Phys 202
Your 'measuring atmospheric pressure' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
** your comment **
** How does the system behave and how does it 'feel'? **
** Length of the air column in units of your measuring device: **
** Your report of the vertical position of the first mark relative to the water surface of the first mark, the length of the air column in units of your selected measuring device, and the two meniscus positions observed with each of your five trials. **
** Your report of the vertical position of the second mark relative to the water surface of the first mark, the length of the air column in units of your selected measuring device, and the two meniscus positions observed with each of your five trials. **
** Your report of the vertical position of the highest mark relative to the water surface of the first mark, the length of the air column in units of your selected measuring device, and the two meniscus positions observed with each of your five trials. **
** Your report of the vertical position of the 'in-between' mark relative to the water surface of the first mark, the length of the air column in units of your selected measuring device, and the two meniscus positions observed with each of your five trials. **
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Your setup for the preceding experiment included a vertical tube, with terminating caps on the other two tubes. You will use the vertical tube again in this experiment.
You will use the remainder of the thin tube supplied with your kit to make the pressure-indicating tube:
First fill the new tube with water:
The easiest way to do this is to temporarily disconnect the vertical tube and replace it with the new tube, so that when you squeeze the container you can fill the new tube. Add water to the container until it is nearly full, then fit the stopper into the hole.
Hold the open end of the pressure-indicating tube a little higher than the top of the container, near the point where you just connected it, and squeeze the bottle so that water fills the tube. Since the water level in the container is higher than in the preceding experiment, and since the end of the new tube isn't much higher than the water level, this shouldn't require a very hard squeeze.
When the tube is full, maintain the squeeze so the water doesn't return to the container and disconnect it. You will have a tube full of water.
Now empty about half the water in the tube, cap it and connect it to the system, and replace the vertical tube:
Just raise one end and/or lower the other, and water will flow out.
Place a terminating cap on one end of this tube.
Open a second tube in the stopper by removing the terminating caps at both of its ends.
Using a connector, connect the pressure-measuring tube to newly opened tube coming out of the stopper.
Replace the vertical tube.
Fit the stopper back into the hole in the container.
You should at this point have:
• The vertical tube, extended down into the water and out of the top of the container
• The extended pressure-measuring tube, open on one end (through its connection to the newly opened tube in the stopper) to the air inside the container, half full of water, and capped at the other end.
• A third tube through the stopper, still capped.
In the picture below you see:
• the short capped tube (the 'third' tube) sticking out of the top of the stopper,
• the 'vertical' tube not yet in a vertical position but extending forward and to the right into a graduated cylinder, and
• the pressure-indicating tube half full of caramel-colored liquid (the liquid is a cheap cola) and draped over a second graduated cylinder toward the back left. The pressure-indicating tube is capped at its end (hanging down near the tabletop), and the last 25 cm segment of the tube contains no liquid.
The picture below shows how the liquid in the tube comes to a point just below the 'peak' of the tube. This leaves an air column about 25 cm long in the capped end of the tube.
In the figure below the pressure-indicating tube is simply lying on the tabletop so the air column at the capped end is clearly visible.
The figure below shows a sketch of a tube which rises out of the bottle at left, then bends to form a U, then to the right of the U again levels off. The tube continues a ways to the right and is sealed at its right end. Liquid occupies the U up to almost the point of leveling, so that an increase in the pressure of the container will cause the liquid to move into the level region. As is the case in our experiment, the tube is assumed thin enough that the plane of the meniscus remains parallel to the cross-section of the tube (i.e., the meniscus doesn't 'level off' when it moves into a horizontal section of tube).
You should manipulate the pressure tube until its configuration resembles the one shown. The length and depth of the U can vary from that depicted, but the air column at the end of the tube should be at least 15 (actual) cm long. The liquid levels at the left and right ends of U should be very nearly equal.
Support the end of the vertical tube so that it is more or less vertical, as it was in the first experiment done with this setup.
The bottle should be pretty full, but not so full that it covers the open end of the tube to which the pressure tube is connected; the left end of the pressure tube should have an 'open path' to the gas inside the bottle, so that the pressure on the left-hand side of the water column in that tube is essentially equal to the pressure in the bottle.
If you squeeze the container a little, water will rise a little way in the vertical tube and the water in the pressure tube will also move is such a way as to slightly shorten the air column. The harder you squeeze the higher water will rise in the vertical tube and the shorter the air column will become.
Go ahead and observe this phenomenon. There is no need to measure anything yet, just get the 'feel' of the system.
In the box below indicate how the system behaves (what changes when you do what, how the system's reactions to your actions appear to be related to one another) and how it 'feels'.
When the bottle is squeezed a small amount of water moves up the vertical tube while the air in the pressure tube moves further away from the bottle by about the same amount. With more pressure or squeezing harder the water and air move further but it gets harder to squeeze the bottle. It seems like as you squeeze the bottle, the air pressure increases forcing both air and water out of the way. Water moves up the vertical tube while air moves out the pressure tube.
