Zeroth Law of Thermodynamics
The scientific field of thermodynamics and the laws of thermodynamics all deal with the various aspects of heat energy and its interactions with matter.
Established after the first three laws, the Zeroth Law of Thermodynamics is actually the fourth law to be developed. However, it was found to underpin all of the other established laws. So, instead of making it the First Law and renaming the others, it was dubbed the “Zeroth Law” and the other laws retained their names.
The Zeroth Law of Thermodynamics is actually an observation. It states that if two thermodynamic systems are in thermal equilibrium with a third system, the first two system are also in thermal equilibrium with each other. Mathematically, this can be represented as: If A equals C, and B equals C, then A must also equal C.
In general, the Zeroth Law describes thermal equilibrium; a balanced state that does not alter over the course of time. If two thermodynamic systems are in contact with each other to the degree that they can transfer heat between each other, but they do not, the Zeroth Law says that they are in thermal equilibrium.
This equilibrium is quite common in everyday life. If you put a tray full of water into a freezer, eventually the water will freeze into ice and that ice will reach the same temperature as the air around it. At this point, the ice and the air in the freezer are in thermal equilibrium.
So, what is the point?
The concept behind the Zeroth Law seems so obvious, it can appear trivial, even pointless. Ice in a freezer does not exchange heat with the air surrounding it. So, what exactly is the point of that observation?
The Zeroth Law is significant because it lets us establish temperature as scientifically measurable and a universal property of matter. If something is scientifically measurable, then we can create mathematically based scales for it.
The popular temperature scales of Fahrenheit and Celsius were actually established long before the Zeroth Law was conceptualized. The problem with both Fahrenheit and even the more scientific Celsius is that they arbitrarily define the lowest possible ‘zero’ temperature, referred to as absolute zero.
In a scientific laboratory, a measuring scale must have a zero point that isn’t arbitrary. For instance, length, mass and time all have zero values, which all happen to be easily defined. Hence, a scale that measures heat energy must have a zero value, or a point at which there is zero heat energy. Because earlier scientists could not measure absolute zero, or even attain that temperature, they had to use other reference points for their scales, such as the freezing point of water.
Based on the Zeroth Law of Thermodynamics, the Kelvin scale defines the bottom of a temperature scale as absolute zero and as a result, establishes temperature as a universal property of matter.
The concept of the Zeroth Law of Thermodynamics may seem trivial, but it lets us define temperature, and that allows us to quantify the other three laws of thermodynamics.