Unit 4Sections A.4-A.6 In which you will learn about: Definition of pressure Units of pressure Applications of pressure Atmospheric pressure
A.4 Pressure • In everyday language, the word pressure can have many meanings • Feeling too busy • Feeling forced to behave a certain way • The greater the pressure, the more “boxed in” you feel • To scientists, pressure also refers to force and space, but not in the same way!
Pressure is the force applied divided by the surface area upon which the force is applied • Pressure = force/area • Pressure is directly proportional to the force applied • Pressure is inversely proportional to the area upon which the force is applied P P F A
Think back to a common experience: someone steps on your foot • The pressure (or maybe pain) you feel depends on two variables: force and area • You can feel the difference between a large person and a smaller person (force) • If same weight (force) is applied, then shoe size will give different effects (area)
In the same way someone exerts pressure if they step on your foot, gas molecules exert pressure on the walls of their container • Gas pressure = # of collisions between gas particles and container wall
Units of Pressure • Pressure has several different units • millimeters of mercury (mm Hg) • torr • atmospheres (atm) • pascals (Pa) • pounds per square inch (psi)
SI Base Units & Derived Units • Remember that the SI system creates standards for measurements and there are seven base units for quantities of length, mass, temperature, luminosity, amount of substance, electric current, and time. • Any units that are a combination of SI base units are considered derived units
Revisiting all of the pressure units: • mm Hg = SI base units of length (refers to barometers) • torr are based on mm Hg (Torricelli was the scientist that invented barometers) • Atmospheres are based on Pascals • pascals are N/m2 where N= newton which is a unit of force used by physicists • 1 N = 1 Pa = 1 kg∙m/s2 (note that these are all SI base units) • psi is the English (non-metric) way to measure things that we see in the U.S. (think of tires)
Which units should we use? • Well, unfortunately, we use all of them! • SI unit is the pascal or kilopascal • Atmospheres are the easiest numbers to work with • mm Hg come from barometer readings (and so do torr) • psi is used in the U.S. • To convert from one unit to the other, use the following equivalents for standard pressure at sea level: • 101,325 Pa = 101.325 kPa = 1.00 atm = 760 mm Hg = 760 torr = 14.7 psi
A.5 Applications of Pressure • Please answer the following questions as part of your homework: • Two bricks of the same mass are lying on the ground. One is standing upright, and the other is lying flat. • Is each brick exerting the same total force upon the ground? Explain. • Is each brick exerting the same pressure upon the ground? Explain.
2) Calculate the pressure, expressed in pascals, that the upright brick exerts upon the ground. It is exerting a force of 18 N, and the dimensions of the brick surface touching the ground are 9.3 cm x 5.5 cm. 3) You need to chop some wood to build a fire. You quite obviously reach for an axe rather than a hammer to complete this task. Explain your choice in terms of the concept of pressure.
A.6 Atmospheric Pressure • The unit mm Hg (or sometimes inches of Hg, used by weather forecasters) suggest that pressure is measured from the height of a column of mercury • How is this possible? • Recall that in Activity 7, you covered a test tube filled with water and inverted it into a container of water. • You then uncovered the test tube and the air pressure forces more water into the test tube so the water level doesn’t change • Now imagine the tube is taller – eventually it gets to a point where the water level doesn’t reach the top
The puzzling effect of the water level not changing until the tube is large enough was first observed by scientists in the mid-1600s. • Scientists discovered that 1 atm of pressure can only support a column of water 10.3 m (33.9 ft) high, even if the column was taller. • A barometer (a device that measures atmospheric pressure) based on a tube filled with water would be much too tall to be useful. • Scientists replaced water with mercury, a liquid 13.6x more dense than water. • The resulting column is 13.6 times shorter. • Thus, at 1 atm of pressure, the mercury column has a height of 760 mm; 1 atm = 760 mm Hg
HOMEWORK QUESTIONS 4) U.S. weather reports generally express air pressure in units of inches of mercury. During a severe storm, the barometric pressure can drop to as low as 27.2 inches of mercury. Convert this air-pressure value to A. mm Hg (HINT: 1 inch = 25.4 mm) B. atmospheres (atm) C. kilopascals (kPa) 5) Heavy vehicles that must move easily over loose sand are often equipped with special tires. A. Would you expect these tires to be wide or narrow? B. Explain your answer using the concept of pressure.