treated by electronics to present the pressure in a display </ul>
  4. 4. IES Auringis. Jaén. José Cayetano Bautista Expósito 4º ESO. Physics and Chemistry. Unit 3. <ul>Variation of air pressure with altitude Just as a diver experiences a decrease in pressure as he or she rises toward the surface, we experience a similar decrease in pressure as we climb to higher altitudes in our planes, or climb to the top of a mountain, or go from the basement to upstairs at home. Atmospheric pressure varies somewhat on a day-to-day basis. When the readings are taken over a long period of time in many different locations and then averaged, it is found that the average pressure at sea level is very nearly 760 millimetres (mm) of mercury. This value has been taken (somewhat arbitrarily) as the &quot;standard value.&quot; A standard atmosphere calculator with graphs and other information can be found at http://www.digitaldutch.com/atmoscalc/ The calculator lets you insert an altitude, and it then gives you the standard pressure, temperature, density, and speed of sound at that altitude. Activities 1.- Find the altitude of Jaén. Go to http://www.digitaldutch.com/atmoscalc/ and introduce the found altitude to get the pressure in Jaén. Do the same with the altitude of Mulhacen and Himalayas and compare the results. 2.- Find information about Otto von Guericke. How did he demonstrate the power of a vacuum? </ul>