Under Pressure - By MITK12Videos
Transcript
| 00:13 | do you feel that do you feel the weight of | |
| 00:16 | the atmosphere pushing down on you ? Actually , you | |
| 00:20 | like most people have probably not thought about how heavy | |
| 00:24 | the atmosphere actually is and about the pressure it pushes | |
| 00:29 | on you every day . In fact , You have | |
| 00:33 | about 14.7 lbs of force pushing on you per every | |
| 00:39 | square inch of you . But where does this pressure | |
| 00:43 | come from ? For the answer to that question . | |
| 00:46 | Science had a way to the middle of the 17th | |
| 00:48 | century when Pascal had hypothesis . You see , he | |
| 00:51 | thought that the atmospheric pressure that we experience every day | |
| 00:54 | is actually due to the massive column of air , | |
| 00:57 | the towers above us , and the way that it | |
| 00:59 | exerts by gravity . To test this , He devised | |
| 01:02 | a very simple experiment . He took an instrument called | |
| 01:06 | a barometer , the measures atmospheric pressure , and made | |
| 01:09 | measurements at several locations in the lowest part of town | |
| 01:11 | , at the top of a church tower , and | |
| 01:13 | even climbed a local mountain to measure the pressure at | |
| 01:15 | the top with the idea that if the height of | |
| 01:18 | the column of air stayed , the same pressure should | |
| 01:21 | stay the same each of those locations . But at | |
| 01:24 | the height of the column decreases at each location , | |
| 01:27 | then the pressure should decrease as well . I'm gonna | |
| 01:31 | show you how to build a simple pressure measuring device | |
| 01:33 | called a manama Ter . First I need to relatively | |
| 01:36 | stiff tubes . I use to plastic pipettes I had | |
| 01:38 | lying around the lab . I cut one end off | |
| 01:42 | to make room for the plug . You also need | |
| 01:45 | a plug to plug one end and a piece of | |
| 01:48 | relatively stiff tube . Now to make the manama ter | |
| 01:52 | , you need to connect the tubing to each of | |
| 01:54 | the pipe . It's or plastic tubes . Once they're | |
| 01:59 | connected , you'll have a U . Shaped device . | |
| 02:01 | You can then fill it with water , add some | |
| 02:03 | food coloring if you like , and then plug one | |
| 02:05 | end to fix the pressure . There you go . | |
| 02:08 | You have a phenomena and you're ready to make pressure | |
| 02:10 | measurements at the beginning , the pressure exerted on each | |
| 02:13 | column of liquid is equal so the liquid levels and | |
| 02:16 | tube A . And B . Are equal as well | |
| 02:19 | . If the pressure on tube A . Increases the | |
| 02:22 | water and tube , they will fall and the water | |
| 02:24 | and to be will rise . If the pressure over | |
| 02:27 | tube A decreases then the water and to be will | |
| 02:30 | fall and the water and tube they will rise . | |
| 02:34 | For our first measurement we went to Constitution Park in | |
| 02:36 | boston here we show that the pressure is greater than | |
| 02:39 | it was in the lab . But using our phenomena | |
| 02:41 | to show that column A . Is 12 millimeters below | |
| 02:44 | column B . But what's the pressure going to be | |
| 02:47 | up on bunker hill monument ? Well let's find out | |
| 02:51 | After climbing 275 stairs were able to take from measurement | |
| 02:56 | at the top of Bunker Hill Monument Here , we're | |
| 02:59 | able to show that the column a . Is just | |
| 03:01 | six below column B . For our final measurement , | |
| 03:05 | we drove about 20 miles south to the Great Blue | |
| 03:07 | Hill . After climbing for about a mile and a | |
| 03:10 | half , we reached the top with stunning views of | |
| 03:13 | boston and the ocean . Here we get our lowest | |
| 03:16 | measurement yet . Column a is actually three above column | |
| 03:20 | B . Now , for some results in calculations getting | |
| 03:24 | from changes in water levels to pressure calculations for starters | |
| 03:29 | , the air and to be is constrained by the | |
| 03:31 | plug Because of this , it's an ideal gas and | |
| 03:34 | obeys the ideal gas law , which states the pressure | |
| 03:37 | times volume equals . NRT . Now NRT remains constant | |
| 03:41 | through all measurements . Because of this , we can | |
| 03:44 | then produce a nice relationship and that is P one | |
| 03:47 | times V one equals P . T becomes V . | |
| 03:49 | Two . Or in other words , the pressure times | |
| 03:51 | volume at one state must equal the pressure and volume | |
| 03:55 | at another state . But how do we get from | |
| 03:59 | differences in water levels that we measured in the field | |
| 04:01 | to changes in volume of to be . Which is | |
| 04:04 | what we need for our actual calculations . Well , | |
| 04:07 | from my device , I've calculated that for each one | |
| 04:10 | millimeter change in water level , that corresponds to a | |
| 04:14 | change in volume of 0.133 mills with half of that | |
| 04:19 | volume change occurring in column A . And the other | |
| 04:21 | half occurring in column B . So to calculate the | |
| 04:24 | change in volume of to be only , we can | |
| 04:26 | take my conversion factor of 0.133 mils per millimeter and | |
| 04:30 | multiply it by half of the high change , Therefore | |
| 04:34 | the final volume in to be that we saw in | |
| 04:36 | the field is V two equals the initial volume . | |
| 04:40 | V one of 22 mills minus the change in volume | |
| 04:44 | . So using this formula , I went ahead and | |
| 04:46 | calculated the final volume to be each of our locations | |
| 04:50 | . A Constitution Park . The final volume of to | |
| 04:52 | be was 20.4 mills at Bunker Hill Monument 21.2 mills | |
| 04:57 | and at Great Blue Hill 22.4 miles . Now we | |
| 05:00 | can take this relationship and rearrange it to actually solve | |
| 05:04 | for the pressure in each of these locations using the | |
| 05:06 | initial conditions of the air and to be a P | |
| 05:09 | one equals 101.6 killer pascal's , which is the pressure | |
| 05:13 | the day that I built my manama Ter and the | |
| 05:15 | initial volume of to be That is the air volume | |
| 05:19 | of 22 mills . Using this equation , we get | |
| 05:22 | an actual atmospheric pressure , a Constitution Park 109.6 killer | |
| 05:26 | Pascal's at Bunker Hill Monument . 105.5 killer pascal's another | |
| 05:30 | Great Blue hill of 99.8 killer pascal's . But how | |
| 05:34 | do these atmospheric pressures relate to the elevations that we | |
| 05:38 | measured them ? Well , Just as pascal showed , | |
| 05:41 | we see that the pressure decreases with increased elevation at | |
| 05:45 | a very linear relationship over the elevations that we measured | |
| 05:49 | . Finally , I'd like to leave you with some | |
| 05:51 | tips for when you make your own measurements . First | |
| 05:53 | . Be patient . It can take time for the | |
| 05:56 | manama tower to reach equilibrium as you change pressures . | |
| 06:00 | Second , take multiple measurements . This will greatly increase | |
| 06:03 | your accuracy . And third pay attention to whether as | |
| 06:06 | things like changes in temperature or incoming weather systems can | |
| 06:10 | have a dramatic effect on the pressures that you measure | |
| 06:14 | . Some final questions to think about , based on | |
| 06:15 | what you've learned today . First , what would happen | |
| 06:19 | to the water in the Manama as I went to | |
| 06:20 | 5000 m ? Or what about the moon ? And | |
| 06:23 | second ? What would happen if you made a measurement | |
| 06:26 | on an airplane at 10,000 m ? Think about these | |
| 06:29 | things , as you think about pressure . |
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