The first law of thermodynamics is one of the more well-known laws of physics, especially in its generalized form of conservation of energy. It provides us with the power to see through stupid ideas and scams. It’s so important, in fact, that even Homer Simpson invokes the laws of thermodynamics when Lisa violated the first law:
But the first law led to a crisis of sorts as scientists looked into it further.
The second law of thermodynamics
The problem was that the first law of thermodynamics would allow all manner of bizarre things to happen. As we did in the previous article, let’s consider a glass of lukewarm water. It would not violate the first law for the water in the bottom half of the glass to get cold, possibly even freezing into a block of ice. For that to happen without violating the first law, the water in the top part of the glass would have to get hot, maintaining the same amount of thermal energy in the glass.
Have you ever seen that happen? No. It may not violate the first law, but we know from our own direct experience that things like that just don’t happen. That’s where the second law of thermodynamics comes in. This one is a bit more complex and has several, equivalent, formulations. We could geek out over heat engines, Carnot efficiency, entropy, and the arrow of time, but for the purposes of building science, the statement of the second law by Rudolf Clausius is the one of most significance:
Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.
In winter, the heat from a fireplace naturally wants to move to the outdoors. Hot to cold. Heat in the outdoor air doesn’t spontaneously make its way into our house “without some other change.” We can get heat from the outdoor air to heat our home with a heat pump, but that piece of equipment has other stuff going on and doesn’t violate the second law. But even then, when you look at how it really works, heat still flows from the warmer to the cooler object. (For more details, see my article on how a heat pump gets heat from cold.)
Heat moving from hot to cold is the form of the second law of thermodynamics I alluded to in the first paragraph of the first article of this series. This meaning of the second law, as well as its application to the flow of moisture (wet to dry) and air (high pressure to low pressure), is taught in many introductory building science classes.
Safety in numbers
A powerful way to understand the second law of thermodynamics comes from statistical mechanics. That glass of lukewarm water, for example, contains gazillions of water molecules. We’re talking lots and lots of zeros. Multiples of Avogadro’s number, which is:
Each one of those molecules has a particular velocity and position at any given time. The potential combinations of molecular states yield almost incomprehensibly large numbers. The reason we don’t see that lukewarm water separate into a hot section and a cold section is that the odds are against it. Way, way, way against it.
Shirts, pants, and friends
Here’s an example that might help you understand the power of large numbers. Let’s say you have five shirts in five different colors. That’s five “states” you can be in. You can go to work or school each weekday wearing a different color shirt. Or, assuming you do laundry each evening and dress in the dark the next morning, pulling a shirt randomly from the drawer gives you a one in five chance of wearing the red shirt on any given day.
Now, if you also have five pairs of pants in those same colors, the number of days you can go without repeating your outfit is much greater. For each pair of pants, you’ve got five different shirts so you have a total of 5 x 5 combinations. You can go 25 days without repeating an outfit. Or, if you dress in the dark, now there’s one chance in 25 you’ll wear your red shirt and red pants on the same day.
Expanding the example, if you have a friend with the same set of clothing options, she also has 25 possible combinations. For each of your 25 combinations, she could pick any one of hers. The total number of combinations here is 25 x 25, or 625. There’s one chance in 625 that you’ll both show up dressed all in red if you both choose your clothes randomly. That’s not a likely outcome, but it could happen. Right?
Let’s grow the group to five people now, each with five different shirts and five different pairs of pants. For each person we add to the group, we multiply by 25 again. The number of possible combinations is now nearly 10 million! If you all choose your outfits randomly, there’s only one chance in about 10 million that you’ll all show up dressed in red shirts and red pants. (But wouldn’t you stand out if you did!)
If we expand again to everyone in the world, all 7.6 billion of us, I think you can see that that all-red possibility is never gonna happen.
The punishing power of large numbers
Going back to that glass of lukewarm water again, the probability of getting separation of temperatures is far less likely than that of 7.6 billion people randomly picking red shirts and pants on the same day. We’re talking numbers of particles that are on the order of 10 to the 23rd power now, not just 10 to the 9th power. With numbers this large, the overwhelming majority of states would yield uniform temperature throughout the glass.
The second law states that heat never moves from a colder to a warmer object. The statistical mechanics version of it says that it could happen. It just doesn’t because the odds in its favor are infinitesimally tiny.
Building science applications
The second law is referenced widely in building science. To have any hope of designing building enclosures that work, we need to know which direction heat, air, and moisture are likely to flow. In analyzing failures, we can rely on that direction of flow to help us understand what happened. Stack effect, hygrothermal analysis, and energy modeling would be entirely different if we lived in a universe that had the first law but not the second. How would we calculate heat loss and heat gain if we didn’t know which way it might flow?
And then on the mechanical equipment side, the heat engine formulations of the second law carry great weight. They say we can’t get 100% efficiency from a heat engine (which turns heat into work) or a refrigerator (which uses work to move heat). The equation for the Carnot efficiency even gives us a way to check the claims of efficiency for such devices. All we need to know are the two temperatures it operates between, the sink and the source temperatures.
Here’s kind of tongue-in-cheek version of the first, second, and third laws of thermodynamics:
- You can’t win.
- You can’t break even.
- You can’t get out of the game.
I’ve heard this many times but just today found it’s attributed to the poet Allen Ginsberg and called Ginsberg’s Theorem.
The plight of the universe
Going just a wee bit beyond building science, the second law of thermodynamics implies that the ultimate fate of the universe is heat death. Given sufficient time, all the energy of the universe will end up spread out uniformly, with everything at the same temperature. That assumes the universe is an isolated system, of course. If our universe is simply a speck of matter in some larger universe, our fate may depend on what happens there, much as an ant’s fate might depend on which way that kid points the magnifying glass.
As you can see, the second law is arguably the most interesting of the laws of thermodynamics.
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.
Other Articles in This Series
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