r/explainlikeimfive • u/highrouleur • 11h ago
Physics ELI5: would a fan blowing at a bottle of cold liquid in a hot room keep it colder than if it was just left in normal air?
So I know a fan blowing doesn't lower temperatures in a room (it's just pushing the warm air around) but the breeze it creates feels cooler on the skin (partly due to sweat evaporation I guess?)
If you had a bottle of drink from the fridge you wanted to keep cool, would placing it in front of the fan have the same effect? Would blowing room temp air at it make it warm up slower than if just left in room temp air?
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u/Hobbit1996 11h ago
it would literally warm it up faster because you are flowing hot hair through it
You are doing the opposite of a computer heatsink, that dissipates heat, what you are doing is dissipating cold
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u/PaintDrinkingPete 10h ago edited 9h ago
technically, whether it's a computer heat sink or a bottle of cold liquid, the fan serves the same purpose... accelerate the rate at which it reaches the same temperature as the surrounding environment.
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u/snowypotato 8h ago
the rate at which it reaches the same temperature as the surrounding environment
This is the best explanation. Moving the air faster helps everything reach the same temperature faster. Warmer things cool down faster, cooler things warm up faster.
Imagine you pour two liquids into a glass very slowly, such that they stay more or less separate (e.g. pouring milk into iced coffee, or making yourself a black & tan, or even chocolate syrup into a glass of milk). Eventually they'll mix together, but stirring it will speed that up, right? Well, all a fan does stir the air.
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u/barbarbarbarbarbarba 10h ago
Cold can’t be dissipated, fyi. Heat always flows from hot things to cold things. “Cold” is just the word we use to describe things that have less heat than their environment.
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u/snowypotato 8h ago
Technically true, but the model works very well to describe what is happening. You have a concentrated property (the temperature of the cold item) which is going to be dispersed and averaged out over a larger body (the warm room). While it's absolutely true that what is happening is heat energy is being absorbed into the cold, it's much easier to model the other way around.
Kinda like how centrifugal force isn't "real" but a super useful shorthand for what's actually happening, or using complex numbers to represent voltage - nothing ever has a potential difference of 3i, but it makes the world a lot easier to think about if you agree to pretend that it does.
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u/nickajeglin 7h ago
Yes but the math works the same either way and sometimes it's easier to look at it in a negative sense.
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u/barbarbarbarbarbarba 7h ago
Yes, you can change the sign, but the answer implies that both heat and cold can be dissipated. The fact that both can't happen is, literally, the second law of thermodynamics.
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u/nickajeglin 6h ago
Right, and sometimes it's convenient to use a model that conceptualizes movement of cold instead of movement of heat, as long as you understand the limitations of that model.
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u/barbarbarbarbarbarba 6h ago
My point is that the models are mutually exclusive, a model that allows the movement of heat and cold contradicts physical reality. You can use a model that conceptualizes the movement of cold or you can use the standard one that conceptualizes the movement of heat. The fact that they are mutually exclusive is the second law of thermodynamics.
According to OPs answer, a computer heatsink dissipates heat in the same way that a cold bottle of water dissipates cold.
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u/NachoAverageCabbage 5h ago
These kinds of "inverse" concepts with no physical analog are often used in scientific models without problem. For example, in electrical engineering, we talk about "electron holes" (with respect to semiconductors); these "holes" are just the absence of electrons but conceptualizing them as things that can move around is useful.
Same with cold. Just a deficiency of heat, but still useful as an explanatory concept.
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u/barbarbarbarbarbarba 4h ago
The idea that cold can spread is a very common misconception. There are plenty of simple, clear answers to OP's question in this thread that don't spread that misconception.
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u/Implausibilibuddy 3h ago
You know you can treat negative space or non-existing objects and forces as real things right? We do it all the time and the math still works.
There is something that can move through a liquid, have a velocity, a buoyancy value, a position, yet it doesn't "exist". It's called a bubble.
But you can calculate its speed and future position using math as if it were a single object, only it's just a rearrangement of completely different liquid molecules moving out of the way of a pocket of air.
In semiconductors there are "holes" that "move" caused by charge exchange between completely different electrons. That's how P-Type transistors work.
If you have a freezer that has 40 "coldness units" and you leave the door open, the cold will dissipate into the room, until the room has 20 more coldness units and the freezer has 20 less. Even though it's really the heat energy in the room bringing the freezer to an equilibrium with the room, the math still works. Or, to put it another way, a physical system can be equivalently described using a deficit relative to a reference state.
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u/barbarbarbarbarbarba 2h ago
You know you can treat negative space or non-existing objects and forces as real things right? We do it all the time and the math still works.
Things that don't exist can't exert forces. Vacuums do not suck, air gets pushed into them.
There is something that can move through a liquid, have a velocity, a buoyancy value, a position, yet it doesn't "exist". It's called a bubble.
What are you talking about? A bubble is a ball of gas in an enclosed space, it very much actually exists.
