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The Role Played by GalileoGalileo was the court mathematician for the Duke of Tuscany during the early 1600s. When faced with the question from the Duke of Tuscany about why the pumps wouldn't "lift" water more than 34 feet; Galileo is supposed to have replied...."It seems that Nature abhors a vacuum only to a height of 34 feet." Galileo was already in trouble with the Church on the matter of Cosmology and World View. He may have been a bit reserved about plunging into another mess about the existence of vacuums because this topic was also in conflict with current Church doctrine about creation and the possibility of worlds beyond Earth.
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.
The Torricellian TubeThe person who did come to understand the role of air pressure and the existence of a vacuum was Evanglista Torricelli. Torricelli was a pupil of Galileo during the last year of Galileo's life and also inherited his position as Court Mathematician for the Duke of Tuscany. It should be noted that other people in Europe where doing related experiments on creating a vacuum using water columns in tubes or siphons and demonstrating the elasticity of air (Isaac Beeckman, Giovanni Baliani, Rafael Magiotti, and Gasparo Berti). These experiments or demonstrations focused mainly on trying to produce a vacuum. If a vacuum could actually be continuously maintained then Aristotle's framework was fallible and new conceptual models could take its place.
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.
Conclusion and educational implicationsIn a short span of about 50 years, researchers understood the effect of air pressure on the movement of fluids in tubes, pumps, and siphons. By creating an experiment to measure the effect of air pressure on a column of mercury, Torricelli set the stage for an explosion of scientific thought by introducing a mechanical apparatus that could be used to measure pressure. By challenging the validity of a conceptual framework, he freed others to see the world in a new way.
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. |
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发表于 2011-6-20 08:41:34
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