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ElizabethE:Pressure Inside Static Fluids

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What do you think your students should know about the pressure inside static fluids?

1) Fluids exert pressure in all directions

2) The presssure increases with depth.

3) The net pressure of of the fluid on an object is in the upward direction.

4)P = Patm +pgh

How will they learn it?

1) Fluids exert pressure in all directions

Students will learn this through observational experiments. Water bottle with pin holes at the same height work well. You may also use Pascal's law apparatus. Ask students to exlain what thier observations can tell them about the pressure. Connect this to students's knowlege about gasses.


2) The presssure increases with depth.

Students will learn through observational experiments and by constructing a qualitative argument. Water bottles with pin holes at different height s work well for this. In the qualitative argument students should analize the fluid by consudering several layers of the fluid. What is the relative magnitude of downward pressure of a layer on the layer below it. What about on the layer above it?


3) The net pressure of of the fluid on an object is in the upward direction.

Students can learn this from qualitative reasoning. (Relate to the balloon in a vacume experiment done earlier.) This can be easily done as a continuation of qualitative argument above that used layers. Now have students consider small volumes of the fluid. Have the students draw in pressure arrows of appropreate lengths. What direction is the net pressure in?


4) Testing experiment for 1-3

Students should now come up with a testing experiment that will test the ideas that they have developped in the first three sections. For example a bottle with a closed lid and two holes. What will happen when the tacks are removed from the holes? Have students create a prediction based on their ideas.


5) P = Patm +pgh

Students will construct the equation through quantitative analysis. To do this experiments can be done using a vaneer pressure probe (A table of experimental data can be given to the students instead.). In the first experiment students will observe that the pressure increasis proportionaly to the depth and that the minimum pressure is the atmospheric pressure. They should come up with the expression P = Patm + Ch where C is a constant. The same experiment can then be repeated with a fluid of different density. The students will analize why C is different for the two fluids and what C is.

Once students have come up with the mathimatical expression, they must test it. A testing experiment may consist of repeating the same experiment with a third fluid.

How will you assess them?

To assess students knowlege you may ask them to explain whay hapens to the size of a baloon when it is taken from hight altitudes to low altitudes. If you want, since students have already done the ideal gass law, they can calculate the change in the balloon's volume. Questions about water bottes at high and low altitudes can also be used. Questions about why wine bubles at high altitudes or why soda is carbinated may be discussed.

To assess their knowledge you may also ask about weather balloons. Why do they float at high altitudes, but not at ground level?

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