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course Phy242
2/7 10
Screw the bottlecap onto a bottle and squeeze the bottle. It should be no surprise that if the tube isn't capped, this will force air out of the tube.Comparing the state of the bottle before and after you squeeze:
Does the amount of air in the bottle increase or decrease?
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decrease, you're forcing the air out
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Does the volume of air enclosed in the bottle increase or decrease?
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decrease, bottle is getting smaller
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Does the pressure in the bottle increase or decrease?
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decrease, less air
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@& The bottle remains open to the atmosphere, through the tube.
The pressure at the beginning is the same as the pressure at the end. In between there is a slight increase in pressure, which causes air to be displaced out of the bottle.*@
Does the temperature of the air in the bottle increase or decrease?
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decrease, temperature is directly related to pressure
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@& If n and V are constant that is so.
However neither is constant, and there is nothing to significantly heat or cool the gas during the time frame of the squeeze.*@
Be sure you have explained all your answers.
Now cap the end of the tube and give the bottle a good squeeze, without straining yourself.
Comparing the state of the bottle before and after you squeeze:
Does the amount of air in the system increase or decrease?
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neither,its sealed
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Does the volume of air enclosed in the system increase or decrease?
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decrease, smaller area
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Does the pressure in the system increase or decrease?
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increase, youve have the same amount of air but in a smaller space therefore more pressure
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Does the temperature of the air in the system increase or decrease?
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increase, more pressure and pressure and temperature are directly related
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@& That relationship holds only for constant volume and amount of gas.
Temperature will increase slightly from the compression; work is being done on the gas, which increases the total KE of the molecules.*@
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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5%
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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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same, same amount of air just in a smaller area
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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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same, same amount of air
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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, 5%
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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, the system is sealed so the air has nowhere to go, 10%
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@& The pressure in the bottle will be very nearly the same as the pressure in the air column.*@
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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theyre equal, because the plug is held stationary
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Which do you think is greater, the pressure in the bottle or the pressure in the air column?
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Air column, because it is a smaller space
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@& If equal and opposite forces are exerted on the plug, which has the same area at both ends, it's because the pressures on the ends are equal and opposite.*@
Measure the length of the air column.
What is the length of the air column?
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approx 5 small 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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half a paperclip
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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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6,10%
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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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3%,8%,11%
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Self-critique (if necessary):
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Self-critique rating:
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&#Good responses on this lab exercise. See my notes and let me know if you have questions.
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