Gas Trapped in a Capillary Tube


  1. The pressure of the gas trapped in a capillary tube depends on the position of the tube.
  2. Figure below shows the pressure of the gas when the capillary tube is horizontal, vertical and vertically upside down.

Example:

Figure above shows 3 identical capillary tubes with one end sealed and containing a column of mercury. PA, PB and PC are the gas pressure in the capillary tubes respectively. Find the value of PA, PB and PC. [Atmospheric pressure = 76cmHg]

Answer:

PA = 76cmHg

PB = 76cmHg + 2cmHg = 78cmHg

PC = 76cmHg – 2cmHg = 74cmHg

Example:

Figure above shows some air trapped in a J-tube. Find the pressure of the trapped air. [Density of water = 1000 kg/m³; Atmospheric pressure = 100,000 Pa]
Answer:

Pgas = Patm +hρg
Pgas = (100,000)+(0.2)(1000)(10)
Pgas = 102,000Pa