Air Pressure and Water Flow

AIR PRESSURE AND WATER FLOW

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EXPLORATION

Materials

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3 suction cups (different sizes)

1 flexible transparent tube

2 buckets of water

1 spring scale

1 plastic plate with string

1 ruler

1 calculator

Soapy water

Dye packets

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1. Push a dry suction cup onto the table, then pull to remove it. Repeat the experiment, but wet the cup before pushing it onto the table with soapy water. Which is harder to pull? Discuss with your group why this is and record your observations below.

2. The force required to remove the suction cups from a surface represents the difference in pressure between the inside of the suction cup and the ambient air pressure. The pressure inside a well-sealed (wet) suction cup is very low, so the force you exerted was almost exclusively counteracting the ambient air pressure.

· We can determine the magnitude of air pressure at ground level from the suction cups. Start by measuring the diameter of your smallest suction cups.

· Calculate the area of the suction cups: and record. Remember: radius is half the diameter and is roughly equal to 22/7. Measure the cavity of the cup, not the whole cup.

Suction Cup Size / Diameter (in mm) / Area (in mm2)
Small
Medium

3. Take a quantitative measurement of how hard it is to pull the medium sized wet suction cup off the plastic plate. Three group members are needed. One holds the plastic plate down on the table firmly, providing a smooth surface. The second partner suctions the cup to the plastic plate with soapy water, and then hooks the spring scale onto the suction cup and prepares to slowly pull the suction cup off the plastic. These two group members should avert their eyes from the cup for safety. The final group member observes the mass on the spring scale from the side.

· Note the force on the scale in Newtons (N) right before the cup releases the plate. That force is the most the suction cup can lift.

· Calculate pressure given the equation: The desired units are m2, so you must convert from mm2 to m2 (divide by 1,000,000).

Do the measurement three times for each cup to get a more precise measurement.

Small
Suction Cup / Force (in Newtons): / Pressure
(in Newtons/ m2):
1st run
2nd run
3rd run
average value
Medium
Suction Cup / Force (in Newtons): / Pressure
(in Newtons/ m2):
1st run
2nd run
3rd run
average value

Discuss your observations with your group. Do you observe any trends? How does surface area relate to the force you must exert to pull the suction cup?

4. Pour dye pack into a bucket of water. Form an upright U-shape with the flexible transparent tube and fill it ¾ of the way with the dyed water. Compare the height of the water in each upright part of the tube. Now change the relative water level by raising and lowering the tube ends, and bending or looping your tube. Record what shapes you try and the water levels at each attempt. What factors contribute to water level changes?

Challenge Work:

1. The large suction cup is extremely hard to pull straight off of the table. Try sliding the cup along the table and off the edge. Explain what you observe.

2. State your step 4 observations as a basic principle of water behavior.

Everyday Applications

· When dams are built, they must be reinforced at their base. You can see this feature in dams, such as this one from Australia. The angled base is strong enough to support the higher pressure at the floor of the reservoir.

· Siphons are frequently used to move massive volumes of water easily (such as in aquariums).

· Flexible tubing filled with water (just like you used today) is used by contractors to verify that a foundation is truly level.

· Siphoning is a highly dangerous tool used by some to steal gasoline when there are high gas prices.

· Your ears pop when you travel through elevation in planes or cars

APPLICATION

1. Take your flexible tube and fill it ¾ with water in case you emptied it. Cover or crimp one exposed end of the tube with your finger, and change the height of that end of the U. What do you observe? Are the two water surfaces level? Discuss with your group and record your explanation.

2. Fill a soda bottle from another bucket of water using a siphon. Place the source bucket on the table, and predict which height will fill the soda bottle more quickly (or if they will fill at the same time): if the soda bottle is on the chair or the floor.

Note: you do not need to put your lips on the tube; you can submerge the tube then cap the ends with your fingers so they are full of water.

3. Place the soda bottle on the floor and fill it from both heights. Record the height that resulted in a faster fill time. Explain your observations.

4. Soda Bottle Diver

Fill a soda bottle full of water. Insert the air-filled eyedropper into the bottle pointed-tip down. Twist the lid on the bottle tightly making sure the bottle is still full to the brim. What happens when you squeeze the bottle? Discuss with your group what you’ve observed and explain the effect. (*Hint: Pay attention to the water level inside the eyedropper)

Challenge Work:

1. What would happen if the air in the eyedropper were sealed in with some putty at the tip?

Summary

Final Clean-up

Please clean all table surfaces you used and replace equipment on the carts. Empty the soda-bottle diver, and separate the pieces.

Bibliography and recommendations for further reading:

Wikipedia contributors, "Fluid statics," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Fluid_statics&oldid=53180054 (accessed May 19, 2006).

Wikipedia contributors, "Dam," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Dam&oldid=53809203 (accessed May 19, 2006).

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