# 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 -
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]

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. 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]

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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