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:

1. The relationship between the length of the spouts and the height of the column of water to the hole in the side. (It should be noted that the water will not flow at a constant rate unless you continually add water to the system to replace the water going out the hole)
2. Determine if pressure depends on depth of the hole or the total volume of the liquid. (You will need different shaped containers with holes at the same height. Have students bring in various types of bottles before this unit starts and try to make the holes all the same diameter)
3. Determine if the pressure in a fluid is equal on all sides of a fluid at the same depth. Will water volume or rate drained out of a hole on the bottom will be the same as a hole out the side next to the bottom or on the opposite side.

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

1. Give students task of seeing which team can siphon water from one container to another the fastest or slowest. Ask them to describe the conditions and limits. When does it not work? How many different ways can a siphon be started? What factors control the rate of flow? Develop an explanation that includes a diagram or model of the forces you perceive acting on the water, tube, and containers. Compare student explanations of how siphons work. Record theories and inferences about the observations that have been made. Define controversial ideas and models of how a siphon works.
2. Ask students to predict what will happen if you try to siphon out of a sealed container? Using the information generated previously, students should work with a group/partner to compare ideas and record a reason/hypothesis to explain their prediction. Using two liter pop bottles with tubes sealed through the top try getting the siphon to work. (use clay to make a seal) Record observations and rework hypothesis.
3. Assign an out of class activity of designing and building a device that uses a siphon or principles of a siphon. Copies of Hero's practical devices could serve as an idea starter. Give the groups the task of describing how they think it works or why it doesn't work and analyze each others systems and explanations. This could be used as an assessment activity where by students have to show general knowledge of the topic to supply an analysis.

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

• The U Tube

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...

• The HEIGHT of water in each arm of the tube is EQUAL if:
1. No pressure is applied to both ends.
2. If equal pressure is applied to both ends.
3. If equal suction is applied to both ends.
• Also...The HEIGHT of the water in each arm of the tube is UNEQUAL if:
1. Pressure is applied only to one side.
2. If only one side is "sucked" on.
• Also...The column of water can defy gravity if you blow it to one end and seal the arm of the tube and stop blowing. The column will fill back down if you break the seal of the arm with the water in it.
• Also...A vacuum or "empty space" can be created if you blow the water into one arm, seal it with your finger, and then suck on the other end. Try pinching off the tube with a clamp to keep the vacuum from collapsing. What's inside the empty space? What causes the water to stay up? What causes the water to rush out when you let go of the seal?
• Revisit the siphon and use air pressure to help explain its function

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 idea of suction.

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:

1. Challenge students to a drinking race using pop bottles with tops sealed with clay and straws inserted. Also, use unsealed bottles, and straws with pinholes in them.
2. Show them a balloon that is inflated in a pop bottle and supply them with a balloon, penny, paperclip, straw, string, and bottle. Have groups of students work to inflate the balloon in the bottle.
3. Use 2 liter pop bottles that have an extra screw top opening glue at the bottom. Have students explore ways to inflate a balloon inside the pop bottle and record explanations of their findings. This double or triple screw top system allows for further combinations of blowing, sucking and the inflation or deflation of balloons. You are only limited by your students imagination.
4. Demonstrate suction cups or dent pullers. What creates the force that hold them down? How much force/weight can the dent puller/ suction cup hold? Is there a relationship to the size or surface area?
5. Try pulling a plastic/playtex glove out of a gallon mason jar that has been sealed to the opening with duct tape. Reverse the situation using a glove that has been inflated outside of the jar and sealed to the opening. Can't pull it out and you can't push it in. Why????
6. Crush popcans with air pressure by heating up water in the can and inverting them in cold water or make water go up an inverted testube that has been heated. Ask students to describe the forces involved in crushing the can or moving the water. What does heating have to do with it? Attach a balloon to the top of a pop bottle that has been heated in a pan of boiling water and allow to cool. Observe your results and provide an explanation.

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!