#$&*
Phy 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.
** Measuring Atmospheric Pressure_labelMessages **
There is no text box below to submit work.
** Measuring Atmospheric Pressure_labelMessages **
Be very sure to keep a copy of your data for this experiment, since you will probably be using it in
Part 2. It is recommended that you keep a word-processing document open, copy this document into that
document, and copy any data you put into these boxes in the appropriate place in that document. That
document can then be used as a course from which you can easily access the necessary data to copy into
the data processing program.
Your setup for the preceding experiment Raising Water in a Vertical Tube included a vertical tube, with
terminating caps on the other two tubes. You will use the vertical tube again in this experiment.
Your kit included two bottlecaps connected by a long tube. The long tube is to be used as a vertical
tube, as in the previous experiment.
Each bottlecap has three tubes. One is a short tube; its intended use is to release pressure in the
system when and if this becomes necessary. The third is fairly long. This tube is to be used as a
'pressure tube'.
First fill the 'pressure tube' with water. You can do this in any way you wish. One way:
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 screw on the bottlecap.
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 from the 'pressure tube'. Cap it and connect it to the system, and
replace the vertical tube. You can do this in any way you wish, but one way is described below:
Just raise one end of the full 'pressure tube' and/or lower the other, and some water will flow out.
Once the tube is about half full, place a terminating cap on one end of this tube. This will hold the
water in the 'pressure tube'.
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 short through the bottlecap, still closed off at the 'top end'.
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 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 new picture 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.
The basic idea is that as you squeeze the system to raise water in the vertical tube, as in your
previous experiment, the pressure in the system increases and compresses the 'air column' in the
pressure tube. By measuring the lengths of this 'air column' you can determine relative pressures, and
by measuring the heights of the water column in the 'vertical tube' you can determine the actual
pressure differences required to support those columns.
Support the end of the vertical tube so that it is more or less vertical, as it was in the previous
experiment.
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.
Indicate below 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'.
----->>>>> behavior
Your answer (start in the next line):
The system behaves such that when the bottle is squeezed water moves up the vertical column which is
open at the top end. As more and more water is forced up the vertical tube, the pressure tube water
also moves. So, it looks as though we can measure the amount of pressure exerted to move the water up
the vertical 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.
Indicate below the level of reduction you have chosen, and your reasons for this choice.
----->>>>> level of reduction and reasons
Your answer (start in the next line):
I used the regular size one,it is easier for me to read with my poor eyesight.
#$&*
In the units of the measuring device you have chosen, write down in your lab notebook the readings you
used to indicate length of the air column, from the meniscus to the barrier at the capped end. No
conversion of the units of your device to standard units (e.g., millimeters or centimeters) is
required. Your information should include the marking at one end of the measuring device, and the
marking at the other. If necessary two or more copies of paper rulers may be carefully taped together.
Indicate in the first line below the length of the air column in the units of your measuring device.
In the second line explain how you obtained your result, including the readings at the two ends and how
you used those readings to indicate the length.
----->>>>> air column length, how obtained incl readings and how used
Your answer (start in the next line):
33.5
I used one of the rulers that I cut off in the lab kit. I placed the 0cm mark at the meniscus and
measured from there to the plug at the end of the air column.
#$&*
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).
Write down the position of the meniscus in the pressure tube. 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 making an
observation and recording it.
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 in the pressure tube. 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 repetition you will write down two more numbers.
Record your information below:
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.
----->>>>> vert pos mark vert tube, air column lgth, meniscus pos 1st trial, same 2d, same 3d, same
4th, same 5th trial, meaning
Your answer (start in the next line):
10
33.5
20.0, 20.6
20.0, 20.6
20.2, 20.8
20.2, 20.8
20.2, 20.8
line 1 is the number of cm the first mark on the vertical tube is above water level, 2nd line is length
of air column in pressure tube, 3-7 are positions of meniscus before and after squeezing.
#$&*
Now repeat the 5-trial process, this time raising water to the second mark. Write down everything as
before.
In the space below report your results, using the same format as before:
----->>>>> same for 2d vert pos
Your answer (start in the next line):
20.3, 20.9
20.1, 20.9
20.0, 20.9
20.2, 20.9
20.2, 20.9
#$&*
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 space below report your results, using the same format as before:
----->>>>> same for 3d pos
Your answer (start in the next line):
20.0, 22.7
19.6, 22.6
19.9, 22.7
19.9, 22.7
19.9,22.7
#$&*
If the highest mark you can easily manage is the third mark, then you may stop. If you have raised the
water to a mark higher than the third, then do one more series of 5 trials, this time choosing a mark
about halfway between the second and the highest mark.
In the space below report your results, using the same format as before. If you were not able to raise
the water higher than your third mark, simply leave these lines empty.
Then report the approximate percent change in the length of the water column for each of the three
vertical heights. Report in a single line separated by commas, and in the last line indicate how you
got these results, including a sample calculation for the second set of trials.
----->>>>> same for 4th pos if possible
Your answer (start in the next line):
19.9, 21.7
19.8, 21.7
19.7, 21.6
19.8, 21.7
19.8, 21.7
Water column? in the vertical tube? how do I calculate the percent of change. The 10cm mark resulted in
a 4%increase, the 20cm mark resulted in a 5% increase, and the 120cm mark resulted in a 14% increase,
the last mark at 60cm resulted in 10% increase. I found these percentages by taking the new position of
the meniscus - the original position of the meniscus and the divided that result by the orginal
position of the meniscus. (new-Original)/original
(20.9-20.0)/20.0=.05
@&
That is correct. This particular calculation showed a 5% change, as I know you are aware.
*@
#$&*
Make your estimate of atmospheric pressure:
What was the maximum height to which the water column was raised?
How much pressure was required to support the column?
Based on the behavior of the air column in the pressure tube, what percent do you think this is of
atmospheric pressure?
What therefore do you conclude is atmospheric pressure?
----->>>>> max ht, pressure to support column, percent of atm pressure, conclusion atm pressure
Your answer (start in the next line):
130
1.2kPa
I must be missing something, I don't know how to compute this. I used Bernoulli's for the 1.2kPa
@&
1.2 kPa would not raise the water 130 cm. 12 kPa would raise it close to that high, but 1.2 kPa would not.
You didn't include the details of your calculation, but Bernoulli's equation would certainly work. I suspect you either made a factor-of-10 error or left something out of your calculation.
Can you clarify?
*@
@&
According to the pressure tube, by what percent did the pressure change in order to raise the water to the 130 cm mark?
How does this help you answer the question about atmospheric pressure?
Note also that I've just today added a sort of a preamble to this lab. You aren't required to do it, but it was added in order to further spell out these relationships. You are welcome to take a look, in case that information might be helpful.
*@
#$&*
*#&!*#&!
@&
This looks good, but your answer regarding the pressure on the last set of questions is off by a factor of about 10.
There are a couple of other questions to answer there. Check my notes.
&#Please see my notes and submit a copy of this document with revisions, comments and/or questions, and mark your insertions with &&&& (please mark each insertion at the beginning and at the end).
Be sure to include the entire document, including my notes. &#
*@