While you slept last night, Santa Claus was putting his knowledge of physics to work. No, not with that silly anti-gravity stuff. Everyone knows the whole reindeer thing is just a cover for the way he really gets to all those houses in just one night. He uses one of the original Time Turners. In fact, Professor McGonagall got her first Time Turner from Santa himself.

No, the physics of Santa’s job is dealing with all those fireplaces and the fires that he encounters in so many of them. It’s understanding heat that saves his hide — and all those presents!

Back in grad school, I was taking a class in surface physics, and my friend Steve asked, “What is temperature?” On first hearing that question, you might think it’s so obvious that it doesn’t even bear asking, and anyone who does give voice to it must be an idiot. But have you really thought about it?

Perhaps the most important lesson I learned in my seven years of grad school was to ask questions, even about things that seem so basic as not to need questioning. There a lot of insights and deeper understanding available when you start questioning your basic beliefs.

Temperature, thermal energy, and heat

So, let’s look at a question related to Steve’s: What is heat? All of us who work or play in the world of building science talk about it, so it’s a good idea to know a little about this basic concept.

Actually, the two questions are related. Heat and temperature go together, but they’re not the same thing. Before we can understand heat and temperature, though, we’ve got to talk about thermal energy. It sounds like thermal energy and heat might be the same thing, too, but they’re not.

Thermal energy is the combined energy of all the jiggling going on in the molecules of an object.

Temperature is a measure of the average kinetic energy (energy of motion) of the molecules.

Heat is a measure of how much thermal energy transfers from one object to another because of a temperature difference.

Of the three quantities, temperature is the one we have direct access to. It’s the one we learn about at an early age, when we touch something hot and get burned. It’s also the key to understanding heat because heat flows only when there’s a temperature difference. You don’t learn any life lessons by touching something that’s the same temperature as you because no temperature difference means no heat flow.

The term “heat” should be thought of only in conjunction with “flow” or “transfer” because it’s what happens when there’s a temperature difference, and one body (at a higher temperature) transfers thermal energy to another (at a lower temperature). We don’t really care about thermal energy in building science because heat flow is what matters.

Calculating heat flow

We can easily calculate the amount of heat that we lose through the building enclosure (walls, floors, and ceilings that separate conditioned from unconditioned space) by conduction on a cold night. The equation is Q = U x A x ΔT.

Q is the amount of heat flow (in BTU per hour), U is the conductance, A is the area, and ΔT is the temperature difference. (For more on the details of this equation, see my articles U R A ΔT, and Other Building Science Blandishments and Flat or Lumpy.)

Now, where this really comes in handy is when you want to do some energy modeling or heating and cooling load calculations for the purpose of sizing the heating and cooling equipment. You use that formula for all parts of the building enclosure, add in your other loads (ducts, people, appliances…), and you can find out how much energy a home uses or needs for heating and cooling. It’s all about heat flow, which is all about temperature difference, which is all about thermal energy.

Let me emphasize one point here before I end: Heat flows only when there’s a temperature difference, and it naturally flows from a warmer body to a cooler body. You can pump it the other way, and that’s exactly what refrigerators, air conditioners, and heat pumps do. But the Second Law of Thermodynamics says heat wants to flow from warmer to cooler bodies.

That law is one of the most important and interesting in all of physics, and we could spend many hours engrossed in a discussion of the Second Law topics of entropy and the arrow of time, the heat death of the universe, why some processes are irreversible, and how it’s possible, according to statistical mechanics, for all the air in the room to be suddenly in one little corner, leaving you gasping in your chair as you read this blog. Of course, we’d need plenty of adult beverages on hand. Even better would be to have that discussion at the end of a long day of skiing.

And speaking of skiing, Santa has put his Time Turner back in its box and is now heading out to the slopes himself. That’s how he keeps in shape during the off-season, you know.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.