The physics of keeping cool
REFRIGERATION: THE process of decreasing the temperature of some thing. Air conditioning (AC) can be a form of refrigeration.
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.
HUMANS DISCOVER SWEAT
Humans just can’t help it. Sometimes they get hot. But humans have a built-in cooling system. It’s called sweat. In order to understand how it works, maybe we should first look at temperature. You can measure temperature in degrees Celsius (ºC) or Fahrenheit (ºF), but what are you actually measuring?
If I were to give a simple definition of temperature, I would say that it is the measure of the average motion energy of the particles that make up an 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.
Suppose we have some water at room temperature (about 23ºC). This means that the water molecules in this 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 still water, but now with a lower average kinetic energy since the
‘Humans just can’t help it. Sometimes they get hot. But humans have a built-in cooling system. It’s called sweat.’
highest-kinetic energy molecules just left.
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.
If your body sweat is working correctly, all of this sweat your body produces will evaporate and you will be dry (but still stinky). In very humid regions,
the sweat 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. You can even use hot water on wet cloth and it will still cool down the water bottle (or beer).
HUMANS LEARN TO STORE ICE
It doesn’t take a genius to realise that in the winter there is ice on the lakes but not in the summer. 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?
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.
Temperature is the quantity that is the same for two objects in contact for a long time.
Ice doesn’t just cool by contact. Well, it does until it reaches its melting point at 0ºC (32ºF). 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? It comes from the surroundings.
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 is a demo you can do on your own to get an idea of how this would work. 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 that 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.
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 tyre, you might have noticed that the tyre 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 its 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.