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.


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]


P= 76cmHg

PB = 76cmHg + 2cmHg = 78cmHg

P= 76cmHg – 2cmHg = 74cmHg


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]

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