4 ms·
I have a vivid memory of being taught this in 2nd grade, probably because it seemed (and still seems) extremely counterintuitive. I think the intuition given wa
by jointpdf 6y ago
I have a vivid memory of being taught this in 2nd grade, probably because it seemed (and still seems) extremely counterintuitive. I think the intuition given was that hot water is less dense than cold water, so the coldness can more easily permeate through the liquid (I imagine a vein-like pattern). Something like that.
At least according to Wikipedia, it seems that it’s a bit more complex: https://en.m.wikipedia.org/wiki/Mpemba_effect https://en.m.wikipedia.org/wiki/Mpemba_effect
Anyways, I’ve been repeating this fact for 2.5 decades, so shout out to my teacher for blowing my young mind.
- copperx 6y agoIn a similar way, I find the idea of cold permeating a liquid counterintuitive, because I can picture heat going into cold, but not cold into heat. The energy has to go somewhere!
- roelschroeven 6y agoIt's probably better to think of it as heat leaving the liquid rather than cold entering it.
- gvjddbnvdrbv 6y agoThis is what is physically happening so it is a much better mental model.
- gvjddbnvdrbv 6y agoCold is not something that can penetrate. Cold is merely a relatively low amount of heat energy per unit of mass. When something cools heat energy is conducted/convected/radiated away from an object causing it to have a lower amount of heat energy per unit mass.
- jointpdf 6y agoAdmittedly, my understanding of thermodynamics has not progressed much beyond the 2nd grade level (aside from a class on PDEs giving me at least some enhanced intuition). My takeaway from the PDE class was that anything beyond a simple textbook problem needs to be simulated and/or observed experimentally, rather than solved for analytically. But other things being equal, wouldn’t cold air permeate a less dense barrier (say a layer of insulation in a home) more easily than a dense barrier? Just as water (or cold air) would permeate soil more readily than rock.
- gvjddbnvdrbv 6y agoCold AIR permeates. That is correct. But there is no physical concept of COLD. The physical concept that exists is heat energy. Temperature is defined as the amount of heat energy per unit of matter. When something gets colder the physical process happening is that heat energy is leaving the thing and being transferred somewhere else/being converted to another form of energy. So when cold air comes into a house heat energy is transferred from e.g. the material that makes up the wall into the air warming the air and cooling the walls. The cold air is replacing warm air which moves out of the building.
- beervirus 6y agoAnd conventional current doesn’t exist because electrons are what’s really moving. It’s still not a bad mental shortcut.
- gvjddbnvdrbv 6y agoThinking of cold penetrating/moving is a terrible mental model. First it is physically wrong. Second the more physically correct mental model of heat moving is just as conceptually easy. Third once you start thinking of heat energy moving classical thermodynamics becomes quite a lot simpler to understand. I'm not sure about the best mental model for electric circuits. I never understood electricity.
- BlueTemplar 6y agoFunny how it's a parallel to electric current having the "wrong" sign...
- gvjddbnvdrbv 6y agoNot always. In an ionic fluid electric current can be transmitted via movement of positive ions.
- beervirus 6y agoEverything you just said applies equally well to conventional current. What kind of situation is there in which thinking about cold propagating instead of heat will give the wrong physical intuition?
- gvjddbnvdrbv 6y agoMaybe that's why I never understood electricity if the mental model they teach is just physically wrong... The OP is an example when they talk about cold penentrating which makes no sense.
- 6y ago