A common phrase used by people to describe or explain the movement of a fluid in tube by air pressure is that it is being "sucked up." This implies that you are pulling up the fluid with a force that is transmitted through the air. This pulling force is initiated by placing your mouth at the opposite end of the tube and "sucking" on it. Just what suction is and how it causes fluids to rise in tubes is perfect example of a discrepant event that can be the focus of a unit on pressure and the behavior of fluids. This curriculum module focuses on the historical context of our discoveries about air pressure and vacuums as a way to "suck" students into rediscovering these often counterintuitive concepts on their own.Level: Grades 6-9
In explaining, how water would go up a tube or straw if you sucked on one end, Aristotle might say: every thing moves by displacement. Since matter fills the world completely, matter that flows into one vessel must be displacing the matter that is already filling the space. It will not flow unless that matter can be displaced so other matter can be moved to fill in behind. He might also have used the idea that Nature could sense that you were trying to create a vacuum by expanding your lungs and the water and air in the tube was reacting by rushing in so the vacuum wouldn't happen, since "Nature abhors a vacuum".
Aristotle would not have employed the idea of air pressure because he did not recognize that it had weight in its "natural state" or exerted pressure on objects in it. The force moving the fluids up would have come from the fluids themselves and not from any external force outside the tube.
It was not until the late 1500's when Simon Stevin published a reworking of Archimedes' Hydrostatics in 1575, that the analogy of water was used by others as a basis to explain the effects of air pressure. During this same period, a Treatise on Pneumatics by Hero of Alexander (100 A.D.) was being republished and added to the interest in the role of air in the movement of fluids in tubes.
Hero's work focused on the functioning of siphons. He was a follower the Atomist's concept of matter and believed that minute particles and voids did exist. However, he did not accept the idea that a continuous vacuum or void could be sustained. His work is mostly concerned with producing mechanical apparatus that function on air and water pressure to produce effects from opening doors to making fountains.
The solution to this well known problem was of great importance to those involved in the economics of this industry. If water could be pumped higher with less energy and materials, then profits could be increased and more ore could be recovered. The limit to the height to which water could be suction pumped was thought to be caused by imperfections in the pumps themselves or by a semi-material fluid or ether seeping in from outside the pump. The belief Aristotle's conception of matter eliminated the possibility of using air pressure and vacuums to explain the limits to their pumps. They chose to create an alternative explanation for their experiences rather than seek a new conceptual framework to view the problem.
It appears that Galileo never really understood about the effect of air pressure or even its existence. However, he did write about his belief in the possible existence of a vacuum. Galileo wrote: "If the vacuum cannot be recognized either by the senses or by intellect, how have you managed to find out that it does not exist?" Galileo's role seems to have been one of questioner. Because of his status in the scientific community or perhaps his smudged reputation by the Church, he was able to question the validity of Aristotle's logic and ideas. This was the spark that would ignite others.
Torricelli did not need to be convinced of the existence of a vacuum. It seems that from studying Galileo's work he accepted its existence and proceeded to create an experiment to measure the effect of air pressure on a column of mercury. Torricelli designed an experiment (he did not perform it) in which he would fill a glass tube four feet long and sealed at one end with mercury. Placing his finger over the opening he would invert the tube in a bowl of mercury with the sealed end up and measure the resulting height of the column. The mercury would fall to about 30 inches in height and an empty space or vacuum was created in the top of the tube.
He perceived that the weight of the column of mercury was equal to the weight of the air column pushing down on the bowl of mercury. The two columns were in equilibrium. If the weight of the column or pressure it exerted on the surface of the mercury were to increase the height of the column of mercury should also increase and vice versa. Torricelli envisioned that the air exerts pressure due to weight of the vast ocean of it over the surface of the earth. This pressure should be like the pressure you experience when diving into water. The deeper you go the greater the effect. He seems to have been one of the first people to see the world in this way.
In 1648, Blaise Pascal confirmed Torricelli's idea (1643) of the variability of air pressure. Pascal's experiment took a Torricellian Tube (barometer) from sea level to the top of a mountain and observed the predicted movement of the mercury in the tube as falling when it increase in altitude.
The path of events culminating in Torricelli's experiment and Pascal's confirmation of the idea that air exerts pressure, provides a framework by which you can help your students experience the process of science and wrestle with restructuring their own world view. The basic notion is that there is a dynamic relationship between perception, experience, and explanation. Your students use their direct observations and common sense notions of the world to explain the events they encounter. What they can not see or experience directly (atmospheric pressure or vacuums) is very difficult for them to use in explaining the world around them. When faced with contradictory evidence, most students will try to "invent" a rationale to explain away the facts. The last thing that they will do is to restructure the way they conceptualize things in order to match the evidence collected.
The task of a teacher is to provide students with opportunities to challenge their ideas and allow them to participate in the scientific processes of collecting information, making arguments, and reworking their perception. The process is not swift or straight forward. In fact, it can be quite messy and confusing. The rewards come from the effort and ownership that students make in trying to convince others of their ideas and involve themselves in an activity that they help direct. Hopefully, they will gain an appreciation of the challenging work involved in explaining the simplest things and learn some science concepts, while having fun doing it.