May 14, 2020 9:00 AM

The Science of Temperature Is Weirder Than You Think

Getting warm is easy—like falling off a bike. So why is it so hard to be cool?

It’s still spring, but it’s starting to get hotter each day. Fairly soon it will be full-blast summer—at least here in Louisiana, where I live. But humans like to change the environment around them. When it’s cold, we want to heat things up. When it’s hot, we want to cool things down. Humans are difficult creatures.

What’s kind of weird, if you think about it, is that going one way is much harder than going the other way. Making stuff warm isn’t a problem. Just about anything you do will cause something to increase in temperature, even if you don’t want it to. But making stuff cold is trickier.

That seems surprising, because we think of temperature as a dimensional thing, where you can just raise or lower the level—the way you use a slider control to adjust the brightness of your screen. But that’s a false comparison, as you know if you have a thermostat in your house: At some point in the spring, you have to switch it from furnace to AC. They’re two different processes.

I’m going to look at a bunch of different ways that humans have invented to increase or decrease temperature. But first we need to talk about what the heck temperature is. No, it’s not a dimension. It’s something much more complicated. And it’s probably not what you think it is.

What Is Temperature Really?

My favorite definition is this:

Temperature is a quantity that will be the same for two things that are in contact for a long time.

If you put a fresh, hot cup of coffee on the table, and then get distracted by something on social media, the coffee will soon have the same temperature as the table. Gah! They won’t have the same amount of thermal energy, but they will have the same temperature.

Right. Then what about thermal energy? This goes along with another rough definition of temperature:

Temperature is a measure of the average kinetic energy of the particles in an object (where kinetic energy depends on both the mass and the velocity).

It’s not crazy to think of thermal energy as the sum of all the kinetic energies of the particles. (I’m oversimplifying a bit.)

But the main point is that two things can have the same temperature but different thermal energies. If you put pizza on aluminum foil in the oven, they will both reach the same temperature. However, the foil is very low mass and has much less thermal energy—that’s why it doesn’t burn your hands when you pull it out.

Did you notice I haven’t used the word heat here? I avoid this word because people think they know what it means, and it hinders their understanding of thermodynamic situations. We normally use it as a verb: The sun heats up our bodies. You heat the water to make tea. But it’s also used as though it were an actual thing that we can move around. We (stupidly) say “add heat” or we talk about “heat transfer.” Technically speaking, you cannot “bring the heat.”

How to Make Stuff Hotter

There are many different ways to increase the thermal energy of something. But basically there needs to be some kind of energy transfer. Let’s look at some of the ways this can happen:

Shine Light on It

One way to transfer energy is with electromagnetic waves. Visible light is one type of electromagnetic radiation; there’s also infrared, x-rays, gamma rays. These are all the same type of waves but with different wavelengths. And they can all transfer energy.

This is why things warm up when you leave them out in the sun. Of course, humans have been using the sun to increase the temperature of things for eons. Now we also use fry lamps. Same idea.

Sunlight is the most low-tech method here. But your microwave oven essentially works the same way. It has electromagnetic radiation with a different wavelength (12 cm instead of 500 nm), but that energy is still absorbed by the material such that it warms up.

Burn It

There are many different kinds of fire. However, the most common is a chemical reaction between carbon and oxygen. During this interaction, oxygen forms a bond with the carbon to create carbon dioxide. When a chemical bond is formed between these two elements, you get a bunch of energy. Yes, you get energy from creating bonds—not by breaking bonds.

You’ve used this method. You gather some carbon, in the form of old tree branches, then increase its temperature enough that this interaction with the oxygen in the air can take place. And just like that, you have a campfire, suitable for increasing the temperature of marshmallows.

It doesn’t have to be wood. Fossil fuels work well (if you ignore the part about ruining the planet for future generations). You can also burn metals like iron or aluminum, but that’s a little more complicated.

Shoot a Current Through It

An electric circuit is super easy to make from scratch. All you need is a battery and conducting wire, and you will have an electrical current. A very basic battery just consists of two different metals along with some type of acid between them. You can even make a simple battery from some pennies.

The battery creates an electric field inside the wire, and this field pushes free electrons so they accelerate. However, the electrons eventually collide with the atoms that make up the metal wire, thus slowing down and losing kinetic energy. But since energy is conserved, it has to go somewhere. Yup, the electrons’ loss of kinetic energy raises the thermal energy of the wire.

Every wire in your house that carries electrical current gets warm. Oh, maybe not super hot, but it does indeed rise in temperature. Next time you use your vacuum cleaner (which draws lots of current), put your hand on the power cable; you can feel the warmth.

Well, no one wants their power cords to get hot. But what about a toaster? Inside, it’s just a wire with electricity running through it that gets hot, and your bread sits next to the wire to get crispy. The same thing works with an electric oven or space heater.

You have to choose your wire carefully to get the performance you want. If you have a thick wire, it doesn’t rise in temperature as much as thin wire. The material matters also. Nichrome warms up more than copper.

Other Ways

That’s not all. Here are some other ways to increase the temperature of something:

Compress a gas. This is the basic idea for a heat pump.
Split (or combine) atoms. Nuclear fission (or fusion) warms stuff up. We use that to boil water in a nuclear power plant.
Rub it. Friction between two surfaces makes them both hotter—like the brake pads in your car. Usually this is an unwanted side effect, but it’s used on purpose in friction welding.
Drop it. When things fall, they speed up. When they then hit the ground, that kinetic energy transfers into thermal energy. So yes, you could try to heat your home by throwing things on the floor, but the methods above are better.

