I get that hot air is less dense, because the molecules are moving faster. But given random Brownian motion of the molecules, why do they move up? What causes the molecules with higher kinetic energy to sort themselves to the top of an air column?
In: Physics
Some answers here talk about buoyancy or pressure, but I think that’s really the end result of what’s happening to these individual molecules. Here’s my understanding of what’s happening at that molecular level.
In that column of air, from the point of view of individual molecules, there’s really two important things: gravity and the energy of each molecule. Those molecules with low energy will act mainly under the influence of gravity and so slowly settle to the bottom (although lots of jostling will still be happening). Those molecules with higher energy will be racing around in random directions, and so will be more likely to find and occupy the larger spaces that emerge as the slower particles drop down the column. The faster the molecule, the more likely it will be to move into areas with more gaps, and so the warmer air will rise.
This is all me trying to interpret my high-school physics lessons, so corrections would be very welcome.
Latest Answers