Air Pressure Curriculum

Section 2—Lesson 3: Passive Causal Agents and the Omnidirectional Nature of Air Pressure

Background Information

Passive Versus Active Causal Agents

When we envision causality, most of us picture some type of action leading to some type of effect. In this image, the cause is seen as active—it does something to lead to a result. However, some forms of causality do not neatly fit this image. Instead, they bring about outcomes (sometimes obvious and sometimes not) in ways that one might call passive. What do we mean by this? Here is an example:

When riding in a car at fifty miles per hour, everything in the vehicle is moving at 50 miles per hour, including us. The car has brakes to stop it. What stops us? Well, as long as the stopping of the car is gradual, we might not notice the role that our seatbelt plays in helping to stop us. It is a passive restraint system. When a car stops short (amplifying the effect), we are more likely to recognize our seatbelt as the cause that holds us in our seats each time the car stops. A more active causal agent would have a greater chance of being recognized. For instance, if a hand came out of the dashboard and pushed passengers into their seats each time the car came to a stop, students would certainly recognize the hand as a causal factor in the system.

Unidirectionality Versus Omnidirectionality

In order to understand air pressure as scientists do, students need to think about air pressure as something that exists all around them and works in an omnidirectional manner. However, as soon as we begin to use language like "push," students begin to associate this with a force. This notion is very appealing because it portrays the air pressure as "doing something." It is seen as an active causal agent. This typically leads students to the idea of air pressure pushing down in a unidirectional manner. Confusing the matter further, we do speak of individual molecules that make up the air as forceful—bumping into and bouncing off of things. Air pressure is due to the collective effect of individual molecules colliding and bouncing around. It is well documented that students have difficulty reasoning about collective behaviors and moving back and forth between levels of individual interaction and collective, emergent outcomes.¹ A further type of confusion may arise, in that some students think of the molecules that make up the air as passive, but visualize pressure as a distinct and active agent that is separate from the molecules, and pushes them along. These students do not envision the molecules bouncing off of the objects in the system.

Beliefs About Gravity and Density Can Inadvertently Reinforce a Unidirectional View

Understanding air pressure as omnidirectional is especially hard when students understand some things about gravity and density. The earth's gravity holds the molecules of the atmosphere close to Earth. The molecules are most dense closest to the earth's surface, and least dense at higher altitudes. When students try to integrate these ideas with concepts of air pressure, it results in the belief that air pressure just pushes down because gravity is pulling molecules that make up the air towards the earth's surface. However, this view leaves out part of the story of the atmosphere. The particles are energized by sunlight and are bouncing all around.² Therefore, pressure acts omnidirectionally instead of unidirectionally. It does follow from the logic above that there are different amounts of pressure at different altitudes, although this doesn't mean that pressure only pushes down.