The point I’m making in this section is that it’s super easy to make things hot—because it’s probably going to happen anyway. No matter what you do, something is likely to increase in temperature, because your actions tend to transfer energy to other objects.

How to Make Stuff Colder

But what about making things colder? It turns out that’s a lot more complicated. Here are some of the cooling methods we can use:

Put It Next to Something Cold

OK, this one’s easy. If you put your drinks in a tub of ice, there is a thermal interaction between the two things. The ice gets warmer and your drinks get colder, thanks to the magic of thermodynamics.

Only one problem. To use this method, you need to already have something cold. Ice is an excellent choice—it takes a bunch of energy to increase in temperature and even more energy to change phase from solid to liquid. This means that your warm beer can transfer a lot of energy to the ice and in the process cool off.

Of course you can just go buy a bag of ice at the store, but in the old days they had to find ways of making winter ice last all year. There were ice houses with insulation. But the real secret was to get huge ice cubes. A big enough chunk of ice takes a really long time to melt. This is old tech. Even in the movie Frozen, Kristoff’s job was to harvest lake ice and store it for summer.

Use Evaporation

This is my favorite cooling method. It happens when a liquid gets enough energy to become a gas; if it’s water, it turns into water vapor. Since this phase transition takes energy, that leaves the remaining water with lower energy and thus colder temperature.

This is exactly what your body does when you sweat. Your skin secretes water, and when the moisture evaporates, it cools you off. It’s genius. Oh, you don’t like to get all sweaty? Yeah, that happens when the air is humid. The sweat still evaporates from your skin, but water vapor also condenses on your skin from the air. Net-net: wet shirt.

You can also use evaporative cooling on other objects. In fact, there’s an ancient device for keeping food called a clay pot cooler. Basically, you put food inside a clay pot, and that pot goes inside another pot with a layer of sand and water between them. When the water evaporates, it cools the inner pot. Of course, this only works in dry areas where you can get the water to evaporate.

What about fans? Yes, if you sit in front of a fan on a hot day, it feels nice and cool. But for the most part, fans don’t actually reduce the temperature of things, they just move air around. That increases the evaporation rate on your skin. But if your skin isn’t sweaty, it won't do much for you.

There is a situation in which fans can work for non-sweaty objects. The fan in your computer increases the air flow over the hot CPU. This has the effect of increasing the thermal contact between the air and the much warmer processor to help bring it closer to room temperature. But it can’t get colder than the surrounding air.

Compress and Expand

Now you are ready for some more modern methods of cooling. What about the air conditioner in your home or car? What about your refrigerator? Both of these things work in pretty much the same way—by heating stuff up. Yes, you can make things cold by first heating them up.

How about a demonstration? Grab a rubber band—one of those thicker ones will work best. Then quickly stretch it out, and keep it stretched as you touch it to your upper lip (which is sensitive to temperature). You should feel that the rubber band gets warm. Now keep it stretched as it cools off. Once it reaches room temperature, let the band relax back to its normal length. If you put it to your lip again, you’ll feel that it actually gets cold.

Basically, that’s a tiny air conditioner. When you stretch the rubber band, you are adding energy, which turns into increased thermal energy. However, this now hot rubber band can interact with the surrounding air because the air is cooler. Eventually, the two will reach an equilibrium temperature. Then, when you relax the rubber band, it goes back to its lower energy state, resulting in a decrease in thermal energy.

Your AC doesn't use rubber bands. (It would work, it just wouldn’t be efficient.) Instead there’s a refrigerant—R-134A is fairly common—that moves in a closed circuit from outside of your house to inside. Not only does it get hotter when you compress it (instead of stretching like a rubber band), it can also go through a phase change from gas to liquid. This means you can compress the stuff and get it hot. It will cool down to the ambient temperature (I assume this part happens outside), and then you can bring it inside the house to let it expand and cool the surroundings. That’s how it works.

Now you can see why you can’t cool off your house by opening your fridge door. Yes, the inside of the refrigerator is cold, but that’s only because there is stuff on the back of the fridge that gets hot. Also, there are electrical wires and motors that add to that. So overall, leaving the door open would increase the temperature in the room.

Do Weird Metal Stuff

This one still blows my mind. It’s called a Peltier cooler. It’s basically just two different metals connected together. When an electric current is run through the interface between these metals, one side gets hot and the other side gets cold. The effect is very small, so you need a whole bunch of these junctions to have an observable effect—but it’s real.

Wait. This device gets even crazier. Instead of running current through a Peltier device, if you make one side hot and the other side cold, you can generate electricity. This is exactly what happens in a thermoelectric generator. It has no moving parts, which is sort of wild. It’s not very efficient, but if you already have something hot (like a radioactive source) and something cold (like the vacuum of space) you can seemingly get free energy. That’s why these are useful in spacecraft.

There are a few other cooling methods out there. You can make atoms decrease in energy by using a laser and the Doppler effect. Also, there is magnetic cooling. It’s basically like an AC except that, instead a refrigerant, something gets warm in a magnetic field.

So to circle back, what did we learn? It’s super easy to make things hot, because it’s probably going to happen anyway. In order to cool things off, you basically need to make something hot and something cold. That’s where it gets complicated. But remember, temperature is not like distance. It’s easier to go one way (increase in temperature) than the other way (decrease in temperature). That’s because temperature is weirder than you think.