Systems where there are two linked attributes, and change in either attribute induces change in the other, are really interesting.
I'm specifically thinking of air masses here— if air gets hotter, then it expands and induces less pressure on its surroundings. Conversely, if air has more pressure induced upon it by its surroundings, it's going to get hotter. And correspondingly temperature drops cause contraction and expansion causes temperature increasing.
This sort of system in which two attributes have a deep interplay with each other is pretty interesting. If there's a change in one, it'll change the other, and then because of that other things might happen to change the first one, causing a cycle of changes of states.
Dynamical systems are pretty interesting, and I don't have a lot of deeper understanding of them from a mathematical perspective— so I might mess around with a feedback system like this in @SnepGem's (still work-in-progress with a fractal-visual drawing feature in the works!) L Toy tomorrow.
The thermoelectric effect consists of two separate ideas— you can use a temperature differential to induce a current (called the Seebeck Effect) or you can use a current to induce a temperature differential (the Peltier Effect). this Steve mould video explains these effects pretty well.
in that video he also mentions a similar interplay can be seen with Stirling engines— you can use rotational kinetic energy to induce heat or you can use heat to induce rotation. The video where he talks about them is here though I skimmed through and didn't hear much mention of the rotational -> heat component.
I suppose this interplay just very often exists with heat (and more specifically heat differentials perhaps?), maybe just because thermal energy permeates everything and is pretty easy to affect or tap into. I'm curious if there are examples of this dynamic that don't involve heat, or ones that are less physical.
These are known as Onsager-Prigozhin's equations in nonequilibrium thermodynamics.
Non-physical examples include osmosis and reverse osmosis (duh), where pressure is related to chemical potential, membrane electric potentials where electricity and contentration gradients are coupled, or basically any plane of dimensional stability of successive biogeocenosis. Physical non-heat examples include electromagnetism (obvious, yeah?), mechanoelectrical couplers (RAILGUN), Einstein effect of photovoltaics fame, and many others. Funny heat examples are thermodiffusion/ thermophoresis, and the fact that to heat up a star you should cool it.