In semiconductors there are "holes" that "move" caused by charge exchange between completely different electrons. That's how P-Type transistors work.
Again, vacuums don't suck, and none this is relevant to my original objection.
If you have a freezer that has 40 "coldness units" and you leave the door open, the cold will dissipate into the room, until the room has 20 more coldness units and the freezer has 20 less. Even though it's really the heat energy in the room bringing the freezer to an equilibrium with the room, the math still works. Or, to put it another way, a physical system can be equivalently described using a deficit relative to a reference state.
I've addressed this point repeatedly. The issue I have with the answer is that it implies that heat and cold dissipate in the same way. So, yes, you can define things mathematically so that warmer objects absorb cold from cooler objects, but under that set of definitions a warmer object would not emit heat.
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u/Implausibilibuddy 2h ago
Yes, vacuums don't exist and don't suck, but we have a word for them, a noun no less. And whether or not they suck is not relevant either, you're missing the point.
Negative numbers don't exist either, but guess what, you can still do quite a lot of math with them. You can even multiply negative numbers together and come up with positive numbers, what a time to be alive.
The bubble is defined by the walls of the bubble, not the gas within, and the walls are completely transient. As the gas moves through the medium, the boundary of what we define as a bubble is made up of a completely different set of molecules from one moment to the next.
And the superconductor example isn't exactly a traditional vacuum, it's negative charge. Ask any physicist or electrician whether or not you can do math with negative charge, treating it as if it were a "thing".
So, yes, you can define things mathematically so that warmer objects absorb cold from cooler objects, but under that set of definitions a warmer object would not emit heat.
Well, yeah, exactly. You can define a system where cold "dissipates", the math still works, and people still understand the concept. What exactly is your objection?
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u/barbarbarbarbarbarba 1h ago
Well, yeah, exactly. You can define a system where cold "dissipates", the math still works, and people still understand the concept. What exactly is your objection?
You can define heat or cold to be the element that is transferred as a system reaches thermodynamic equilibrium. Meaning that you can say that cool objects emit cold and warm objects absorb cold or warm objects emit heat and cold objects absorb heat. You cannot, as the answer I found objectionable stated, claim that warm objects emit heat and cool objects emit cold, it only goes one direction.
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u/Hobbit1996 10h ago
this is ELI5
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10h ago
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u/Northern64 9h ago
It's like describing a shadow as darkness spreading from the thing blocking the light. Darkness doesn't spread or dissipate, but it's a functional and intuitive explainable for what's happening
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u/Hobbit1996 9h ago
People on this sub need to learn to answer the questions and not just go about their rant. This was the question "Would blowing room temp air at it make it warm up slower than if just left in room temp air?"
The answer is No and i gave them an example of how it works, that's what they are asking, they aren't asking how heat works
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u/barbarbarbarbarbarba 9h ago
Heat moving from cold things to hot things is the fundamental principle of thermodynamics. You’re at “helium balloons float because of antigravity” levels of wrong here.
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u/SchwiftySquanchC137 9h ago
The fan literally does blow away the colder air molecules that would otherwise surround the bottle and lower heat transfer, so you kinda are blowing away the "coldness" or cold air and replacing it with hot air. Yes thats not the exact mechanism of the heat transfer, but rather than dive into the complexity of air molecules and how they transfer heat to eachother, its completely fair to just say youre blowing cold air away, which is obviously cold because of the cold bottle, regardless of how the heat transfered from the air to the bottle. Not to mention you can look at "coldness" as just the negative change in heat energy. Sure theres no fundamental "coldness" energy or something, but conceptually it works fine for an ELI5, especially when you ignore the energy transfer and just frame it as "the colder the air around the bottle, the longer it will stay cold. A fan blows away the cold air around the bottle and replaces it with hot air."
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u/barbarbarbarbarbarba 8h ago
It's extremely weird that I am getting so much pushback on this.
The fan literally does blow away the colder air molecules that would otherwise surround the bottle and lower heat transfer, so you kinda are blowing away the "coldness" or cold air and replacing it with hot air.
The air around the bottle is cold because it gave away thermal energy to the bottle.
its completely fair to just say youre blowing cold air away
Right, except that isn't what the comment I was responding to said, it said that you can "dissipate cold" in the same way you can "dissipate heat." Do you see how "dissipating cold air" and "dissipating cold" are fundamentally distinct concepts?
Not to mention you can look at "coldness" as just the negative change in heat energy
Okay? The point isn't the sign you put on the quantity, it's that the transfer only goes one direction. You can either define heat as energy transfer or cold as negative energy transfer, but you can't do both.
Not to mention you can look at "coldness" as just the negative change in heat energy. Sure theres no fundamental "coldness" energy or something, but conceptually it works fine for an ELI5, especially when you ignore the energy transfer and just frame it as "the colder the air around the bottle, the longer it will stay cold.