Using a measuring device you will measure the relative positions of the meniscus as you vary your squeeze:
• One of the ruler copies used in the previous experiment on the distortion of paper rulers should be used here; a reduced copy should be used for greater precision. You may choose the level of reduction at which you think you will achieve the greatest level of precision. Only relative measurements will be important here; it will not be necessary to convert your units to actual millimeters or centimeters.
In the box below indicate the level of reduction you have chosen, and your reasons for this choice.
I chose to use a normal ruler and measure in mm since I did not have a paper reduced ruler.
In the units of the measuring device you have chosen, write down in your lab notebook the readings you used to indicate length of the air column, from the meniscus to the barrier at the capped end. No conversion of the units of your device to standard units (e.g., millimeters or centimeters) is required. Your information should include the marking at one end of the measuring device, and the marking at the other. If necessary two or more copies of the measuring device may be carefully taped together.
Indicate in the first line the length of the air column in the units of your measuring device.
In the second line explain how you obtained your result, including the readings at the two ends and how you used those readings to indicate the length.
40.9 cm
I measured from the end of the capped end to the beginning of the water meniscus after the air column.
Now place the same measuring device along the tube, positioned so you can observe as accurately as possible the relative positions of the meniscus in the pressure tube.
• It is recommended that the initial position of the meniscus be in the vicinity of the center of the measuring device, so that position changes in both directions can be observed.
• It is not necessary for the measuring device to extend the entire length of the air column, as long as you know the reading on the measuring device that corresponds to the initial position of the meniscus. From this information and from subsequent readings it will be easy to determine the varying lengths of the air column.
• Take whatever precautions are necessary to make sure neither the measuring device nor the pressure tube can move until you have completed the necessary trials.
Mark positions along the vertical tube at 10-cm intervals (actual 10-cm intervals as indicated by a full-sized ruler) above the surface of the water in the bottle.
• If the bottle is pretty full, as described before, it might be possible to make the first mark on the vertical tube at 10 or 15 cm above the water surface.
• Marks may be made using an actual marker, or pieces of tape, or anything else that happens to be convenient.
Write your information in your lab notebook:
• Write down the position of the first mark on the vertical tube with respect to the water surface (e.g., 10 cm or 15 cm). 21.5 cm
• Write down the position of the meniscus. 8cm This position will simply be the reading on your measuring device. For example if the meniscus is at marking 17.35, that is what you write down.
• As in all labs, you directly record what you read. Never do any arithmetic between your reading and your recording of what you read.
You will now conduct 5 trials, raising water to the first mark on your vertical tube and reading the position of the meniscus before the squeeze and while water is at the given level.
• Squeeze the bottle until water reaches the first mark in the vertical tube, and carefully read the position of the meniscus. Release the bottle and immediately write down that position.
• Repeat, being sure to again write down the position of the meniscus before squeezing the bottle (this position might or might not be the same as before) and the position of the meniscus when the water is at the first mark in the vertical tube.
• Repeat three more times, so that you have a total of five trials in which the water was raised to the first mark in the vertical tube. With each repeat you will write down two more numbers.
In the box below record your information:
• Indicate on the first line the vertical position of the first mark on the vertical tube, relative to the water surface, giving a single number in the first line.
• On the second line give the length of the air column, as measured in units of the device you used to measure it.
• On the third line, give the position of the meniscus before the first squeeze then the position of the meniscus when the water in the vertical tube was at the first mark. Give this information as two numbers, delimited by commas.
• On lines four through seven, give the same information for the second through the fifth trials.
Starting in the eighth line give a brief synopsis of the meaning of the information you have given and how you obtained it.
21.5 cm
40.9 cm
17, 6.5
17, 7.9
17.8, 7.5
18, 8
20, 8.5
The first line is the distance my first mark on the vertical tube is from the surface of the water in the bottle. The second line is the length of the air column as measured from the tip of the column tube to the start of the water. The 3rd through 7th lines give the differences the water travelled by the starting point and ending point of the meniscus.
Now repeat the 5-trial process, this time raising water to the second mark. Write down everything as before.
In the box below report your results, using the same format as before
31.5 cm
40.9 cm
33, 14.5
34, 12
34.5, 11
34, 10.7
34, 10.2
Again, the first line is the distance my second mark on the vertical tube is from the surface of the water in the bottle. The second line is the length of the air column as measured from the tip of the column tube to the start of the water. The 3rd through 7th lines give the differences the water travelled by the starting point and ending point of the meniscus.
Repeat again, raising water to the highest mark you can manage with normal effort. Remember that this isn't supposed to be a test of strength.
In the box below report your results, using the same format as before:
33.5 cm
40.9 cm
30, 10
31, 11
31.5, 10
32, 10.2
33, 9.5
The first line is the distance I could raise the water on the vertical tube from the surface of the water in the bottle. The second line is the length of the air column as measured from the tip of the column tube to the start of the water. The 3rd through 7th lines give the differences the water travelled by the starting point and ending point of the meniscus.
If the highest mark you can easily manage is the third mark, then you may stop. If you have raised the water to a mark higher than the third, then do one more series of 5 trials, this time choosing a mark about halfway between the second and the highest mark.
In the box below report your results, using the same format as before. If you were not able to raise the water higher than your third mark, simply leave this box empty.
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1 hour
&#Good work. Let me know if you have questions. &#