| || ||Sucking Students Into Understanding Air Pressure and Vacuums|
Basic Hydrostatic Concepts
Historically, the comparison of the atmosphere to the ocean is key to the understanding of air pressure. Many of the stumbling blocks to making the analogy of air to water is that air was not conceived to have mass or weight. A volume of air or water in its natural state being surrounded by itself was not thought to have weight or be able to exert pressure. People could NOT SENSE the weight or pressure, so it was not thought to exist. Fluids exert pressure uniformly on all surfaces at the same depth. You don't feel the pressure because it is pushing on you equally on all sides. It was also hard to see that the air was different than water in that it could be compressed, whereas water basically is NOT compressible. Hence, the density of water is constant as depth changes. This is NOT true for air. Pressure is equal to force divided by surface area: P = F/sa. In a liquid, the pressure is equal to the height of the column times the density: P = h x d
Observations of water flowing out of holes in plastic containers
Students to observe the flow of water out of various plastic containers that have been punctured near the bottom. (use a hot nail) The water spouts are different lengths and decrease in length as water flows out of container. Use student observations to discuss the cause of different spout lengths. (Ideas such as pressure, weight, density, and force should be discussed) This exploratory activity is ideal for allowing small group interaction and discussion. Students should be encouraged to draw diagrams or models and describe the forces involved in causing the observations that they make. They should also be making hypotheses and inferences to find help find FOCUS QUESTIONS for the next step. The Teacher can help by recording observations and explanations on the board or large pieces of paper to be saved and referred to later.
The two basic ideas that this activity should bring to the surface is the relationship between depth and pressure in a fluid. The volume of water is NOT a factor. It should also focus on the idea of pressure being equal in all directions. The students should be encouraged to work in groups to select a focus question and method of gathering information to provide support for their ideas. Groups should be asked to present their ideas to the class. The Teacher again serves as a supervisor in helping students design experiments and moderator in discussions. Introduction to basic science concepts and or historical ideas can be related to the findings of the class after THEY have expressed THEIR ideas.
Based on their input, students can investigate a variety of things, such as:
Further demonstrations with drops of food coloring in oil or the round shape of balloon may serve as a way to visualize the equal pressure being exerted on the surface. You might also try watching bubbles form in soda pop in tall bottles or graduates and see how the size changes from small to large as they rise to the top. This will help them to see how pressure decreases as the bubble rises and allows it to expand. Try blowing up a balloon that is sealed to a tube at various depths in the swimming pool. Will the balloon be harder to inflate in deeper water?
Linking Hydrostatics and Air Pressure
The concept of pressure at equilibrium states for fluids was worked out by Archimedes (250 B.C.) No significant work was done until Simon Stevin (1575) published his hydrostatics treatises. During this same period the work of Hero of Alexandria (100 A.D.) was being republished and played apart in linking pressure in water to pressure in air. These were used by Blaise Pascal and others to develop an understanding of air pressure and the functioning of the Torricellian Tube.
The basic explanation of the functioniong of a siphon is that the shortest arm of the siphon exerts the least force downwards. The weight of the water in the arm is a force in opposition to the force of air pressure pushing the water up. So, the net force up is greater in the shorter arm of the tube. This unbalanced force causes the water to flow up the shorter arm and out the longer arm.
Students will have problems visualizing the air as being a force pushing on the surface of the water. They will have difficulty in thinking about the ability of water to transmit force throughout a container. The goal in this section is to let them "mess around" with this simple device and make observations and conclusions based on the information THEY collect. The telling and modelling of "proper" science ideas should wait until they have some experiences to relate it to.
Practical experiments using a siphon
The Idea of Suction and the Existence of a Vacuum
Students are to investigate the phenomenon of "suction," compressibility and pressure of air in various settings. Up to now the idea that air exerts pressure by the fact that it has mass and we are at the bottom of an "ocean" of it, probably has only been hinted at by your students. In trying to explain the siphon some of them probably used air pressure as an idea to help explain the observations they made. You've probably had to bite your tongue to keep from just telling them about it. Try to hold on if its not too sore, we are getting close to using this in our explanation.
