Gas Pressure Quiz

PDF page 406: The earth’s atmosphere exerts a pressure, as does any other gas. Although we do not normally notice atmospheric pressure, we are sensitive to pressure changes—for example, when your ears “pop” during take-off and landing while flying, or when you dive underwater. Gas pressure is caused by the force exerted by gas molecules colliding with the surfaces of objects (Figure 8.2). Although the force of each collision is very small, any surface of appreciable area experiences a large number of collisions in a short time, which can result in a high pressure. In fact, normal air pressure is strong enough to crush a metal container when not balanced by equal pressure from inside the container.

PDF page 407: F In general, pressure is defined as the force exerted on a given area: P = . Note that pressure is directly A proportional to force and inversely proportional to area. Thus, pressure can be increased either by increasing the amount of force or by decreasing the area over which it is applied; pressure can be decreased by decreasing the force or increasing the area. Let’s apply this concept to determine which would be more likely to fall through thin ice in Figure 8.3—the elephant or the figure skater? A large African elephant can weigh 7 tons, supported on four feet, each with a diameter of about

PDF page 407: F In general, pressure is defined as the force exerted on a given area: P = . Note that pressure is directly A proportional to force and inversely proportional to area. Thus, pressure can be increased either by increasing the amount of force or by decreasing the area over which it is applied; pressure can be decreased by decreasing the force or increasing the area. Let’s apply this concept to determine which would be more likely to fall through thin ice in Figure 8.3—the elephant or the figure skater? A large African elephant can weigh 7 tons, supported on four feet, each with a diameter of about

PDF page 407: The SI unit of pressure is the pascal (Pa), with 1 Pa = 1 N/m , where N is the newton, a unit of force defined as 1

PDF page 407: The SI unit of pressure is the pascal (Pa), with 1 Pa = 1 N/m , where N is the newton, a unit of force defined as 1

PDF page 408: Definition or Relation to Other Unit kilopascal (kPa) 1 kPa = 1000 Pa pounds per square inch (psi) air pressure at sea level is ~14.7 psi 1 atm = 101,325 Pa atmosphere (atm) air pressure at sea level is ~1 atm 1 bar = 100,000 Pa (exactly) bar (bar, or b) commonly used in meteorology millibar (mbar, or mb) 1000 mbar = 1 bar 1 in. Hg = 3386 Pa inches of mercury (in. Hg) used by aviation industry, also some weather reports 1 1 torr = atm 760 torr named after Evangelista Torricelli, inventor of the barometer millimeters of mercury (mm Hg) 1 mm Hg ~1 torr

PDF page 408: Definition or Relation to Other Unit kilopascal (kPa) 1 kPa = 1000 Pa pounds per square inch (psi) air pressure at sea level is ~14.7 psi 1 atm = 101,325 Pa atmosphere (atm) air pressure at sea level is ~1 atm 1 bar = 100,000 Pa (exactly) bar (bar, or b) commonly used in meteorology millibar (mbar, or mb) 1000 mbar = 1 bar 1 in. Hg = 3386 Pa inches of mercury (in. Hg) used by aviation industry, also some weather reports 1 1 torr = atm 760 torr named after Evangelista Torricelli, inventor of the barometer millimeters of mercury (mm Hg) 1 mm Hg ~1 torr

PDF page 407: kg m/s . One pascal is a small pressure; in many cases, it is more convenient to use units of kilopascal (1 kPa = 1000 Pa) or bar (1 bar = 100,000 Pa). In the United States, pressure is often measured in pounds of force on an area of one square inch—pounds per square inch (psi)—for example, in car tires. Pressure can also be measured using the

PDF page 408: Definition or Relation to Other Unit kilopascal (kPa) 1 kPa = 1000 Pa pounds per square inch (psi) air pressure at sea level is ~14.7 psi 1 atm = 101,325 Pa atmosphere (atm) air pressure at sea level is ~1 atm 1 bar = 100,000 Pa (exactly) bar (bar, or b) commonly used in meteorology millibar (mbar, or mb) 1000 mbar = 1 bar 1 in. Hg = 3386 Pa inches of mercury (in. Hg) used by aviation industry, also some weather reports 1 1 torr = atm 760 torr named after Evangelista Torricelli, inventor of the barometer millimeters of mercury (mm Hg) 1 mm Hg ~1 torr

PDF page 408: Definition or Relation to Other Unit kilopascal (kPa) 1 kPa = 1000 Pa pounds per square inch (psi) air pressure at sea level is ~14.7 psi 1 atm = 101,325 Pa atmosphere (atm) air pressure at sea level is ~1 atm 1 bar = 100,000 Pa (exactly) bar (bar, or b) commonly used in meteorology millibar (mbar, or mb) 1000 mbar = 1 bar 1 in. Hg = 3386 Pa inches of mercury (in. Hg) used by aviation industry, also some weather reports 1 1 torr = atm 760 torr named after Evangelista Torricelli, inventor of the barometer millimeters of mercury (mm Hg) 1 mm Hg ~1 torr

PDF page 409: We can measure atmospheric pressure, the force exerted by the atmosphere on the earth’s surface, with a barometer (Figure 8.4). A barometer is a glass tube that is closed at one end, filled with a nonvolatile liquid such as mercury, and then inverted and immersed in a container of that liquid. The atmosphere exerts pressure on the liquid outside the tube, the column of liquid exerts pressure inside the tube, and the pressure at the liquid surface is the same inside and outside the tube. The height of the liquid in the tube is therefore proportional to the pressure exerted by the atmosphere.

PDF page 409: If the liquid is water, normal atmospheric pressure will support a column of water over 10 meters high, which is rather inconvenient for making (and reading) a barometer. Because mercury (Hg) is about 13.6-times denser than water, a 1 mercury barometer only needs to be as tall as a water barometer—a more suitable size. Standard atmospheric 13.6 pressure of 1 atm at sea level (101,325 Pa) corresponds to a column of mercury that is about 760 mm (29.92 in.) high. The torr was originally intended to be a unit equal to one millimeter of mercury, but it no longer corresponds exactly. The pressure exerted by a fluid due to gravity is known as hydrostatic pressure, p: p = hρg where h is the height of the fluid, ρ is the density of the fluid, and g is acceleration due to gravity.