Air Pressure Curriculum

Section 1—Lesson 1: Air Pressure is Non-obvious but Causes Effects

Background Information

The Non-obvious Nature of Air Pressure

Just like fish in water, we are so accustomed to the medium in which we live that we typically fail to notice it. Our bodies continually adapt to the sea of air around us; however, we are usually unaware of these adaptations. They only become obvious when air pressure changes rapidly, such as when we ascend in an airplane and our ears pop. Even in these situations, most people typically seek out obvious explanations before looking for non-obvious ones. This tendency can make sense in everyday life because it uses the most efficient search strategies, but it limits the variables one considers.

Scientifically accepted explanations often involve causes that are hard to notice. You can't observe the causes directly; you have to figure out that they exist. Students need to know that causes can be hard to notice, or even impossible to perceive, so that they don't limit their search for explanations to obvious variables.

Air pressure is a non-obvious variable. It was not formally recognized until 1643 when the mathematician Evangelista Torricelli discovered that air pressure accounted for the height to which water could be pumped out of mineshafts.¹ So it is no surprise that students and laypersons often fail to note air pressure's existence or contribution to an effect. Students have difficulty shifting their focus from the apparent features of tasks to less obvious variables, such as the behavior of air or water.

An Historical Puzzle

This lesson begins by introducing the historical puzzle that led to Torricelli's discovery. In the late 1500's, people tried to understand why pumps could not pull water up out of mines to a height greater than 32 feet. The cause was entirely non-obvious—air pressure. Picture the mine in a sea of air. The weight of the air or the atmospheric pressure pushing on the pool of water at the foot of the mineshafts dictates the height to which the column of water could rise. (For further background information read The Discovery of Air Pressure: A Brief History (PDF 62 KB).) We can't see air pressure. We are so used to it around us that we can't really feel it either. Typically, most people consider primarily obvious causes in their search for a solution. Only when the answer continues to be elusive are less obvious explanations considered. In the lesson, students are asked to generate a list of possible problems that could interfere with removing water from mines. Students typically begin by focusing on obvious variables, such as the tools used or the technology at that time, rather than considering non-obvious variables, such as air pressure, in attempting to explain the water's height.

Note to Teacher: An excellent resource on the history of this problem is the book, Connections, by James Burke, particularly pages 72-73.²

Inferring the Existence of a Non-obvious Cause

The lesson then goes on to have students construct a type of barometer (although they are not named as such) and to observe what happens inside it. Students notice that the oil stays in the straw at a certain level. Highlighting obvious effects that have no obvious cause can push students to begin to examine non-obvious causes. The lesson uses this strategy to engage students in recognizing the existence of a non-obvious variable—air pressure. After students consider what is going on with the barometers, they go back to the problem of the mines and consider if any connections can be made between the problems in the mines and their barometers. Students should realize that air pressure, a non-obvious variable, is the cause in both situations.

Note to Teacher: This lesson will work best if you do the first two steps, then teach the next lesson, and then come back to the remaining steps. Alternatively, it is possible to teach the lessons in sequence and revisit the ideas from the first lesson in the second one.