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Comment by extractionmech | Hacker News Reader

> On the other hand, cooling at the microscopic level involves the release of energy from individual particles, resulting in a dampening of their motion. This process corresponds to the system losing energy, leading to a decrease in the intensity of particle movement.

It actually makes sense, doesn’t it? Heating the object adds energy constructively. In cooling, energy removed from one particle may in fact be absorbed again by neighboring particles, so it is not ‘efficient’. So I’d venture a guess at saying the object cools from outside until it is entirely cooled.

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Heating and cooling are rarely nicely split up in time. While you're heating something it is also cooling and while you're cooling something it is also warming up. This usually limits your ability to heat something up or to cool it because at some point these two are in balance.

No. Put a cold thing into something hotter and the thing will heat up without cooling.

the hot thing will get cooler.

like to make something clean, you have to make something else dirty.

Is that true though? If I have say a box with mixed red and blue bricks and I sort them by color, both sides are "clean" now.

Sorting something is not the same as cleaning it, just like sorting an array doesn't sanitize its contents.

In a sense that misses the metaphor, sure.

But the thing which is "dirtier" is the wider universe, where the energy you used to do that sorting has higher entropy.

At the scale of atoms and molecules in a gas, you can also sort them into high-energy particles on one side of a barrier and low-energy particles on the other side, and now there's a heat difference you can run a heat engine. This is totally a thing you can do with the right devices — but those devices will necessarily consume more energy than you get from a heat engine running on that heat difference.

https://en.wikipedia.org/wiki/Maxwell%27s_demon

That’s because the box isn’t a closed system, you’ve interacted with in and you have spent energy sorting the bricks. It’s the same story with humans on earth, things get sorted because of the energy inout from the Sun.

You've just marginally warmed up everything else including yourself.

Red bricks won't magically disappear by sorting, just like dirt won't disappear when you wipe it off a surface.

Cleaning is usually relocating dirt.

If either red or blue were dirt, one of the sides is dirty, and the other is clean.

If neither red or blue were dirt, then nothing was dirty, it was just mixed up.

You've left out the entropy you've added to the universe by ordering the system. That entropy is in the form of wasted heat.

No. The outer layer of the hot thing will quickly heat up to the temperature of the hot medium, then it will cool down as is stays in contact with the middle layer, which will heat up itself. The same thing repeats in layers all over the object.

Your idea is inconsistent with the heat equation and with basic materials science. What do you derive this model from?

No, why do you think the outer layer will cool down? There will be heat transfer between the outer layer and the layer within, but at no point in time will the temperature of the outer layer be cooler than it was at an earlier point in time, so it will never cool down.

Other than the outer surface of the object there are no layers.

At the boundary between water and air, molecules are constantly snapping back and forth between being liquid and vapor. It's just that an equilibrium has been reached that makes the system appear static.

A molecule isn’t liquid or vapour. A molecule doesn’t have a temperature.

You are using a didactic oversimplification as an absolute truth while it is more nuanced.

An individual molecule in an open system can freely exchange energy with its surroundings and can/does have a temperature.

You can call the limitations of your knowledge about the exact state of a molecule a temperature, but that doesn't mean that any single molecule has an actual physical temperature in the same way as a collection of particles. You can say that a passing car had a speed between 60 mph and 70 mph because you couldn't measure it more exact, but the car has at any given moment an exact speed independent of your knowledge of it, not a distribution of speeds.

You may be right about the molecule, but the car absolutely has a distribution of speeds depending on what you are measuring in relation to.

That is exactly GPS point: a car doesn't have a distribution of speeds unless you measure it relative to something else whereas a liquid or a solid does have a temperature, regardless of anything else. Depending on what the something else is that you measure your car's speed by you may decide it goes forward, backward or is motionless.

When you start talking about individual molecules, atoms or particles the whole concept of temperature becomes very counterintuitive. Think of it as a substitute for motion or vibration if you wish and even that is grossly inaccurate (but less so...).

Maybe this will help: a gas in a container has a pressure and a density as well as a temperature, all of which are properties of the gas and not of the individual gas molecules. A single molecule that the gas is made up from does not have a density, it doesn't have a temperature and it doesn't have a pressure. What it may have though is a speed relative to something else, and when it hits the something else it may impart some energy relative to that speed difference.

It sounds like you're enthusiastically agreeing with me?

Molecules don't have innate solid or liquid properties, but they do operate as a solid or liquid, correct? It has nothing to do with them having a temperature of their own, and everything to do with how they are currently acting with their peers. IIUC.

You mean putting cold ice cubes into a warmer drink doesn't cool it down?

The ice cubes will only get warmer and will never get cooler.

I don't think you understood me. I meant the drink gets cooler.

I told you what I meant. Of course the drink gets colder, but that is not what we discuss.

Wait so hot objects have molecules with greater nuclear energy? This seems wrong. Are they emitting energy like radiation? I suppose that makes some sense, and would in fact support the asymmetry.

Without thinking about it I thought heat was kinetic energy. And I don't see how collisions would transfer kinetic energy in positive direction any better than negative direction.

I can't see any mention or implication of 'nuclear' - where do you see that?

Hot molecules move faster, it’s just kinetic energy.

If this were the case, then how can a solid have a temperature at all? Especially crystalline solids, where all the molecules/elements are bound in a lattice?

The molecules in a crystal still vibrate kinetically. A solid with no molecular motion would be at absolute zero temperature.

Wouldn't a fair amount of the thermal energy be hanging out in the bonds themselves? The stress in the system has to count for something.

I'm imagining a bunch of potential energy being stored in the fields at any given moment.

Absolutely, but that’s not heat. Things like heat produced from burning wood is an example of potential energy of binds getting released as kinetic energy + photons.

And heating/cooling is just transfer of kinetic energy? Seems strange if so.

Black body radiation, the emission of photons or EM radiation is one area to dig into for more information.

But also note the all mater you interact with is almost entirely comprised of empty space, despite the illusion of solidness at our scale.

But isn't that passing on of energy from one particle to the next, just conduction? Why wouldn't it apply in the same way when heating?

Particles in heat are more active, as in brownian motion puts them allover the place, the cooler things get, the smaller the ripples, the less likely to interact with another particle, by transfering that ripple.

Now in solid materials, that neighbour is always there to distribute any energy to everyone equally. But from solid to gas, there is only the surface and a gases density is lower, so the transfer propability shrinks again.

But that's hot vs cold. As I understood it, this article is talking about difference in rate of heating vs. cooling, from the same starting temperature.

I'm not saying that it doesn't make sense. What I am getting at is that I thought this was already known.

active vs passive, it makes sense to me that they would only be symmetric if the active side (heating) was done at a lower threshold than the upper threshold for cooling.

But it makes sense as to why heating would potentially be faster than cooling.

And heating in that coming only from outside. Hah. I never realized that.

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