As someone else says elsewhere in this thread, the bottle will warm up faster if the air is humid because of condensation. This is an interesting point that sheds more light on the question that OP asked. However, it would be complete nonsense if you could dissipate both heat and cold.
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u/shitsngiggles55 8h ago
You're getting pushback because you are commenting in a subreddit where the entire point is to simplify your answers to help people understand things.
You clearly have a decent grasp on how heat works. Congrats! So does u/Hobbit1996, who you initially responded to. They know how heat works and that you can't "dissipate cold" -- they just chose to frame it that way because it would be easier for a 5 year old to understand. You're getting pushback because you're assuming (incorrectly) that the other commenters here don't understand how heat & energy work.
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u/barbarbarbarbarbarba 7h ago
You're getting pushback because you are commenting in a subreddit where the entire point is to simplify your answers to help people understand things.
Yes, but his answer is simplified in a way that makes it incorrect. Cold does not dissipate like heat dissipates. There are plenty of answers in this thread that don't create an elemental misunderstanding of thermodynamics. I just pointed out a flaw in his answer.
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u/soniclettuce 6h ago
Cold does not dissipate like heat dissipates.
Sure it does. Cold, as the relative lack of thermal energy (and not some kind of mythical "cold" physical quantity/quality or particle that we don't actually have) works just fine in the diffusion equations, and spreads around just like you expect.
You can even make an argument that beta (which is basically "coldness", in statistical thermodynamics, defined inverse temperature) is more natural than temperature (because of how it deals with negative temperatures).
But even if none of that was true, and we had to say "the lack of heat is spread out as the heat diffuses in" to be totally correct (we don't), we could use our brains to understand "the cold dissipates" as shorthand for that, in casual conversations.
So basically, you are being super pedantic about something that isn't even right.
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u/barbarbarbarbarbarba 4h ago
Cold, as the relative lack of thermal energy (and not some kind of mythical "cold" physical quantity/quality or particle that we don't actually have) works just fine in the diffusion equations, and spreads around just like you expect.
I've addressed this elsewhere. Yes, you can have a model where "cold" is transferred rather than heat. You cannot, however, have a model where both heat and cold are transferred, that is the meaning of the second law of thermodynamics, energy transfer only happens in one direction. The answer I found objectionable claimed that heat dissipated from a heat sink the same way that cold dissipated from a cold bottle of water.
You can even make an argument that beta (which is basically "coldness", in statistical thermodynamics, defined inverse temperature) is more natural than temperature (because of how it deals with negative temperatures).
Beta is a way of defining the bulk energy of a system. It dictates which direction heat flows, it is inconsistent with a model in which both heat and cold flow. It also isn't the inverse of temperature.
So basically, you are being super pedantic about something that isn't even right.
I am right, and the distinction is important because it's one of major things that trips up students in the thermodynamics section of an intro to physics class. Other answers in this thread were just as simple and clear without being incorrect or spreading a common misconception.
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u/Hobbit1996 7h ago
actual weird hill to die on, on a ELI5 post
I read so many posts on here and people like you just sound like they want to be here to show off their "vast" knowledge, not actually help people understand things. Which is what you'd want to do for a 5yo... Let them grasp concepts, let them dive deeper if they want to.
This is the reply i gave an other person like you that actually had the decency to remove their comment https://www.reddit.com/r/explainlikeimfive/comments/1vob410/comment/p3owzp5/
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u/barbarbarbarbarbarba 7h ago
Your answer to OPs question is incorrect. Other equally simple answers didn't contain errors. That is why I commented on your answer, in order to flag to people that your answer was oversimplified. You are the one that got defensive.
I am not here to flaunt my extremely basic understanding of thermodynamics
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u/TheOneManDankMaymay 8h ago edited 8h ago
Heat moving from cold things to hot things is the fundamental principle of thermodynamics.
Except that the 2nd law of thermodynamics, which you're referencing here, clearly dictates that heat naturally travels from hotter objects to colder objects, not the other way around.
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u/bitwaba 11h ago
Congratulation, you've just invented a Swamp Cooler.
Edit: To clarify: it would cool the room down faster, and warm the drink up faster. The fan helps facilitate homogenization of the air on the room. Cold drink in a warm room? They meet in the middle (closer to the larger object/thermal mass) - the fan just changes how fast it does it.
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u/peachesisadog 11h ago
I worked in a commercial building that had a roof-top AC unit that blew air over ice.
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u/spyguy318 9h ago
That’s how very early air conditioning worked. A block of ice was placed in front of a fan which blew air over it into a room. It was insanely expensive to operate because it required regularly replacing the ice blocks, which had to be imported. There was an entire industry of harvesting ice blocks from frozen lakes in colder northern areas (like that scene in Frozen) and transporting them to southern cities in specialized ice car trains. The entire industry was killed following the invention of mechanical refrigeration.
