(a) 26.4 cm Hg × 10 mm Hg 1 cm Hg × 1 torr 1 mm Hg = 264 torr 26.4 cm Hg × 10 mm Hg 1 cm Hg × 1 torr 1 mm Hg = 264 torr The pressure on the left is due to the gas and the pressure on the right is due to 26.4 cm Hg, or mercury.) We could use the equation p = hρg as in Example 23.2, but it is simpler to just convert between units using Table 23.1. (The pressure at the bottom horizontal line is equal on both sides of the tube. (c) bar SolutionThe pressure of the gas is equal to a column of mercury of height 26.4 cm. Table 23.1 provides some information on these and a few other common units for pressure measurementsĬalculation of Pressure Using a Closed-End ManometerThe pressure of a sample of gas is measured with a closed-end manometer, as shown to the right. Pressure can also be measured using the unit atmosphere (atm), which originally represented the average sea level air pressure at the approximate latitude of Paris (45°). 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. 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). The SI unit of pressure is the pascal (Pa), with 1 Pa = 1 N/m 2, where N is the newton, a unit of force defined as 1 kg m/s 2. (credit a: modification of work by Guido da Rozze credit b: modification of work by Ryosuke Yagi) Let’s apply this concept to determine which exerts a greater pressure in Figure 23.2-the elephant or the figure skater? A large African elephant can weigh 7 tons, supported on four feet, each with a diameter of about 1.5 ft (footprint area of 250 in 2), so the pressure exerted by each foot is about 14 lb/in 2:Īlthough (a) an elephant’s weight is large, creating a very large force on the ground, (b) the figure skater exerts a much higher pressure on the ice due to the small surface area of the skates.

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. Note that pressure is directly proportional to force and inversely proportional to area. In general, pressure is defined as the force exerted on a given area: P = F A. If you actually perch a bowling ball on your thumbnail, the pressure experienced is twice the usual pressure, and the sensation is unpleasant. These may seem like huge amounts, and they are, but life on earth has evolved under such atmospheric pressure. At sea level, this pressure is roughly the same as that exerted by a full-grown African elephant standing on a doormat, or a typical bowling ball resting on your thumbnail. You can watch an explanation of this phenomenon in the next video.Ītmospheric pressure is caused by the weight of the column of air molecules in the atmosphere above an object, such as the tanker car. A dramatic illustration of atmospheric pressure is provided in this brief video, which shows a railway tanker car imploding when its internal pressure is decreased.

1 atm to psi proof