Gas Pressure

Card 1: How gas pressure is produced?

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.

Card 2: Instruments used to measure gas pressure

Instruments used to measure gas pressure

-Manometer
- Bourdon Gauge

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Card 3: Using Manometer

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

Example 1

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

Card 4: U-Tube

U-Tube

A U-tube can be used to determine density

Example 2

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:

Card 5: Gas Pressure In A Capillary Tube

Gas Pressure In A Capillary Tube

Pgas = gas pressure in the capillary tube
Patm = atmospheric pressure
h = length of the captured mercury column
ρ = density of mercury
g = gravitational field strength

Example 3

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

Card 6: Bourdon Pressure Gauge

Bourdon Pressure Gauge

  • If pressure is applied, the copper tube will be straightened slightly.
  • The copper tube pushes the lever system and hence rotates the pointer.
  • Gas pressure can be read directly from the scale.
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