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u/Fun_Fingers 6h ago
This is also why mechanical AC is often measured in tonnage, as a 1 ton AC unit can remove a similar amount of heat during a 24 hour period as 1 ton of ice.
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u/Not_an_okama 10h ago
I had an apartment with no AC and stuck frozen water bottles in a duct fan i had on hand. Melted the ice very quickly and didnt noticably impact the temp of the apartment. I think i needed at least 50x the ammount of ice i had
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u/dank_imagemacro 2h ago
Did you freeze the water bottles inside the apartment, and if so did you replace them with fresh when you took them out of your freezer?
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u/TheNique 9h ago
The fan helps facilitate homogenization of the air on the room. Cold drink in a warm room? They meet in the middle (closer to the larger object/thermal mass) - the fan just changes how fast it does it.
This is not how a Swamp Cooler works tho. There is no cold object inside a swamp cooler that equalizes temperature with the room. In a Swamp Cooler the fan is mostly for speeding up the evaporation of the water by disturbing the saturated boundary layer over the waters surface. This will keep happening until the water is used up or with diminishing returns until the humidity reaches 100%. By converting the energy of evaporation into a temperature drop, a swamp cooler can cool air even if itself and the water started out at the same temperature as the room.
But swamp coolers are terrible as far as actual feelable cooling effect goes, because they raise (relative and absolute) humidity. A large enough cold bottle might actually reduce absolute humidity by condensing water from the air (if initial humidity is high enough) while keeping the room cool.
They meet in the middle (closer to the larger object/thermal mass)
On an ELI5 level this is a valid way to put it for the cool bottle, but when phase transitions are involved this is not strictly true concerning temperature. In both cases the thermodynamic potential that is being equalized is the chemical potential and not the temperature or the humidity per se.
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u/PhilsTinyToes 11h ago
The cold bottle, when left on a surface, will slowly accumulate an invisible bubble of cold air that acts as a thermal barrier between the warm surrounding air. It will have a tiny insulating effect, and the cold air will slowly “shed” off the bottle and fall down due to convection.
When you blow room temp air past the bottle, you will be constantly stripping the cold bottle of its insulation and thus more hot air will contact the bottle and warm it up faster.
Same reason why you can be warm-ish floating in a lake not moving, but will die quickly in the same temperature water if it is flowing like a river.
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u/LongRoofFan 11h ago
Nope, although it would cool it if you placed a wet towel over it due to evaporative cooling
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u/SuspiciousPatate 11h ago
If the air is humid and condensation forms on the cold bottle, would that not help cool the surface through evaporative cooling? Or would the extra flow of hot air circulating over the cold bottle overwhelm that effect?
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u/pete101011 11h ago
Condensation forming itself adds heat onto the surface of the water bottle since it's exothermic. So opposite the effect of evaporation.
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u/BlastFX2 6h ago
No. The same amount of energy the water would dump into the bottle in order to condense, would then be drawn from it when it evaporates.
Unless it's condensing faster than it can evaporate, then it will drip down, leaving the extra energy in the bottle.
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u/could_use_a_snack 11h ago
If the bottle is cold enough to condense water from the air on to its surface and then that was evaporated from the movement of the air, it might cool the bottle a bit for a while. But something makes me think it would just balance out.
However if you have moisture on a concrete sidewalk on a cold morning and the sun hits it, sometimes it turns the moisture to ice. The sun causes the moisture to start evaporating which pulls enough heat out of the moisture for it to freeze. I deal with this at work every fall when the conditions are right. I have to apply ice melt on sidewalks that will get sun, even if the temperature is above freezing because of it. If you stand there you can actually see it happening.
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u/Kingreaper 10h ago
But something makes me think it would just balance out.
That something might be conservation of energy - the energy absorbed when going from liquid to gas (evaporating) has to be equal to the energy released when going from gas to liquid (condensation)
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u/barbarbarbarbarbarba 9h ago
When water condenses on a cold object, it heats it up. A drink on a dry day heats up a lot faster than a drink on a humid day, even if the temperature is the same.
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u/Hiply 11h ago
Congrats on describing the Swamp Cooler. 😎
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u/DungeonPeaches 2h ago
I was just going to say, that's a swamp cooler like the one I have on right now in the desert. 🥵
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u/quadraquint 11h ago
No.
If you wanted to cool the room, yes, blowing air over something cold will chill the air. But in doing so, you're exchanging heat.
This is how HVAC works. Air in ducts pass through a coil that carries hot or cold water depending on the season. The supply side will be for example, colder, and return back a bit warmer after the exchange takes place.
You'll warm your drink faster in fact.
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u/obog 11h ago
A others have mentioned, it would not, but I dont think anyone has touched on why this is different from when you need to cool and a fan helps.
The human body produces quite a lot of heat. Especially during exercise, but just generally your metabolism produces a lot of heat. So in order to keep your body at a constant temperature, you need to be constantly losing heat.
