Joule Thomson Effect: Overview, Questions, Preparation

For thermodynamics, the Joule-Thomson effect plays an important role in it. The very foundation of thermodynamics lies on this effect, and it is very crucial for you to understand it.

The Joule-Thomson effect displays the decrease or increase in temperature of the gas or liquid when moving freely through some valve or through other restraining devices while keeping the entire process insulated so that no heat escapes or enters the device.

The change in temperature during the process of expansion depends not only upon the pressure but also on the manner of expansion. For instance, if the expansion process is reversible, the gas would be in a state of thermodynamic equilibrium, and the expansion is isentropic.

The Joule-Thomson effect can be described through the Joule-Thomson coefficient. The formula for the Joule-Thomson effect is μJT = (∂T/∂P)H 

When there is no change in pressure, even when the temperature is decreased, that temperature is known as inversion temperature.

1. The Joule-Thomson effect is based on the principle of transfer of heat.
2. For an ideal gas, the Joule-Thomson coefficient is always zero because the enthalpy of the gas is dependent on the temperature. 
3. The Joule-Thomson effect does not affect hydrogen gas in higher temperatures because, at high temperatures, the gas behaves as an ideal gas. So, it affects hydrogen at lower temperatures.

1. The Joule-Thomson effect is used in the petrochemical industry in the Linde technique, where the cooling effect is used to liquefy the gases.
2. The Joule-Thomson effect is an effective cooling agent.
3. It is also used in various cryogenic processes.

In the chapter, ‘Thermodynamics’, you would get to know about thermal equilibrium, laws of thermodynamics, thermodynamic processes, heat engines, reversible and irreversible processes, along with the Joule-Thomson effect.

1. A geyser heats water flowing at 3 litres/minute from 270C to 770C. If the geyser operates on a gas burner, what is the rate of consumption of the fuel if it's heat of combustion is 4 x 104 J/g?

Ans: T1 = 27 0C and T2 = 77 0C, Specific heat of water = 4.2 J/g0C

Rise in temp = 77 - 27 = 50 0C

Total heat, Q = mcT

  = 3000 x 4.2 x 50

  = 6.3 x 105 J/min

Rate of consumption= 6.3 x 105/(4 x 104) = 15.75g/min

2. Why does the air pressure in a car tyre increase during driving?

Ans: As the car moves, the temperature of the gas inside the tyre increases. Pv=nRT. Since the volume remains constant, the air pressure increases.

3. Why is the climate of a harbour town more temperate than of a town in a desert at the same latitude?

Ans: Water has a high value of specific heat capacity, whereas sand has a very low value of specific heat capacity. Because of this reason, the temperature of water bodies does not change easily, but the temperature of sand changes swiftly. So, the climate near harbour remains moderate, whereas it is extreme in deserts.

Q: What is the Joule-Thomson effect?

Q: What is the Joule-Thomson coefficient?

Q: What would happen when the expansion process is reversible?

Q: State some properties of the Joule-Thomson effect?

Q: State some applications of Joule-Thomson effect?