Experimenting with tubes, bottles and balloons
Using a U shaped flexible tube 150 centimeters long and partially filled with water and colored with food coloring: Students are asked to explore all the various way to get the liquid in the tube to move, stay in balance or be unbalanced.
Numerous varieties exist... Blow in one end and close the other. Blow in both ends. Suck in both ends. Suck in one end and close the other. Have contests of who can exert the most pressure by sucking or blowing. Students should be given the task record the set ups and supply an explanation of the motion of liquid in the tube.
The point of this activity is to get students to think in terms of the motion of the fluid being related to the existence of balanced and unbalanced pressure in the tube. Also, they will experience the creation of a vacuum or "empty space". Students should be encouraged to have FUN! while also being responsible for recording observations and reflecting on finding patterns or relationships between the movement of liquid in the tube and forces being applied.
Things to observe in this activity...
Use the results and ideas of this activity to reflect on the explanations for the functioning of the siphon. Student should discuss the following questions and rework their ideas about how the siphon works. Students might publish a treatise on the function of siphons recording and documenting their ideas and have a debate about how they work.
Why doesn't a siphon work in a closed container? When does a siphon not work? What causes the rates or direction of flow to change? How does blowing and sucking in a tube (inverted siphon) relate to the siphon. In the experiments on hydrostatics we showed that water exerts pressure relative to its depth; does air?
The notion that suction is a force PULLING the water UP will come out of this activity if it hasn't already. Challenge students to find proof for this idea. Many activities can be used to investigate the idea of suction and air pressure. Here are a few I like:
Creating Closure and Applying Concepts
Comparing a straw to a suction pump: What's the limit?
By now you've pretty much exhausted the ideas necessary to explain the motion of liquids in tubes and the phenomenon related to suction and vacuums.
This would be a good point to start tying up the loose ends and relate it to the historical mystery of suction pumps and the debate about the existence of vacuums. The question The Duke of Tuscany posed to Galileo about the limit of a suction pump could serve as point of departure to discuss ideas about matter and the existence of vacuums and the role of air in explaining the phenomenon.
Basically, a suction pump work just like a straw. If you have access to a stairwell or vertical space of 35 feet or more. Students can experiment with the limits of a straw. Using a flexible tube students can try "lifting" water to the highest height. Is there a limit? What causes the limit? If more suction force could be applied could we get the water higher? Is it a problem with the materials or does it violate some kind of Natural Law? Does "Nature Abhor a Vacuum?" Is there a subtle matter that is able to move through the pores in the tube to fill the "empty space?"
You could also create a Torricellian Tube using water instead of mercury. The limit to the suction pump and the height to which water will extend in a closed vertical tube is about 34 feet. This being due to the force of the weight air on the surface of the liquid is equal to the force of the weight of the column of water in the tube. All you have to do is find a way to fill the tube. Seal one end and lift the sealed end to the top. Meanwhile, keep the other end in the water. An empty space or vacuum should form at the top. Students could be asked to calculate the volume and mass of water in the tube and create a measuring scale to observe the changes in air pressure. Using the idea of balanced forces you should be able to estimate the force or weight of the column of air above the surface of the container of water (14.7 lbs/sq.in.).
If you can leave the tube up, you could use it in Earth Science class to measure changes in air pressure as long as it is in a place where temperature is constant.
The mystery of the snorkel
As a cumulative activity you can have your students wrestle with this Question??? Skin divers snorkel with a twelve inch Snorkel to breath through. Why aren't snorkels longer? How long can a snorkel be? If there is a limit..What limits the length? Experiment with students trying to breath through a snorkel made of plastic tubing of varying lengths in the swimming pool. Supply an explanation for your observations. Try it!
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