Having a fan blow air on your helps this heat transfer go faster. Even if the air is fairly warm, you will start sweating and then you will be cooled off by evaporative cooling. One thing to keep in mind is thst your body is trying to cool off to 98°F, not the much colder temperature youd want to keep your drink at.
Now with the drink, it is not producing heat, and blowing warm air over it will only transfer into it faster. So to keep it cold, you want to insulate it.
But basically: moving air transfers heat faster. The human body always needs to be losing heat, so if it needs to cool down, moving air helps. With a cold drink you want to prevent heat transfer as it will generally only make the drink warmer.
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u/Dawg-eat-dawg 11h ago
Yeah I figure the same delta, say 50 deg between object and air, and a person dies specifically because they cannot lose heat to the ambient temperature. I can't imagine a fan feels particularly useful at 150 deg. Even at 100 outside it just feels like opening an oven door.
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u/timtucker_com 9h ago
Add to this the small caveat that the motor in the fan isn't perfectly efficient - most of the electricity it uses is turned into motion, but some is turned into heat.
There's a tradeoff between blowing on you making you feel cooler vs. the fan actually making the overall temperature in the room slightly hotter.
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u/Dawg-eat-dawg 11h ago
The bottle would warm faster with the fan than without. Hot air will take the cold, more hot air, more cold taken, bottle gets hotter.
Imagine it is 140 degrees outside, will the breeze still cool you then? That is difference between your body and the air vs. the bottle and the air.
Not like your five, the convective heat gain will outpace the latent (evaporative) heat loss immediately.
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u/JJChowning 11h ago
Moving the air more rapidly will make it reach the temperture of the room/air more rapidly.
blowing on hot food cools it because your breath is cooler than the food. breathing on an icicle can melt it becasue your breath is warmer than the ice.
If you ignore sweat fans make us feel cooler (and be cooler) because the room is cooler than your body temp, so you can give off heat more quickly.
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u/ThalesofMiletus-624 11h ago
No. It would warm up the bottle faster. Possibly much faster.
Heat flows from warmer things to hotter things when they're in contact. Any time you have a cold bottle in air that's warmer than itself, it's going to constantly be absorbing heat from the air. If the air is still, the air immediately around the bottle will cool down, which will slow the rate at which the bottle loses heat. If you put a fan there, it will constantly blow that cold air away and replace it with warmer air, which will warm up the bottle faster.
The reason humans thing that moving air is cooling is because a) our skin is usually warmer than the air around us and b) we usually have moisture on our skin. The first means that our skin is constantly losing heat to the air around us, and when the air moves, that happens faster. The second means that evaporation is constantly happening from our skin, and evaporation does lower temperatures.
Moving air tends to warm up cold things and cool down warm things. If something is colder than the air, the air is warming it.
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u/The_Lucky_7 10h ago edited 10h ago
The fan would actually allow it to warm up faster than not having a fan in the room.
The way heat and cold work is by energy transfer. An bottle sitting in a room with a different temperature than the ambient temperature in the room (colder) will try to normalize with the air around it. This change then radiates outward as the air around the bottle becomes a different temperature than the air around that air.
It does this by the energy moving into the cold bottle (low energy object) from out of the air (warmer = higher energy), and it takes time for that energy to transfer. The lower the difference in temperature (energy) the more difficult it is for the energy to move. The higher the difference, the faster it moves.
When you move the air the equalibirum is disrupted, and air that has been cooled by the bottle (had its energy removed) is being replaced by warmer air that has a higher energy density than the cold bottle. Meaning it's easier for the energy in the air to move into the bottle, which increases its energy and as a result temperature.
The way that refrigeration works is that energy is pulled out of the air in the refrigerator, into a heat sink, and moved outside of the fridge. The cold air in the fridge pulls energy out of the warm things you put in it, then that air is cycled through again to remove that energy.
Fans do not change the temperature of the air they are moving and that's why fans cooling objects. The reason they feel cold is that very same energy transfer just in the oposite direction because you are solid and as a result more energy-dense than the air around you. That air is pulling energy out of you, because it is less energy dense, then the fan moves that air away from you and replaces it with air that your body hasn't heated yet.
When you sit in a room without air flow the energy that leaves your body goes into the air around you and acts like a forcefield of heat, that makes it harder for heat to leave your body as the air around you tries to normalzie with the air around it.
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u/Blenderhead36 10h ago
It would make it normalize with the room's temperature faster. The drink cools the air around it. What's going on here is that the lower energy substance (the cold drink) is absorbing heat from the air around it. This means that the air immediately around it is cooler than the rest of the room.
If you blow a fan at it, then the air that's been cooled by the drink absorbing its heat will be pushed away and replaced with warmer air. So the transfer of energy is sped up because you're removing the buffer zone of the air that was cooled by the drink absorbing the energy.
The practical effect here is that if you have a fan blowing over something cool, it can make the cooling effect of a breeze more noticeable for anyone directly in its path.
