Question 7: On removing the valve, the air escaping from the cycle tube cools. Why?

ANSWER

Air in a cycle tube is contained in a smaller space, so having high pressure and temperature. When the valve is opened, the outside atmospheric pressure is less than the pressure in the tube, the air escapes into the atmosphere with a high speed. As no heat enters or leaves the system (cycle tube) the process is adiabatic. Applying First Law of Thermodynamics,

ΔQ = ΔU + ΔW
0 = ΔU + ΔW
– ΔU = ΔW

The negative sign shows the work is done at the cost of internal energy of the system. This decreases the temperature of the air in the tube and the cycle tube cools.