Joule–Thomson Effect
The temperature change of a real gas when it expands through a throttle without exchanging heat or doing external work.
Formula / equation
μ = (∂T/∂p)_H
Unit
K/Pa
In depth
As molecules move apart, work is done against their mutual attraction, drawing on internal energy and cooling the gas. Below its inversion temperature a gas cools on expansion; above it, repulsive effects dominate and it warms. The effect is the basis of most industrial refrigeration and gas liquefaction.
Examples in the real world
An aerosol can chilling in use; hydrogen and helium warm on expansion unless pre-cooled below their inversion temperatures.