Understanding Pressure: Definition, Units, and Applications, Exams of Earth, Atmospheric, and Planetary Sciences

Download Understanding Pressure: Definition, Units, and Applications and more Exams Earth, Atmospheric, and Planetary Sciences in PDF only on Docsity! 11.7Atmospheric Pressure Have you ever felt your ears “pop” in an airplane or when driving over a hilly road? What you are experiencing is a change in air pressure. Like everything else near Earth, the atmosphere is under the in uence of gravity. Gravity pulls this enormous mass toward the centre of Earth. Pressure Pressure is de ned as the force per unit area. Like volume and temperature, pressure is a physical property of a gas. As you will soon learn, there are many important applications of gas pressure. Before discussing air pressure speci cally, we will consider a more visible example of pressure. Figure 1 illustrates some of the factors that determine pressure.  e gravitational pull of Earth exerts a downward force on everyone, including the person in Figure 1. As a result, he exerts a downward force on the nails. However, to minimize pain, this person would be wise to distribute this force over as large an area as possible.  e greater the area, the lower the pressure. He would still exert the same downward force if he were to stand on one foot on the nails. However, the force would then be concentrated into a smaller area.  e smaller the area, the greater the pressure. Mathematically, pressure, P, is expressed as P 5 F A  e SI unit for pressure is the pascal (1 Pa 5 1 N/m2). Pressure is directly related to the size of force applied.  e greater the force, the greater the pressure. Pressure is inversely related to area. A large force applied to a small area will produce a large pressure. If the same force is applied to a large area, the pressure will be less. For example, if a force 100 N were applied to an area of 1.00 cm2 (0.000100 m2 or 1.00 3 1024 m2), the pressure would be 1.00 3 106 Pa (Figure 2(a)). If the same 100 N force were applied to a much larger area of 1.00 m2, the pressure would be only 100 Pa (Figure 2(b)). Figure 1 The more nails there are, the less painful this experience will be. pistons compressible gas P = 100 N 0.000100 m2 = 1.00 � 106 Pa P = 100 N 1 m2 = 100 Pa A � 1.00 cm2 � 1.00 � 10�4 m2 A � 1.00 m2 F � 100 N F � 100 N Figure 2 The smaller the surface area on which the mass is resting, the greater the pressure exerted. (a) (b) When a piston applies a pressure to a trapped sample of gas, as in Figure 2, the gas exerts a pressure on the walls of its container. It is the force exerted by the gas mol- ecules as they collide with the inner walls of the container that results in the observed pressure.  ese collisions are what keep bicycle tires hard when they are in ated. 11.7 Atmospheric Pressure 541NEL Measuring Atmospheric Pressure Atmospheric pressure is the force per unit area exerted by air on all objects. It is com- monly reported in kilopascals, kPa. At sea level, the pressure exerted by a column of air with a base of one square metre is equal to 101.325 kPa (oft en rounded to 101 kPa). Th is pressure is known as standard pressure and is the basis for another unit of pressure: the atmosphere. One atmosphere is equal to 101.325 kPa. Traditionally, chemists defi ned the standard conditions for work with gases as the temperature 0 °C and pressure 101.325 kPa. A gas sample at these conditions is said to be at standard temperature and pressure (STP). However, since 0 °C is not a convenient temperature at which to conduct laboratory investigations, scientists have recently defi ned another set of standard conditions. Th ese conditions are called standard ambient temperature and pressure (SATP) and are defi ned as 25 °C and 100 kPa. Th e SATP standard is more convenient than STP because it more closely represents the conditions in a laboratory. Evangelista Torricelli (1608−1674) was the fi rst person to devise a method of mea- suring atmospheric pressure. He was trying to solve a problem. Pump makers in Tuscany could not raise water more than 10 m using a suction pump. Torricelli used mercury, which is denser than water, to investigate the vacuum and atmospheric pressure. He prepared a glass tube similar to an extremely long test tube. He fi lled the tube with mercury and carefully inverted it, submerging the open end into a dish containing more mercury (Figure 3). Th e mercury in the tube was pulled down by gravity. However, the mercury did not all run out of the tube. Why not? Air pressure pushed on the mer- cury in the dish, eff ectively pushing mercury into the tube. A vacuum formed at the top of the tube. Th e vacuum exerted no downward pressure on the mercury inside the tube. Torricelli noticed that the mercury level in the tube changed slightly from day to day. Th e fl uctuating mercury level was due to changes in air pressure. Th is device for measuring atmospheric pressure became known as a barometer. At one time, the standard pressure was defi ned as 760 mm Hg or 760 Torr in honour of Torricelli. Scientists had been investigating gases for many years before there was a standard- ized unit for pressure. Some scientists developed their own ways of measuring pressure. Th is is one reason we now have so many units for pressure. Some of these units are used in a specifi c situation. For example, medical professionals use mm Hg for measuring blood pressure. In Canada we still commonly measure tire pressure in psi (pounds per square inch), even though we use the metric system for many other quantities. Table 1 shows the conversion of several SI and non-SI pressure units. atmospheric pressure the force per unit area exerted by air on all objects standard ambient temperature and pressure (SATP) 25 °C and 100 kPa standard temperature and pressure (STP) 0 °C and 101.325 kPa standard pressure 101.325 kPa (often rounded to 101 kPa) glass tube vacuum air pressure mercury 760 mm Hg Figure 3 Torricelli’s apparatus for measuring atmospheric pressure was based on the work of an earlier scientist: Galileo. Table 1 SI and Non-SI Units of Pressure Unit name Unit symbol Defi nition/conversion pascal Pa 1 Pa 5 1 N/m2 millimetres mercury mm Hg 760 mm Hg = 1 atm = 101.325 kPa torr Torr 1 Torr = 1 mm Hg atmosphere atm 1 atm = 101.325 kPa (exactly) pounds per square inch psi 1 psi = 6895 Pa Tutorial 1 Converting between Units of Pressure Sometimes you are given a measurement of pressure in one unit, such as millimetres of mercury (mm Hg), and you need to convert it into a different unit, such as pascals (Pa). This is a fairly simple mathematical procedure. You can use the defi nitions in Table 1 to write conversion factors that allow you to switch from one unit to another. 