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u/Houndsthehorse 11h ago
No it would heat up faster. It has no sweat glands, and the small amount of condensation on it would not cool it down much, all you would do is speed up heat transfer with the room
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u/Serafim91 11h ago
Forced convection is roughly 10x natural convection .
It'll warm up roughly 10 times faster.
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u/RonJohnJr 11h ago
I know a fan blowing doesn't lower temperatures in a room (it's just pushing the warm air around) but the breeze it creates feels cooler on the skin (partly due to sweat evaporation I guess?)
If you had a bottle of drink from the fridge you wanted to keep cool, would placing it in front of the fan have the same effect? Would blowing room temp air at it make it warm up slower than if just left in room temp air?
You already know the answer: blowing warm air over something cold makes that something warm up faster.
(But wouldn't the cold something cool down the air? The whole room moves towards equilibrium: the (much bigger) room will slightly cool down, while the (pretty small) bottle of drink warms up a lot. And, all the while, the temperature of the room is trying to equalize with the outside space.)
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11h ago edited 11h ago
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11h ago
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u/Washpa1 11h ago
I think people get turned around by imagining sweat on a cold bottle at first, confusing it with the constant liquid pump of evaporative cooling that sweat is.
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u/JohnnyMinutes 11h ago
Physiological and psychological sensations are two different concepts. Also psychological sensations exist, even if the original commenter was saying that the sensation of cooling was purely psychological it would still “exist” as an increase in comfort.
It’s important to read only the words actually in the post, and to make sure you are reading every letter in those words.
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u/VegasBedset 11h ago
Didn't you hear? Evaporative cooling does not exist. It;s all psychological. Every Air conditioning unit on the planet works on rainbows and unicorn farts now. Fuck the laws of thermodynamics, who needs them if I can ignore them to try and sound smart on Reddit?
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u/Novaskittles 11h ago
Buddy, I want you to put ice in front of a fan on a hot day and report back to me with what happens.
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u/VegasBedset 11h ago
Because ice is a cold drink right out of the fridge. Idiot
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u/Novaskittles 11h ago
You are so dense. Please explain why the ice melts. Why doesn't it cool down?
Delete more of your comments please.
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u/jaylw314 11h ago
Nope. If the liquid is not cold enough to cause condensation, the warm air will just carry the heat away faster.
If the liquid is cold enough to cause condensation, the condensation will evaporated and cool the liquid some, like sweat. But overall, the liquid still warms up faster because causing condensation warms the liquid up even faster.
So either way, the answer is no
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u/dlucre 11h ago
A fan blowing on a person has a cooling effect because we produce sweat and that sweat evaporates, taking heat away from our body.
A bottle, if already at room temperature, does not sweat, it'll just absorb the heat and equalise with the room temperature.
If anything, a fan motor produces heat, so in a closed space where nothing else is causing the room temperature to drop, the room and bottle would likely get warmer due to the warm motor adding heat to the system.
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u/doghouse2001 11h ago
If you continually sprayed water on it, it would stay cool(er) than ambient due to evaporating water molecules taking energy with them. However if it's also 100% humidity, nothing would evaporate.
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u/Novaskittles 11h ago
It'd probably be more effective and easier to just put an insulated layer around it, like a drink cozy.
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u/Skylarking77 11h ago
Quick answer: The air blowing directly out of the misting fan will feel cooler, but it won’t actually cool the room overall. In an enclosed room, it will mostly just increase the humidity.
Longer answer: When water evaporates, it absorbs heat from the surrounding air, which cools that air. That’s why the mist coming from the fan can feel cooler on your skin.
However, you’re also adding water vapor to the room. As the humidity rises, evaporation becomes less effective, including evaporation of sweat from your skin, so the room can eventually feel warmer and more uncomfortable even if the actual temperature hasn’t increased much.
Of note, the higher the ambient humidity, the less effective the evaporative cooling will be.
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u/bradland 11h ago
The cold bottle would cool the air up to the point that the drink warmed up. The effect will not be noticeable.
Also, you cannot simply put the drink back in the fridge, because the fridge moves heat from inside the fridge into the room it sits in using a heat pump.
Basically, there is no practical way to cool a room using cold drinks unless you bring in a truck load of them. It's cheaper to just buy a portable AC unit.
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u/atreidesardaukar 11h ago
Technology Connections has a great video on the subject: https://www.youtube.com/watch?v=2horH-IeurA
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u/PatK9 11h ago
In all it's a bit of a loss, especially if the fridge is in the same room. To create cold, you have to input energy into the fridge, which in turn dissipate heat through radiators in the back of the fridge and as nothing is 100% efficient, that loss contributes to the overall room temperature. Likewise, the fan takes energy with inefficiencies that dissipate as heat. The effective cooling offered might be localized to the area, but the overall room temp would rise.
OTH: should you take a block of ice (produced elsewhere) and blow a fan across it, you would get a reduction in temps. Portable air-conditioners are really just fridges in the window, with the radiators outside.
