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Stay Cool

Summer officially started this week. Has anyone checked with Mother Nature? Are we sure she got the memo? Seriously, sometimes I can’t tell where we are with the weather. I don’t really mind the hot weather at all, as long as I can do things like jump in a river, or ride my bike. The warmer days had me thinking about an interesting topic that spans a bit more than just electricity and magnetism, but I thought it might be interesting anyway. How do we cool things down?

I’ve always been fascinated by this actually. Even as a child, I could understand, heating things up was easy. Fire is easy. Warming myself up in winter with a heavier jacket was easy. Cooling things down is always trickier, and there are several ways to do it. To get a better understanding of how we cool things down, we have to start at the very basics. What is temperature? Actually, what we call temperature is a measure of motion, specifically of atoms and particles. They’re not moving from A to B, but rather, vibrating in place, or in the case of gases, moving around haphazardly. The faster they move, the higher the temperature of the substance. When all motion stops, the substance if brutally cold. So cold, in fact, that it has a special name: absolute zero. Absolute zero is approximately -273.15 Celsius or -459.67 Fahrenheit. We even have a temperature scale that starts at absolute zero, called the Kelvin scale. 0 Kelvin is absolute zero. The motion of the atoms or particles also gives off something called black body radiation. This is simply thermal-spectrum electromagnetic radiation. As such, it’s part of the electromagnetic spectrum I wrote about some time ago. This is how infrared cameras work as well as any thermal imaging camera.

With that background in place, it’ll make more sense when I explain certain cooling methods we commonly use. One of the most efficient and effective cooling methods is gas decompression. This is how nearly all refrigerators, air conditioners and heat pumps work. In any gas, the molecules comprising the gas are spaced pretty far apart. When you compress them into a tinier volume of space, the gas will increase in temperature. Is this because the particles are now hitting each other more often and creating more frictional heating? It would be nice if that’s how it worked, but it’s actually a bit more complicated than that. The simplest way to understand heating of compressed gas is to understand conservation of energy. Energy cannot be created or destroyed. Compressing a gas requires some amount of energy, and that energy has to go somewhere. It ends up going into the gas causing the particles to move faster, which we observe as heat. From a physical standpoint, the gas particles are interacting with the boundaries of their space more frequently as the space containing them shrinks. The most scientifically accurate explanation of the temperature increase is that by reducing the available volume of space, you’re increasing your theoretical knowledge of where each particle is. Instead of being somewhere in a huge volume of space, each particle is now in some much smaller volume. You’re decreasing the entropy of the gas. That knowledge doesn’t come free though. The particles essentially say, “ok, we’ll let you know more about where we are, but in turn we’re going to let you know less about our speeds, because we’re going to move faster.”

Whew…all of that. Are you still with me? Hadley, you haven’t mentioned a single thing about cooling yet! Just heating! Yes, but now all the pieces are available. When we compress a gas, it heats up for the reasons stated above, but we can do something with that heat. We don’t have to let the gas just stay hot. This is what refrigerators do. Once they compress a gas, they pass it through some type of heat sink. This is a device that allows the heat of the gas/fluid inside to dissipate as quickly as possible to the ambient environment. Once this happens, we have a roughly room temperature compressed gas. If we allow the gas to decompress, its temperature will fall…to below room temperature. The particles are now saying, “ok, you’re increasing our volume, and this means you’ll know less about where we are. So to compensate, we’ll slow down a bit so you can at least know how fast we’re going.” Obviously the trick in any of these refrigeration systems is being able to compress the gas a lot as well as being able to efficiently remove the heat from the compressed gas.

This process will work for any gas, including air. In fact, I encounter this phenomenon every time I air down the tires on my bike. The tires are filled with ordinary air, compressed to between 90 and 110 PSI. When initially pressurized, they do heat up, but over time, they cool to ambient temperature. When I rapidly release the pressure, the valve becomes noticeably cold to the touch. In most refrigeration, we don’t use air, we use some kind of refrigerant, like Freon. Freon is the trademark name for any number of different gasses used as refrigerants known as halocarbons. You’ve probably heard of at least some of these by their scientific name, chlorofluorocarbons (abbreviated CFCs). These are the same CFCs that scientists discovered were causing ozone depletion in the 1980s and 1990s, so they aren’t in widespread use anymore. All refrigerants are just special types of gas that have properties that are beneficial to what we’ll be doing with them.

The important thing to remember is that if you’re unable to remove the heat from the compressed gas, and you let it decompress, all it will do is decrease back to roughly room temperature where it started. So it won’t be cool the way we want it. If you want the gas to be very cold, you have to make its starting temperature less. There is actually no limit to this (beyond absolute zero) and this is how we create extremely cold substances like liquid nitrogen. In fact, you can sometimes create bits of dry ice (frozen carbon dioxide) by releasing a fire extinguisher into a burlap sack. The CO2 in the fire extinguisher is compressed, and if it’s been sitting long enough, it’s also at room temperature. Releasing the pressure causes a large decrease in temperature. So large that the gas actually solidifies into its solid state.

Hopefully this was an interesting slight deviation from my usual topics of electromagentism.

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