Discrepant Event Title
|Levels of Liquid in a Glass U-Tube |
|Author: Dean Papadakis | | |
|Discrepant Event - Teacher's Guide | | |
|SED 695B; Fall 2007 | | |
|[pic] |The first step in addressing this question | |
| |is to: MAKE OBSERVATIONS | |
|[pic] |
|Here is a picture of the entire U-tube |
| |
|Making OBSERVATIONS of the problem, is the first step to understanding the problem. |
|For example, does the sentence above, “Here is a picture of the entire U-tube”, seem to be angled |
|upward? |
|Doing a simple measurement (e.g. 2 vertical measurements of distance) can show that this is not the case, |
|and in fact, this apparent sloping is merely an optical illusion. |
|However, the two different heights of liquid in the U-tube, is NOT an optical illusion. |
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| | |
|Detailed Explanation of Discrepant Event | |
| | |
|A glass, U-tube (~ 50 cm high) is constructed so that one side is taller on one side compared to the other. | |
|It contains fluid to which food coloring has been added, which appears to reach a higher level on one side, | |
|compared to the other side. | |
|Two small corks are placed in each side of the U-tube. | |
|Students are asked to “explain” why the liquid is higher on one side compared to the other side, using ideas from | |
|DENSITY. | |
|The discrepant event that occurs is that the level of liquid in each side of the tube should be the same, if the pressure of the atmosphere pushing down is pushing down equally | |
|on both sides. | |
| | |
|Principles illustrated | |
|The concept of DENSITY is important to this demonstration. | |
|Also, shown by this demonstration is the relationship between the density of a liquid, its mass and its volume. | |
|The principle of PRESSURE should be understood as being force or weight divided by area. | |
|Standards | |
|Density and Buoyancy (from 8th grade science) | |
|All objects experience a buoyant force when immersed in a fluid. As a basis for understanding this concept: | |
|Students know density is mass per unit volume. | |
|Students know how to calculate the density of substances (regular and irregular solids and liquids) from measurements of mass and volume. | |
|Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced. | |
|Students know how to predict whether an object will float or sink. | |
| |
|Questioning Script |
|Prior knowledge & experience: |
|Students should understand the concept of density. Density = Mass/Volume |
|Students should also have skill in making observations. |
|Students should also understand the difference between an observation and an inference. |
|They should understand how the variables of mass and volume can be varied and how they affect the density. |
|For example, if the density of a liquid changes, what happens to the volume of liquid if the mass is held constant? |
|Root questions: |
|What is the primary cause of the liquid level being higher on one side compared to the other |
|side? |
|Does the difference in height of the glass tubing, affect the results? |
|Do the two corks placed in the tube affect the results? |
|Which scientific principle or concept can help to explain this behavior? |
| |
|Target response: |
|Observers may want to focus attention on different areas, one at a time. For example, what |
|might happen if the corks are removed? |
|Students may want to consider the possibilities first, before the corks are actually removed. |
|If the levels were to change as a result of removing the corks, what would explain this result? |
|If nothing changes when they are removed, we must look elsewhere to explain the problem. |
|Students could then shift their attention on the fluid and make observations about the fluid. |
| |
| |
| |
|Common Misconceptions: |
|When one substance (A) floats in another substance (B), students will say that substance A is |
|lighter than B. |
|Fluids will always seek their own level. |
|[pic] |A close up view of the two levels of liquid. |
|[pic] |Has removing the corks affected our problem? |
| | |
| |Why don’t the levels of fluid in the left and|
| |right sides of the tube, seek the same level?|
| |(misconception #2) |
|Here are some questions to consider: |Answers: |
|Does a brick of lead have the same mass as an equally sized construction brick? |No, a lead brick is heavier. |
|What is the primary reason that the two bricks have unequal mass? |They have different densities. |
|What property must be changed, in order to make the construction brick equal the mass of the lead brick? |The volume of the brick must be changed. |
|[pic] |The top left picture shows two bodies of |
| |liquid separated by a barrier, whose valve is|
|Now, have you figured out the discrepant event? |closed. The top right picture shows what |
|What are some common liquids that are less dense than water? |happens when the valve is opened. |
| |When the valve is opened, the two levels of |
|Solution is shown below! ( |liquid reach the same level, so that the |
| |pressure on both sides is |
| |equal. |
| |The bottom left picture shows a fluid of high|
| |density separated by a barrier with the valve|
| |closed, from a fluid of low density. |
| |The bottom right picture shows what happens |
| |when the valve is opened. The two levels of |
| |liquid reach the SAME PRESSURE, but not the |
| |SAME LEVEL. |
| | |
| |CONCLUSION: Fluids will only seek the same |
| |level when they have the SAME DENSITY. |
|[pic] |
|Here is how this discrepant event was made. The U-tube was filled with water colored with food coloring, to a level several inches from the top. |
|Then methyl alcohol colored with the same color as the water (although here I used yellow color so that it can be seen more clearly), was added to one side of the U-tube. |
|Since the methyl alcohol is less dense than the water, it takes a greater volume of it to equal the same weight of a smaller volume of water. |
References & Links:
Physics Begins with an μ . . . Mysteries, Magic, and Myth by John W. Jewett, Jr. Copyright 1994 by Allyn and Bacon
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