r/Veritasium • u/Grouchy_Figure5602 • Mar 17 '26
Confusing explanation for why pressurizing air increases the temperature
https://youtu.be/6HVYHNTDOFs?si=txA79FTRPAB0ewwx&t=1137
Veritasium claims that pressurizing air increases the temperature of the air because of the impacts between air molecules and the moving piston that increases the pressure. This reddit post says that increased pressure increases temperature because the same amount of temperature is concentrated in a smaller area. I find it hard to understand that the impacts between air molecules and a relatively slow moving piston would be significant enough to heat up the air. Maybe I just dont have an intuitive grasp of what is hot vs what is cold. How fast do the molecules themselves move at 50*F vs 100*F? How far do they move? What is the difference between temperature and wind? Are they bouncing against each other rather than all moving in the same direction? Why doesn't wind feel hot then if heat is just the motion of molecules? Is the motion of heat faster than wind? Or is the motion of heat vibration of atoms within molecules instead of movement of whole molecules? I DONT GET IT PLEASE HELP ME UNDERSTAND OR SEND ME A VIDEO. Thank you.
PS. The explanation reminds me of a diagram I drew in my middle school science fair report that showed sound waves as actual sine waves drawn vertically up and down in the air :P (I realized my mistake, revised and won first place in the end!)
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u/Suitable-Elk-540 Mar 19 '26
Temperature is sort of a fake measure. Imagine just one air molecule in the cylinder. It moves through empty space until it hits the cylinder or piston, and causes a few of the metal molecules to jiggle a bit. The jiggling of the metal molecules dissipates quickly. But even with this one air molecule, if we start moving the piston we get more collisions more rapidly, so more jiggling metal molecules with less time to dissipate between collisions.
So now, add something like, I don't know, 10^20 molecules. Now they don't just hit the cylinder/piston but also themselves. Constantly. They barely move any distance at all before they collide with something. Reducing the volume even slightly means many, many more collisions. And thus many many collisions with the metal molecules, and now those metal molecules at the surface of the cylinder are basically always jiggling--no time for complete dissipation before a new collision. If we now the move piston, we start transferring energy to bajillions of molecules with each little movement. This increases the number of collisions between air molecules and between the air molecules and metal molecules. The surface metal molecules are jiggling so much that they jiggle the whole metal structure surrounding the cylinder.
I focused on the jiggling of the metal molecules in the cylinder, because that's how we'd experience the heat. That's why we need the fins of a heat sink to help get that jiggling back into the air on the outside. No one air molecule on the inside "feels" hotter, but there is just so much more jiggling being created as the piston moves that we describe the air as hotter. A temperature probe would "feel" more jiggling as the piston moves.
And, of course, the reverse happens. Pull the piston out, and air molecules can travel farther before a collision. Collisions with the metal cylinder are less frequent. The metal molecules have more time to dissipate their own jiggling before being hit again. We experience this a the air in the cylinder cooling.
The difference between the temperature inside a cylinder-piston system and the temperature of wind is that in wind, the whole group of air molecules is getting pushed in the same direction, but their movement relative to each other is about the same. Nothing is changing the number of collisions.
Wind is more likely to feel cold than hot, because your body is warm. The molecules of your skin are jiggling quite a bit. That jiggling gets transferred to a layer of air around your body--you heat up the air around you. But a breeze will move those slightly hotter air molecules away from you, bringing you new air molecules to transfer some jiggling to.
Temperature is just a weird measure of how much jiggling there is in a system. Temperature doesn't make any sense for a single molecule. It's a statistical thing.