9.4.1: Lecture Demonstrations
- Page ID
- 50842
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Measuring Pressure with a Syringe and Baby Scale
A free-moving 30 cc glass syringe is plugged with a luer lok stopper with a trapped volume of 20 cc at atmospheric pressure.
The plunger is 7/8" in diameter (0.88") so the area is πr2 = 0.60 in2.
Push down on the scale with the plunger until V = 15 cc and measure weight (force), ~3 lb
New pressure = 3 lb/0.60 in2 external + 14.7 atmospheric = 19.6 lb/in2.
Push down on the scale witht the plunger until V = 10 cc and measure weight (force), ~9 lb
New pressure = 9 lb/0.60 in2 external + 14.7 atmospheric = 29.4 lb/in2.
For the three cases, PV = ~294, so P1V1 = k = P2V2
Measuring Atmospheric pressure with a Manometer and Vacuum Pump
Connect a vacuum pump to the top of a 100 cm tube dipped in mercury in a vial in a sidearm flask, turn on pump, measure height of Hg corresponding to 5 mi of air in atmosphere.
Syringe (etc.) in A Bell Jar
Put a syringe, marshmallow, small bag of chips or pretzels, etc. in a bell jar and turn on vacuum.
