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course Phy 242
Jan 28 around 10:30am.
Brief Bottle Experiment 1bThe Air Column as a measure of Pressure
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Siphon a plug of water into the tube, seal the end of the tube to create an air column between the plug and the sealed end, and screw the cap back on. Give the bottle a moderate squeeze.
Does the air column get longer or shorter? By what percent do you estimate the length of the column changes?
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Shorter, 5%
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Does the volume of the air column increase or decrease? By what percent do you estimate the volume of the column changes?
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Neither
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Does the number of molecules in the air column increase, decrease or remain the same? By what percent do you estimate the number of molecules changes?
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Remains the same, 0%
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Does the mass of the air in the air column increase or decrease? By what percent do you estimate the mass of the air in the column changes?
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Increase, 5%
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@& If the number of molecules remains the same, their total mass remains the same. Molecules don't change their mass unless there's a chemical and/or nuclear reaction.*@
Does the pressure in the air column increase, decrease or remain the same? By what percent do you conjecture the pressure in the column changes?
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Increase?!
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@& By what percent do you think it increases?*@
Does the pressure in the bottle increase, decrease or remain the same? By what percent do you conjecture the pressure in the bottle changes?
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Increase, squeezed, same amount as pressure in the tube.
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When you hold the bottle in the squeezed position, with the water plug stationary, the pressure in the bottle results in a force on the plug which pushes it toward the capped end, while the pressure in the air column results in a force that pushes the plug away from that end. Which force do you think is the greater, or are they equal?
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Equal because cap is staying on.
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@& It's not so much because the cap is staying on. It's designed to hold with a pressure hundreds of times that you're creating.
The point is that the water plug is stationary, meaning it's not accelerating, so that the force from one side is equal to the force from the other.*@
Which do you think is greater, the pressure in the bottle or the pressure in the air column?
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Air column
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@&
The water plug is stationary, meaning it's not accelerating, so that the force from one side is equal to the force from the other. The area of one end of the water plug is equal to that of the other. So the pressures have to be equal.*@
Measure the length of the air column.
What is the length of the air column?
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Approx. 4 paperclips
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How far would the water plug have to move to make the air column 10% shorter?
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¼ of one of the paperclips?!
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Squeeze the bottle so the air column becomes 10% shorter. It's up to you to figure out how to tell when it's 10% shorter. If you can't squeeze hard enough to achieve the 10% difference, then figure out what percent you can manage and note the percent in your answer.
On a 1-10 scale, with 10 the hardest squeeze of which you are capable without risking injury, how hard did you have to squeeze the bottle and what percent change did you achieve in the length of the air column?
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5, 10%
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Now, using the same 1-10 scale, give the bottle squeezes of 2, 5 and 8. Estimate the percent changes in the length of the air column.
What were your percent changes in air column length?
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2%, 10%, 15%
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Now by heating and/or cooling the bottle, what extremes in air column length can you achieve? Careful not to melt the bottle. It won't handle boiling water, and you shouldn't mess with water hot enough to scald you or cold enough to injure you (e.g., don't use dry ice, which in any case is too cold for the bottle, and certainly don't use liquid nitrogen).
Report your results:
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I cooled the bottle by holding it in the freezer for a minute and saw that the air column length increased. The water in the tube gradually started going back down into the bottle, mm by mm.
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@& Good, but be sure to see my notes.*@