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Gas Pressure
How gas pressure is produced?
Gas molecules are at constant and random movements.
When the molecules collide with the wall of the container and bounce back, they experience a change in momentum.
The momentum change exerts a force on the wall.
Force per unit area is the pressure exerted on the wall of the container.
Using Manometer
- Difference of gas pressure at different level can be ignored.
- Pessure on the surface of a liquid is equal to the pressure of the gas in contact.
- For a given liquid (same liquid), the pressure at any point of the same level is the same.
- For different liquid with different density, pressure will be different at the same level.
Pgas = Patm + Pliquid
Pgas = Patm + hρg
Pgas = Gas Pressure
Patm = Atmospheric Pressure
g = Gravitational Field Strength
Figure above shows the water levels in a water manometer used to measure the pressure of a gas supply. Find the pressure of the gas in unit of
i) cm water
ii) Pa
[Atmospheric Pressure = 1000cm Water; Density of water = 1000kg/m3]
Answer:
i)
Gas Pressure,
P = Patm + Pwater
P = 1000 + (30-5) = 1025 cm Water
ii)
h = 1025cm = 10.25m
ρ = 1000kgm3
g = 10ms-2
Gas Pressure,
P = hρg
P = (10.25)(1000)(10)
P = 102500 Pa
Figure above shows a U-tube filled with 2 liquids X and Y that do not mix. Given that the density of liquid Y is 1.6g/cm3. Find the density of liquid X.
Answer:
Figure above shows a capillary containing a column of mercury. If the atmospheric pressure is 100,000Pa, find the pressure of the gas trapped in the capillary tube. [Density of mercury = 13,600kg/m3]
Answer:
Gas Pressure,
P = Patm + hρg
P = 100,000 + (0.005)(13600)(10)
P = 100680Pa
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