Taking a step back: Basic thermal concepts

Article By : Bill Schweber

Even if you understand the basics of thermodynamics, a new book provides added insight and perspective.

Thermal considerations are a major part of many designs. Whether it’s minimizing or managing dissipation, access to various cooling techniques, or even creating cold for refrigeration or superconductivity, heat-related issues are pervasive in engineering considerations.

When engineers think thermal, it’s usually in the context of a broad macroscopic sense. In contrast, many theoretical physicists look at thermal effects from a highly microscopic perspective which I think of as the thermal “life” of atoms and molecules.

Regardless of specific discipline, many engineers take a thermal course at some point. If it’s not in a formal class, there are many good academic and vendor-sourced tutorials online. These courses and tutorials explain basics such as thermal sources, heat flow, impedances, conductivity, and modeling, Figure 1. These are usually precursors for examining passive and active thermal management tools ranging from heat sinks to heat pipes and cold plates, fans, air conditioning, and even fluid-based cooling.

Fig 1: Thermal models such as this one showing heat flow paths and resistances for a packaged semiconductor device are critical parts of the design process. (Image source: Richtek Technology Corporation)

I actually have a love/hate relationship with thermal science. Many years ago, I foolishly signed up for a course on thermal physics expecting to learn about the macro side, but the course was actually about the micro side and pretty much useless for engineering design. Instead, it was a deep dive into fundamental physics; it even included close reading of one of Einstein’s 1905 papers (in translation, of course) on random Brownian motion which invokes complex interactions among abstract theoretical constructs supported by advanced calculus, Reference. The three inescapable pillars of heat flow — conduction, convection, and radiation — were never mentioned in the course, which I soon realized was a bad omen about the entire course, Figure 2.

Fig 2: Understanding the three ways in which heat and energy flow is a critical first step in thermodynamics and thermal analysis. (Image source: solpass.org)

However, on the love side, I have really enjoyed learning—admittedly in a different setting—about the broad thermal concepts and analysis which we use to assess and manage heat in electronic systems, and the practical approaches to understanding and managing the flow of heat.

Due to my mixed emotions about thermal issues, I was somewhat hesitant to read a recently published book on the topic. “Einstein’s Fridge: How the Difference Between Hot and Cold Explains the Universe” by Paul Sen is a holistic look at the basic questions of what we mean by heat; how different experimenters, engineers, and researchers contributed to our present-day understanding of it; misunderstandings and misconceptions from earliest days of inquiry to the present; the three “simple” laws of thermodynamics; and more, Figure 3. It navigates among competing perspectives of theory, history, misconceptions, partial answers, contradictions, practical implications, and big-picture perspectives, with just a few very simple equations and with many analogies.

Fig 3: This recently published book provides a broad perspective and insight into heat and its many effects and implications, as well as historical context. (Image source: Scribner/Simon and Schuster)

There is certainly a need for this level of insight, especially among journalists who purport to inform us on energy-related topics. For example, in a recent article in The Wall Street Journal, “Heating Your Home Is Expensive and Carbon Heavy. Will Heat Pumps Help?” the author cited the flow of heat as “particles,” but I was unaware of the existence of such particles.

In addition, the author cited another possible heat-pump virtue, saying that “unlike traditional furnaces, which burn natural gas, oil or coal, heat pumps are electric and don’t rely on emissions-intensive fuels. If the electricity they receive from the grid is generated from renewable sources, they don’t need fossil fuels at all.” Wait…doesn’t that statement apply to all electrically driven heating systems, not just heat pumps? Many of the 600+ comments to the article pointed out other misunderstandings, gross oversimplifications, and simply misleading statements on energy issues. The reality is that like all heating systems, heat pumps have an array of advantages and disadvantages which are a function of local conditions, environment, house type, and more.

While I thought I knew as much as I needed to know about heat based on academic studies and practical experiences, the book was still a good way to think about issues that fall under “the meaning of it all” category, and so was much worth my time.

However, I do have two minor quibbles about the book: first, there’s the fairly misleading title, which I assume was foisted on the author by the publisher with the intention of better capturing potential reader (and search engine) interest. Einstein did devise and build an efficient refrigerator using his thermal insights, but it is only a small part of the book. Second, the cover graphic is backward: blue (for cold) should be on the left side of the arc and red for warm on the right, while the width of the arc indicator segments should be thinner for blue/cold and wider for red/warm.

What’s been your experience with thermal issues and thermodynamics? Did you have misconceptions that were gradually peeled away? Did you ever delve into thermal physics in the hope of greater insight?

Einstein, Albert; “Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen” [On the Movement of Small Particles Suspended in Stationary Liquids Required by the Molecular-Kinetic Theory of Heat] (PDF), Annalen der Physik (1905) in German; English translation at https://einsteinpapers.press.princeton.edu/vol2-trans/137.

This article was originally published on EE Times.

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical website manager for multiple EE Times sites and as both Executive Editor and Analog Editor at EDN. At Analog Devices, he was in marketing communications; as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these. Prior to the marcom role at Analog, Bill was Associate Editor of its respected technical journal, and also worked in its product marketing and applications engineering groups. Before those roles, he was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls. He has a BSEE from Columbia University and an MSEE from the University of Massachusetts, is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. He has also planned, written, and presented online courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.


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