542 Chapter 11 • The Gas State and Gas laws NEL High-Altitude Training Many endurance athletes train at high altitudes in an attempt to improve their per- formance. Some research shows benefi ts to this type of training, but other studies do not. When athletes train at high altitudes, they generally go to elevations above 2000 m where the air pressure is 77 to 80 kPa (Table 2, page 543). At this altitude there is still 21 % oxygen in the air but all atmospheric gases are at lower density. A lower density means that each breath contains less oxygen than it would at sea level. Aft er three or four weeks the body compensates for lower oxygen levels by making more red blood cells to carry oxygen and producing more enzymes to utilize oxygen. When athletes return to lower altitudes they may feel energized, having an increased ability to use oxygen. Not all athletes perform better aft er altitude training. Disappointing performance may result because the athlete cannot train as rigorously while the body adjusts to the higher altitude and lower oxygen level. Some endurance athletes live at high altitudes but train at low altitudes. Th ey believe that this way they obtain the physiological advantages of high altitudes, but they can still train intensively. To check out an interactive graph that compares the concentration of atmospheric oxygen at various altitudes, WEb LINK Go To nElSon SCiEnCE To fi nd out more about being an athletic trainer, CAREER LINK Go To nElSon SCiEnCE In this investigation you will explore the effect of atmospheric pressure on a pop can. You will boil water inside the can to produce water vapour. Then you will cool the can rapidly by placing it in a pail of water. Equipment and Materials: chemical safety goggles; lab apron; plastic pail; graduated cylinder; empty aluminum pop can; beaker tongs; heat source (hot plate or Bunsen burner clamped to a retort stand); tap water This activity may involve open fl ames and boiling water. Tie back long hair and secure loose clothing and jewellery. 1. Wearing chemical safety goggles and a lab apron, your teacher will fi ll a bucket three-quarters full with cold water. 2. An aluminum pop can will be fi lled with about 10 mL of water. 3. Your teacher will then hold the can over a heat source until the water boils. 4. The can will then be placed over the pail of water, inverted, and submerged in the cold water. 5. Observe any changes in the aluminum can. A. How does heating the water in the can change the conditions inside the can? T/I B. What effect does the water in the pail have on the conditions inside the can? T/I C. Explain the changes that you observed when you inverted the can in the water. T/I How Strong Is your Pop Can? (Teacher demonstration) Mini Investigation Skills: Questioning, Planning, Performing, Observing, Analyzing, Communicating SKILLS HANDBOOK A1.2, A2.1 11.7 Atmospheric Pressure 545NEL 11.7 Summary • Pressure (P) is defi ned as the force (F) exerted per unit area (A). P 5 F A • Th e SI unit for pressure is pascal (Pa). 1 Pa = 1 N/m2 • Atmospheric pressure is the force per unit area exerted by air on all objects. Standard pressure (the pressure exerted at sea level by a column of air with a base of 1 m2) is 101.325 kPa. • Standard temperature and pressure (STP) conditions are 0 °C and 101.325 kPa. • Standard ambient temperature and pressure (SATP) conditions are 25 °C and 100 kPa. • Th e density of atmospheric gases is greatest at sea level. 11.7 Questions 1. Copy and complete Table 3 in your notebook. K/u Table 3 Equivalent Values of Pressure in Different Units Pressure (kPa) Pressure (mm Hg) (a) 58 (b) 125 (c) 130 (d) 950 2. (a) Defi ne STP and SATP. (b) Explain why scientists have defi ned two sets of standard conditions. (c) Which standard is most frequently used, and why? K/u 3. Skydiving is a sport that requires specifi c equipment for the demands of the environment (Figure 8). According to the Canadian Sport Parachuting Association, oxygen is mandatory for jumps that exceed 4572 m. Explain why this is necessary. T/I A Figure 8 At high altitudes, skydivers must wear oxygen masks. 4. Some athletes sleep in low-pressure tents prior to participating in an athletic event at a high altitude. How is this likely to improve athletic performance? Do you think this is likely to be more or less effective than high-altitude training? Explain. K/u A 5. Modern aircraft have pressurized cabins. T/I A (a) What is meant by “pressurized cabin”? (b) What can occur if pressure is lost in the cabin? 6. The Summer Olympic Games were held in Mexico City in 1968. They produced some interesting results. The performance of many endurance athletes fell short of expectations. Many events that involved jumping and throwing, however, produced better than expected results. Use the data in Table 2 to help explain these results. A 7. (a) Create a graph of air pressure against altitude, using the data in Table 2. Label each of the locations on your graph. (b) Describe the trend in the data. Suggest an explanation for this trend. (c) Research the altitude and typical air pressure where you live, and add it to the graph. K/u C A 8. Research altitude sickness. What are the symptoms? Explain this phenomenon. T/I A 9. Research how air is used in pneumatic nail guns. T/I C A 10. High-pressure injectors, sometimes called jet injectors or hyposprays, are used to inject vaccines and other medicines under the skin of patients without using a needle. Research how these devices work and the benefi ts and drawbacks associated with their use. T/I A Go To nElSon SCiEnCE 546 Chapter 11 • The Gas State and Gas laws NEL