http://www.ck12.org Chapter 12. Fluid Mechanics
FIGURE 12.11
straw. The open end has full atmospheric pressure pressing on it, while the sealed side has no pressure. TheFigure
12.11 shows a mercury-filled barometer.
If the tube is filled with mercury and has no air, then the atmospheric pressure pushes on the top of the dish, lifting
up the mercury in the tube that has no air pushing down on it from above. All suction works this way –it is really
atmospheric pressure pushing, rather than the vacuum pulling. The mercury is stable when the atmospheric pressure
on the mercury in the dish is equal to the hydrostatic pressure which the column of mercury exerts at the bottom of
the tube.
Illustrative Example 1
We can calculate the height of mercury lifted up by one standard atmosphere of pressure,Patm. Mercury has a
density ofρ= 13. 6 × 103 kg/m^3. So, if the hydrostatic pressure from this mercury is equal toPatm, we have:
Patm=P=ρgh → h=Patm
ρgh=(
101 , (^396) mN 2
)
(
- 6 × (^103) mkg 3
)(
9. 81 ms 2)=^0 .760 m.
So the pressure of one standard atmosphere hold a column of mercury with a height of 0.76 meters. As the
atmospheric pressure changes, the height of the column of mercury changes.
Check Your Understanding
In the above experiment, if water had been used rather than mercury what would the height of the column of water
have been?
Answer: Mercury is 13.6 times denser than water. So the atmosphere can support a column of water 13.6 times
higher than a column of mercury.
Height of water column:( 0. 76 m)( 13. 6 ) = 10. 34 → 10. 3 m
A column of water 10.3 m high has the same weight as a column of mercury 0.76 m. Mercury barometers are
therefore much more practical than water barometers.
Gauge Pressure
In practice, most examples of pressure happen inside atmospheric pressure. When a faucet pushes out water,
that water is pressing out against atmospheric pressure. Thegauge pressureis the pressure above the current