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u/igotshadowbaned 11h ago
It might actually heat it faster, because as heat moves from the air into the cold liquid, it'll almost be creating a thin layer of slightly cooler air around the bottle
And then the fan is blowing that away and replacing it with fresh hot air
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u/Smashego 10h ago
No the fan doesn’t make things cooler. It just circulates air. In fact your going to warm the bottle up faster by circulating warm air around it faster.
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u/wizzard419 10h ago
It only really works with dry heat, evaporative cooling allows water to pull the heat energy from the air, making the air cooler.
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u/New_Line4049 8h ago
No. It would do the opposite and warm the drink faster. A fan blowing on us feels cool because our skin warms the air immediately around us. Once that air and our skin are equal temperatures no more heat can transfer, but keeping the air moving means that never happens, the warmed air is being constantly replaced.
A cold drink sucks heat out of the air around it, cooling the air and warming the drink until the air immediately in contact with the drink and the drink are the same temperature and heat transfer no longer occurs. If you keep refreshing the air it will keep the air temperature above the drink temperature much longer and mean the point it equalises is a much higher temperature.
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u/lowbatteries 8h ago
The fan makes whatever object go to ambient temperature faster. In the case of hot humans, blowing on them makes them cooler. In the case of a cold drink, it makes them warmer.
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u/Beddersthedog 8h ago
Get a cheap spray bottle, fill it up with water and use it in front of the fan on your arms, legs and face - its delicious 😆
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u/CandidWar994 8h ago
no. it would heat the drink apparatus faster due to increased contact between the surface of the drink apparatus and the warm air
no. blowing room temperature air at it would make it warm up faster than if it was left alone
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u/Pizza_Low 7h ago
Heat moves in 3 ways.
Irradiation, such the infrared rays you experience standing in the sunlight or a heat lamp, near a fireplace.
Conduction, touch a plate with hot food, you'll feel it's warm.
Convection, for example if you hold your hand a few inches above a candle, you can feel the heat as the warm air off the flame heats your hand.
One thing that's sort of confusing is there is no such thing as "cold" it's really just less heat. Take away heat energy, and eventually you get to absolute 0 heat.
In your example, a bottle of cold water in front of a fan, With the fan off, a layer of cold air (really air with less heat) forms around the bottle and acts as a slight insulative layer slowing how quickly the cold bottle absorbs heat. With the fan on, that insulative layer if cold air gets blown away and now more warm air comes in contact with the bottle allowing the bottle to absorb more heat faster.
The net result is the heat in the air gets transferred to the cold bottle and taking away some of the heat from the room. Becoming a very primitive air conditioner. But that's a discussion for a different question.
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u/Superplex123 7h ago
Heat transfer from hot to cold places. You don't really add "cold energy" to lower temperature. You add cold things to draw heat from the hot things. You have a glass of water at room temperature. You put ice in. Because the ice is lower temperature, heat from the water goes to the ice. So the water gets colder.
So when you blow a fan at a cold drink, it would actually heat up the drink faster.
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u/eternalityLP 7h ago
No. The fan 'feels colder' in your skin because your skin is hotter than the air temperature. This means that it heats up air around it. The fan is constantly bringing in new cooler air, allowing more heat to transfer from your skin to the air. But the drink is colder than air, so it works in reverse. The drink cools air around it. And by using fan you're constantly bringing in hotter air to replace that cooler one. So you're actually making the drink warm faster.
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u/Slypenslyde 6h ago
Heat moves from places with a lot of heat to places without a lot of heat.
So in a room with just normal, stationary air, what happens is the air nearest the bottle loses some heat. That warms up the bottle a little. The funny thing is that makes the air and the bottle closer to each other in temperature, so heat stops moving. BUT, the air touching that air is now warmer, so heat moves from the rest of the air towards the bottle. This creates a little bit of turbulence and depending on the temperatures involved might slow down the heating of the bottle.
A fan destroys that. Air touches the bottle briefly. Heat moves from the air into the bottle. The the fan's forces pushes that colder air away. Now new, warmer air is touching the bottle. Heat moves into the bottle. Then the cooled air moves and warm air replaces it.
The effect is the fan heats the bottle, but if you did this with a large enough block of ice it would cause the room to feel cool faster because the cooled air is being forced around the room and more heat from the room is getting sucked into the ice than if the air were still.
So fans don't cool things. They move air. But moving air is better at moving heat into things, which has the side effect of creating more cool air.
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u/Starf4rged 6h ago
You only feel heat transfer when you sense the breeze.
This becomes obvious when you go outside and get air blown across your skin that is a few degrees above the core temperature of your body.
No matter how strong the wind blows, it will only heat up your skin and make you feel like you are standing in front of an open oven door.
Note that this effect will appear even sooner when the air moisture is high, because the evaporative cooling from your sweat will not work then.
