A-level Chemistry/AQA/Module 5/Thermodynamics/Slow and Fast Expansions

Adiabatic and Isothermal Expansions

What is the difference between a slow and fast expansion or compression? Thermodynamically we say that if an expansion is fast it is Adiabatic, i.e. A process where there is no heat flow into the system. A slow expansion is generally an isothermal expansion, where the temperature of the system remains constant throughout the process.

Adiabatic Processes

Adiabatic Process are reversible as the entropy stays the same. No heat flows into the system and all work is done at the expense of Internal Energy.

For an Ideal Gas the equation of a PV equation of an Adiabatic Process is

$PV^{\gamma }\propto 1$

where γ is the ratio of the Specific Heat at Constant Pressure to that of Constant Volume.

Isothermal Processes

Isothermal Processes are a little different, the system must be connected to a heat bath so that heat can flow into or out of the system in order to keep the temperature constant.

For an Ideal Gas the PV plot is hyperbolic, i.e.

$PV\propto 1$