#$&*
course Phy 242
2-20I still have a hard time understanding the relationship of the number of air molecules with respect to pressure increases. I understand that the air density increases because we are compressing same amount of air molecules .into a smaller area and that when pressure increases there is some temp increase. I understand most of the relationship just not the one between increase in pressure and increase in air molecules
PART ONE
____________________________________________________________________________
Screw the bottlecap onto a bottle and squeeze the bottle. It should be no surprise that if the tube isn't capped, this will force air out of the tube.
Comparing the state of the bottle before and after you squeeze:
Does the amount of air in the bottle increase or decrease?
decrease due to the fact c.s. area deceases
#$&*
Does the volume of air enclosed in the bottle increase or decrease?
decrease for the same reason
#$&*
Does the pressure in the bottle increase or decrease?
increase because using Bernoulli’s equation we know for decrease in volume there must be an increase somewhere else which is in part wrt pressure
@& The pressure at the end will equal the pressure at the beginning; since the tube is open the pressure in both states is atmospheric pressure. There is a slight and temporarly pressure increase as air is being forced out of the bottle.*@
#$&*
Does the temperature of the air in the bottle increase or decrease?
increases
#$&*
Be sure you have explained all your answers.
Now cap the end of the tube and give the bottle a good squeeze, without straining yourself.
Comparing the state of the bottle before and after you squeeze:
Does the amount of air in the system increase or decrease?
neither seems to stay fairly constant
#$&*
Does the volume of air enclosed in the system increase or decrease?
because the c.s. area of bottle decreases slightly the volume of air seems to decrease in bottle but all that means is we are forcing the air into closed tube so we are not losing it, just compacting it in the tube
#$&*
Does the pressure in the system increase or decrease?
Niether really, but there may be a very slight increase
#$&*
Does the temperature of the air in the system increase or decrease?
increases slightly
#$&*
@& When the tube is capped you get a significant (i.e., measureable) decrease in volume accompanied by a significant increase in pressure.*@
PART TWO
__________________________________________________________________________
Brief Bottle Experiment 1b
The Air Column as a measure of Pressure
________________________________________
Siphon a plug of water into the tube, seal the end of the tube to create an air column between the plug and the sealed end, and screw the cap back on. Give the bottle a moderate squeeze.
Does the air column get longer or shorter? By what percent do you estimate the length of the column changes?
init air column = 26cm
when squeezed = 22
=approx. 15% decrease
#$&*
Does the volume of the air column increase or decrease? By what percent do you estimate the volume of the column changes?
volume air column decreases by same percent, which is 15%.
#$&*
Does the number of molecules in the air column increase, decrease or remain the same? By what percent do you estimate the number of molecules changes?
air molecules would remain the same they are just being compressed into a smaller area
?????I have problems with this relationship later on, but think this answer is not correct. Not going to go back and change it though because that doesn’t help anyone?????????????????????????????????????
@& The tube is closed, so there is a 0% change in the number of molecules.*@
#$&*
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?
mass increases again due to more air being compressed into smaller area. Should be an approx. 15% increase in the mass of the air column.
#$&*
@& The same number of particles will have the same mass. The decrease in volume increases density, but the density is multiplied by the smaller volume, so it all balances out.*@
Does the pressure in the air column increase, decrease or remain the same? By what percent do you conjecture the pressure in the column changes?
Pressure increases because of the volume decreasing and would experience an approx. 15% increase
#$&*
Does the pressure in the bottle increase, decrease or remain the same? By what percent do you conjecture the pressure in the bottle changes?
Pressure in the would decrease because we are dealing with a close system and if the air comlumn experiences an pressure increase then the bottle would have a pressure decrease. I would believe that the difference in pressure would be very close to the 15% we approx. to gain wrt air column
#$&*
@& The force exerted on the water plug is the same on one side as the other; its cross-sectional area is constant so the pressure is the same on both sides.*@
When you hold the bottle in the squeezed position, with the water plug stationary, the pressure in the bottle results in a force on the plug which pushes it toward the capped end, while the pressure in the air column results in a force that pushes the plug away from that end. Which force do you think is the greater, or are they equal?
Because the water plug is stationary the forces would be equal, no net force or the water would be moving
#$&*
Which do you think is greater, the pressure in the bottle or the pressure in the air column?
Pressure = Force/area
Because the tube contains less area than the bottle, with pressure being equal to force divided by area then the pressure in the air column is greater.
#$&*
@& The water plug has equal areas on both ends. It doesn't know anything about the diameter of the bottle.
Now if was a continuous incompressible fluid extending from the bottle into the tube, the diameter of the bottle could be important.*@
Measure the length of the air column.
What is the length of the air column?
26cm
#$&*
How far would the water plug have to move to make the air column 10% shorter?
