Chapter 1
Atmospheric pressure
(30 in Hg , 34 ft of water)
Patm = force/area = downward force (weight) from all air molecules above you / unit area
o
oo
14.7 lb bowling ball
1 in2 bar
o
o
ouch
How can molecules “weigh” anything if they are floating around in space?
Example: birds in airplane
What direction is pressure pushing?
ANS: all directions!
F
Area A
Piston
Patm
P = Patm + F/A
P
Area A
Glass of water
Experiment:
Water doesn’t fall out !
Thin rigid sheet
Weight of water (W)
Why?
Consider forces on a sheet :
So Patm A must be > W !
F = Patm A
How can we make the water fall out?
(A is glass cross-section)
1. Make glass 34’ tall
2. Reduce Patm → 0 by pumping air out
Bell Jar
gauge
P→0
Sheet falls off as P goes to 0
Pump