The Hydrogen Economy From A to Z

April 21, 2022

Defining the Hydrogen Economy from A-Z: J is for Joule

In Recognition of the 200th year anniversary of Fourier’s Théorie analytique de la chaleur (The Analytical Theory of Heat), we’ll dedicate this week’s blog to the letter J for the “Joule” and our understanding (or lack thereof) of the Joule and heat.

The Joule, which is aptly named after James Joule, the physicist who discovered its relation to “work” in mechanics, is the International System of Unit (SI) unit of energy in relation to heat flow. But that’s not how its formally defined. Joule has been described both mechanically and electrically as equal to the work done by a force of one newton acting through one meter; and the energy released in one second by a current of one ampere through a resistance of one ohm. So where does heat flow fit in? Well, when there is a difference in temperature of two bodies, we observe that heat flow from warm to cool as an energy transmission and quantify that as a Joules. But, in classic calorimetry, we measure heat transmission at a constant temperature, i.e., phase change and define it by its Joules per gram. The total internal energy of a system, i.e., enthalpy, is measured in Joules. The kinetic energy of a body in motion, rotation or vibration is also described in Joules.

So, it begs the question, what really is Joules. That answer would lie in what is heat? And that question would ultimately lie in what is energy? As mentioned above, 200 years ago, the question of what heat is was highly debated, some argued for the theory of the caloric, which quantified heat as an actual substance or gas that would diffuse from warm to cold through solid or liquids. This theory has been largely abandoned since the 19th century in favor of Fourier’s theory on heat, but even Fourier was more concerned in what heat did, rather than what it is. Despite all the different ways in which heat or its unit Joule is described, I think we all agree that its some form of energy. As for what energy is… Who knows, we mainly describe it as the ability to do work. Basically, from a standpoint of use and conversion, rather than a single quantifiable measurement. Although, there are prevailing theories in the scientific community on a unifying theory of energy; energy, and its various forms like heat, and its units like Joules would continue to be a mystery to me. But like Fourier saw, we don’t need to know exactly what it is to use it and understand what it does.

In cryogenics, specifically using liquid hydrogen we take advantage of Joules, energy, and the management of heat to make it work for us. It might sound strange to say that cryogenics has to do with heat management, but it does, and a key takeaway is that heat does not flow as a function of temperature. J is for Joule (“heat”) and after that comes K for Kelvin (“temperature”) which we will talk about in GenH2‘s blog next week.

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