2.6cm
#$&*
Squeeze the bottle so the air column becomes 10% shorter. It's up to you to figure out how to tell when it's 10% shorter. If you can't squeeze hard enough to achieve the 10% difference, then figure out what percent you can manage and note the percent in your answer.
On a 1-10 scale, with 10 the hardest squeeze of which you are capable without risking injury, how hard did you have to squeeze the bottle and what percent change did you achieve in the length of the air column?
4
#$&*
@& percent change in length?*@
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?
For 2 squeeze
1cm = approx. 4%
For 5 squeeze
3cm = approx. 12%
For 8 squeeze
5cm = approx. 20%
#$&*
@& OK, got it here.*@
Now by heating and/or cooling the bottle, what extremes in air column length can you achieve? Careful not to melt the bottle. It won't handle boiling water, and you shouldn't mess with water hot enough to scald you or cold enough to injure you (e.g., don't use dry ice, which in any case is too cold for the bottle, and certainly don't use liquid nitrogen).
Report your results:
Seem to be able to get maybe 1.5cm difference but more like 1cm.
#$&*
PART 3
__________________________________________________________________________
Brief Bottle Experiment 1c
Siphoning water into empty sealed bottle
________________________________________
Starting with the cap in place on an empty bottle, siphon water from an adjacent full bottle. Allow the siphon to run a few minutes until the water levels in the two bottles stabilize.
Estimate the percent change in the volume of the air in the capped bottle.
????I’m not sure if I’m doing this correctly I have tried it several ways and after I init the siphon even with the tube elevated I can’t get any more than a 5% increase from bottle 1 to bottle 2????
5% decrease maybe
#$&*
@& That's about right.*@
Estimate the percent change in the number of molecules in the air within the capped bottle.
decrease of approx. 5%
#$&*
@& Once the bottle is capped no air enters or leaves it.*@
Estimate the percent change in the volume of the water in the open bottle.
approx. 5% decrease
#$&*
What do you think is the percent change in the air pressure in the capped bottle?
Because we decrease in the volume of air the pressure would increase by approx. 5%
#$&*
What is the difference in the two fluid levels?
capped bottle
water level = 3cm
uncapped bottle
water level = 14.5cm
#$&*
What is the percent change in the number of air molecules in the capped bottle?
decrease of approx. 5%
#$&*
@& Once the bottle is capped no air enters or leaves it.*@
Raise the open bottle as high as possible without disturbing the capped bottle. Allow time for the water levels in the two bottles to stabilize.
What percent of the volume of the capped bottle do you now estimate is occupied by water?
17%
#$&*
Estimate the percent change in the number of molecules in the air within the capped bottle.
The increase should be approx. 12%
#$&*
@& There won't be any change in the number of air molecules. *@
By what percent do you estimate the pressure in the capped bottle exceeds the original pressure (i.e., the pressure when the bottle was first capped)?
the pressure should be 17% greater
#$&*
What percent of the uncapped bottle do you estimate is now occupied by air?
20%
#$&*
What is the difference in the two water levels?
13.5cm and 4cm
#$&*
Return the uncapped bottle to the tabletop. What happens?
What is now the difference in the two water levels?
????I messed up and didn’t have my tube far enough down into the liquid for anything to happen????
When I redid the water was sucked back into the bottle and air which came to the was in the form of bubbles and exited the system by rising through the uncapped bottle
#$&*
What do you think is the pressure in the uncapped bottle as a percent of its original pressure (before the bottle was capped)?
The uncapped bottle would have constant pressure equal to atmospheric pressure right, or 100% of its original pressure.
#$&*
PART 4
__________________________________________________________________________
Brief Bottle Experiment 1d
Raising water
________________________________________
Add the extension to the tube, so that by squeezing you can force water from the bottle into the tube. Squeeze hard enough to raise the water to as high as possible into the tube. Evaluate how hard you had to squeeze, on the 1-10 scale you used in part 1b. Measure how far you were able to raise water in the tube above the level of the water in the bottle.
How high did you raise the water, and how hard did you have to squeeze (using the 1-10 scale)?
approx. 160cm(very close approx., might have resulted in a squeeze of 8-9 with one hand
#$&*
Give the bottle a squeeze corresponding to 1 on the 1-10 scale, and observe how high water rises. Then give it another squeeze, halfway between 1 and the squeeze you used to raise water to the top of the tube. Do this blind. Don't look at the tube, just feel the squeeze. Then look at the tube and see where the water is.
Report a table of water column height vs. squeeze.
Squeeze
Squeeze
Height of water in tube
7cm 1
63.5cm 5
160cm 9
#$&*
"
@& Looks good, but be sure to see my notes, especially on the confusion about the number of molecules of air in a system.*@