Laws of Thermodynamics

“The forward momenta of Thermo,
Through progress from Physics and Chem, show
Laws named One, Two, and Three,
Then seek to define T,
So backtrack to a fourth law deemed Zero.”

In learning chemistry, I was often distracted by the etymologies of some of the terms of interest presented in my courses.  The 19 April 2019 limerick addresses one such linguistic oddity, one of the topics I now teach each year: the “zeroth” law of thermodynamics.  

“The forward momenta of Thermo,
Through progress from Physics and Chem, show…”
Thermodynamics is a field that generally examines heat energy and thus is of interest to many types of scientists.  This particular poem highlights the field’s development by physicists and chemists as commemorated in the four laws of thermodynamics, which were articulated during the mid-nineteenth to early-twentieth centuries.  

“Laws named One, Two, and Three,
Then seek to define T,
So backtrack to a fourth law deemed Zero.” 
The fundamental principles of thermodynamics are collected in the four laws of thermodynamics.  The first law explains the conservation of energy: energy cannot be created or destroyed; it can only change forms.  For chemists, this is generally presented as: ΔU = q + w. This equation states that a system exchanges its energy (ΔU) with its surroundings via two energetic “currencies”: heat (q) and work (w).  The second law contextualizes a property called entropy, which has been poetically dubbed “time’s arrow”; in terms of processes, entropy governs their spontaneity: the direction in which these processes naturally occur.  The third law states that as temperature approaches absolute zero (0 K), the entropy of a system approaches zero.   

The meaning of this limerick hinges on the unusual fact that the law of thermodynamics that was fourth to be formulated is named the zeroth law!  This is because the first, second, and third laws all rely on the property of temperature (T)… but to use temperature, it’s necessary to first define it.  The zeroth law defines temperature; when it was formulated, scientists decided it would be simpler to denote it as zeroth rather than renumber the existing three laws.