The Physics of Keeping Cool

The Physics of Keeping Cool

Yes, there is snow and ice in the winter but where is it during the summer when we need it?
Image: Rhett Allain

The Physics of Keeping Cool

Image: Rhett Allain


There are several ways to reduce the temperature of things - like a person or a beer. The history and physics of cooling things can be quite interesting. I'm not a historian, so I am only going to speculate on the timeline of events in the life of refrigeration. However, I feel comfortable explaining the physics in each method.

Humans Discover Sweat ———————

Humans just can't help it. Sometimes they get hot. But alas! Humans have a built in cooling systems. It's called sweat. In order to understand how it works, maybe we should first look at temperature. You can measure the temperature in Celsius (°C) or Fahrenheit (°F), but what are you actually measuring?

If I were to give a simple definition of temperature, I would say that it is a measure of the average motion energy of the particles that make up that object. That's not a perfect definition, but I think it will be fine for now. This means that when you cool something, you decrease the average motion energy (kinetic energy) of its particles.

How does sweat cool you off? It works through the evaporation of water. Let me explain. Suppose we have some water at room temperature (about 23°C). This means that the water molecules in this group of water has an average kinetic energy of some value (it doesn't matter how much). But not all water molecules are the same. Instead there is a distribution of kinetic energies. Some molecules are moving quite slow and some are moving very fast. It's possible that these very fast (and few) molecules can escape the liquid water and become gas water (we call it water vapor). What's left is a water but now with a lower average kinetic energy since the highest KE molecules just left. Here is an older much more detailed post about evaporation.

If this water is a bead of sweat on the skin of a human, the water can be colder than the skin and cool it off through conduction (which I will talk about next).

This sweaty boy is hot and sweaty after soccer practice.

Image: Rhett Allain

Oh, important thing to notice. If your body sweat is working correctly, all of this water-sweat your body produces will evaporate and you will be dry (but still stinky). In very humid regions, the sweat-water doesn't completely evaporate and you are still stinky.

Cooling by evaporation isn't just for living things. You can actually use something like this to cool off something in your house. Here is a video I made showing this effect with a wet cloth and water bottle. Oh, you can even use hot water on the wet cloth and it will still cool down the water bottle (or beer).

Really, you should try that experiment at home. It's awesome.

Humans Learn to Store Ice ————————-

It doesn't take a genius to realize that in the winter, there is ice on the lakes but not in the summer (unless you are in that movie my kids love - Frozen). What if we just take that ice and store it somewhere so it doesn't melt so fast? Then, in the summer we can bring out out to make lemonade?

That's pretty much exactly what happened.

Water is something else you can get cold, but it's not as good as beer.

Image: Rhett Allain

How does ice cool things down? This is called heat conduction. The basic idea is that when two objects are in contact, there will be a transfer of energy from the hotter object to the cooler object until the two reach the same temperature. This leads me to my favorite definition of temperature.

Temperature is the quantity that is the same for two objects in contact for a long time.

That definition seems so obscure, but it's actually a fine way to put it.

Back to ice. Ice doesn't just cool by contact. Well, it does until it reaches it's melting point at 0°C (32°F). In order for the water to make a phase transition from solid to liquid, it requires even more energy. Where does this energy come from? Yup. It comes from the surroundings.

Humans Create the Kegerator —————————

What exactly is a kegerator? This is JUST like a refrigerator except you put a keg of beer in there. Oh, and you add a tap on the top or door so you can get cold beer without even opening the fridge door. Awesome, isn't it? You might think I'm kidding about the kegerator, but I'm not. Keeping drinks (and beer) cold was a consideration for both cooling by ice and refrigerators.

But how does a refrigerator work? It's all about compressing a gas and letting it turn into a liquid and then evaporate back into a gas. That might seem crazy, but here a demo you can do on your own to get an idea of how this would cool. All you need is a rubber band.

Take the rubber band and touch it to your lips to get a feeling of the temperature of the band (lips are more sensitive than your fingers). Now stretch the rubber band as far as you can without breaking it and touch it to your lips again (quickly). You should be able to feel the rubber band is now hotter than it was. Next, just hold it in a stretched position for a short time so that it can cool off to room temperature. Finally, let the rubber band compress back to its original size and feel it again. Guess what? It's cooler than room temperature. Here's the same thing in a video.

Your AC and refrigerator don't use rubber bands. Instead, there is a gas (called a refrigerant). This gas is compressed and gets hot in the process. If you have ever pumped up a bike tire, you might have noticed that the tire gets hot - same idea here. Since this hot compressed gas is hotter than the surrounding air, it transfers (through conduction) energy to the air and decreases it's temperature. This also causes the gas to condense into a liquid.

The next step is to take this liquid and allow it to expand into a gas. This phase transition and expansion into a gas takes energy. Of course the energy comes from the surroundings. This is the cooling part of the AC or refrigerator. The gas then goes back into the compressor and the cycle continues. Yes, I missed some details but that's the basic idea.

The inside of a freezer can get quite cold.

Image: Rhett Allain

I didn't say anything about geothermal cooling. This basically just cooling by conduction expect that you want to make thermal contact with things underground. At some times, the ground temperature is colder than the air temperature and can be used to cool things off. Yes, there are also some other cooling methods. Maybe in the future we will have magnetic based refrigerators.

One final thought: Why is it so much easier to increase the temperature of something than it is to decrease the temperature? Maybe this will be a future blog post.