Gas Pressure Quiz

PDF page 468: 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 9.2). 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 468: 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 9.2). 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 469: 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 9.3—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 469: 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 9.3—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 469: 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 9.3—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 469: pressure exerted by each blade is about 30 lb/in : 2 lb 1 skater 1 blade pressure per skate blade = 120 × × = 30 lb/in 2 skater 2 blades 2 in Even though the elephant is more than one hundred-times heavier than the skater, it exerts less than one-half of the pressure and would therefore be less likely to fall though thin ice. On the other hand, if the skater removes her skates and stands with bare feet (or regular footwear) on the ice, the larger area over which her weight is applied greatly reduces the pressure exerted: 2 lb 1 skater 1 foot pressure per human foot = 120 × × = 2 lb/in 2 skater 2 feet 30 in

PDF page 469: 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 469: 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 470: 1 atm = 101,325 Pa = 760 torr

PDF page 470: 1 atm = 101,325 Pa = 760 torr

PDF page 470: 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

PDF page 470: 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