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Glossary

Isotherm

the temperature is kept constant during a process

Isobar

the pressure is kept constant

Isochore

the volume is being kept constant

Isopleth

the composition is being kept constant

Isolated

the system exchanges neither heat nor mass with the environment

Adiabatic

the system exchanges no heat with the environment

Reversible

the change takes place along a series of states of rest. Action and reaction are only infinitesimally different.

Phenomenological

A phenomenological theory is a theory that takes a phenomenon on face value without attempting to describe it from ‘the ground up’, say from the atomistic principles of quantum mechanics. Most of thermodynamics was developed without the concepts ‘atom’, ‘molecule’ or ‘wave function’ and yes it worked quite well.

Phenomenological theories can be quite far reaching but have their limitations and many physical scientists regard them as somewhat inferior to an ab initio version.

Intensive

An intensive variable is a variable which is independent of the size of the system. The density of a substance is one example of an intensive variable or property. Regardless of the amount of substance present or the size (i.e. volume) of the chosen system, this value does not change. This intensive property is unique to a given material.

Extensive

Many functions or properties can be either attributed to a particular object, say the volume of a particular keg of beer or based on a more standardized entity, say the volume of a mole of water. The first is called an extensive variable, the second an intensive one

In the first case we would write V_keg. In the second case we would write bar over the symbol.

One is volume e.g. in [liter], the other is volume per mole or molar volume in [mole/liter].

In thermodynamics it is ofter necessary to specify a number of attributes that pertain to a quantity. Is it per mole or per keg? Is it isothermal? Is it for the liquid or the solid? Is is for the state of my experiment or for a standard state? Often the number of sub- and superscripts necessary to specify it all makes the notation pretty cumbersome and a little squiggle or sub_i easily gets lost in the shuffle. It is therefore important not to rely too much on that but instead to try and specify in your own mind what a symbol refers to precisely.