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Phy 202
Your 'question form' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
** Question Form_labelMessages **
instructions for next lab
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I'm wondering what lab work I should do now that I've completed your streamlined version of the Measuring Atmospheric Pressure, Part 1 lab.
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Should I backtrack to the Kinetic Model Experiment (or an alternative/streamlined version of it), go ahead to the Bottle Thermometer experiment (or an alternative/streamlined version of it), or do something else?
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See above re: what I do and do not understand about the situation. Thanks again.
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I haven't yet reviewed it, but the streamlined version you did will replace both of the Measuring Atmospheric Pressure labs.
You should do the Kinetic Model Experiment as it is presented. I think you'll like that one.
You can then, if you wish, bypass the Bottle Thermometer, since you have a good handle on the behavior of that system, and investigate the following:
The situation appears in the Class Notes and on the DVD, typically referred to as 'bottle engine'. The idea is to observe the efficiency achievable with an engine consisting of the bottle and tubes, using hot tap water as the energy source.
A 2-liter bottle would be the best choice, since the volume of the tube is proportionally less than it would be for a bottle of lesser capacity. However you will want to use a bottle whose cap seals, and that should be the first priority.
Using the bottle and the vertical tube, you will seal off the other two tubes and see how high you can raise water by running hot water from the tap over the bottle. We'll call this the maximum achievable height.
You won't be timing anything so you don't have to worry about exothermic reactions within your laptop.
Then
1. Cool the system back to its original state. It doesn't take long to raise water to its maximum height so you're unlikly to have warmed the water in the bottle much, but if you did you can replace the water in the bottle with room-temperature water. The bottle itself will return to room temperature quickly.
2. Arrange the vertical tube so that it rises to only half the maximum achievable height. At that height bend the tube at a right angle to carry water horizontally to a cup or other vessel whose upper lip is at that height. If the mid-length tube is long enough, you can use it instead of the previouis vertical tube.
3. Measure how much water the system displaces into the cup when you again heat it with the tap water. You should also include any water left in the horizontal section of the tube (consider this water as having been delivered to the cup).
4. Repeat for water at 1/4 the maximum achievable height, and again at 3/4 the maximum achievable height.
Naturally you'll want to anticipate and control for any significant extraneous variables, but don't go to extremes. The most significant sources of error are either easily controlled or ridiculously difficult to deal with.
To analyze:
a. From the maximum achievable height you can deduce the temperature to which you were able to raise the gas. The same temperature should be achievable in each of the subsequent trials.
b. From the change in temperature, the volume of the gas in the system and the fact that the volume of the gas remains constant (tube volume and expansion of the bottle being considered negligible), you can figure out the thermal energy necessary to heat the gas. Air is primarily diatomic so its molar specific heat at constant volume is very close to 5/2 R, with R = 8.31 Joules / (mole Kelvin).
c. For the trial at half the maximum achievable height you can figure out the temperature at which the water first reached that height. Up to that point you can consider volume to have been constant. After that point the water doesn't rise any higher and you can consider pressure to be constant. This allows you to again figure out how much thermal energy was required to complete the process (molar specific heat at constant pressure is 7/2 R).
A similar analysis can be done for the 1/4 and 3/4 heights.
For each trial the PE change of the system is easily calculated (you know how much water was raise and how high it was raised).
The 'practical efficiency' of the process will be defined as the ratio of PE change to the thermal energy required to heat the gas.
Of course a whole lot of thermal energy went down the drain, but any practical engine of this nature will use a more efficient method of heat transfer.