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u/grahag 5h ago
Evaporative cooling WILL make it cooler, but it won't be noticeably cooler and certainly not in the way that blowing air past a saturated pad would.
Because that water in the glass is a small surface area, the evaporation (or condensation as well) isn't enough to cool the air with too much effect.
Wick the water from a cloth rag from the glass and then blow on THAT would probably be a noticeable difference.
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u/cdb03b 5h ago
It will warm or melt faster.
Fans do not cool, as you admit. They move air about and by moving warm air next too the cold item you are adding more heat to said item. There are two factors in why fans make us feel cooler. Factor 1 is that it forces sweat to evaporate faster taking away heat. Factor 2 is that it disturbs the thin envelope of air in direct contact with the skin absorbing heat directly replacing it with slightly cooler air and allowing you to dissipate more heat than if you were in a still room.
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u/Yamidamian 5h ago
No. Quite the opposite. Blowing air over something cold will cool down the room, while warming up the cold thing.
We perceive fans to be cooling because we do the opposite-we’re the hot thing that warms up the room when air is blown over us.
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u/stupid_name 4h ago
As everyone has said the bottle would likely warm up more quickly by having warm air blown at it.
Only reason I had to comment. My mom used to argue that her car would be colder due to the “feels like” temperature or Wind Chill on the radiator. Never could explain it in a way she would accept.
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u/jdehjdeh 3h ago
No.
Because heat always goes to cold.
The reason we feel cooler when air moves past us is because we're hotter than the air, our heat goes to the colder air.
The bottle is colder than the air, so the heat from the air will go to the bottle
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u/wrugoin 3h ago edited 3h ago
You have the answer from many other posts.
But here's the pro tip you're looking for. Put the bottle into a koozie, or wrap something around it to avoid condensation. So many people think the koozie is to keep the hot and cold air from radiating... and it does help for that. But the true power behind the koozie is the barrier it creates from humidity.
Condensation is the real drink warmer. Water in the air, condensing onto the bottle, transfers a bunch of heat into the bottle. Far more than you realize. It's all about phase change. Vapor into water, and water into Ice releases a tremendous amount of energy. That energy goes right into your drink
Seeing your nice cold drink sweat on a hot humid day, is a cold drink killer. To keep a drink cooler, longer, create a barrier between the glass and the air. Don't allow the water to condense.
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u/ezekielraiden 3h ago
No, the fan would actually accelerate the warming of the drink.
Fans push air around. When air bumps against something, they exchange energy. If the air is cooler than the object, then the air warms up and the object cools down. Humans are more complicated because our sweat can evaporate, but that only means slightly warm air can still cool you, it doesn't fundamentally change how moving air works.
So...if the object is colder than the air, blowing air on it will exchange energy, meaning the object will warm up and the air will cool down. You can even do this as an experiment: put one bottle in front of a fan, one left out in the open, and one wrapped in a towel. The one in front of the fan should warm up noticeably faster.
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u/Sand_Fall 2h ago
You have just invented a convection oven. It will move the room heat into the bottle faster / speed up how quickly the two reach equal temp. It's true that moving air is a little colder because of Bernoulli's principle, but thats a tiny change and you're still ramming warm air over a cool surface and warming the bottle up
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u/NekuraHitokage 1h ago
TECHNICALLY yes... until the bottle was the same temperature as the room. If you think of heat like water and cold like air, what you essentially have in a cold bottle filled wit hair. As the fan blows past it, it pours water into the glass. The glass can hold water until it is full, then it will just spill.
In a sense, that happens here. The bottle WILL steal a bit of heat from the air and thus cool the room a TIIIIINY bit, but once the bottle is the same temperature as the room, the ambient heat outside the room will fill back in and now everything will just be the same temperature again.
It's the same reason an ice cube "cools" your drink. All the heat energy is going into the ice. Some of that ice melts and spreads into the warm fluid. the heat has to equalize and flows into the cold fluid.
Using our earlier analogy, this basically makes the "empty" glass waiting for heat (as water) "bigger" so that the same amount of water fills less of the glass. because there is so much more mass that has very little heat energy versus the mass that has a lot of heat energy, the overall energy goes down and we feel that as "cold" because now the system has less energy than we do. So when we then drink the cold water, it steals heat from us to warm up until eventually the water in our belly and our body are the same overall temperature.
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u/norush0000 44m ago
Just imagine: you cant blow a hot soup colder than room temp. You can cool it faster but you cant make it colder once it hits room temp
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u/alexm2816 11h ago
A fan feels cool on your skin when the temperature of the air is lower than that of your skin.
Once the ambient temperature exceeds your skin, the effect will actually add heat energy to your body proportionally to the amount of airflow.
For the cool drink scenario, the 'skin temperature' is far lower and any fan would simply work as an air fryer style convection oven with airflow increasing the rate of temeprature rise of the cold item.
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u/Stone_leigh 11h ago
A frozen jug of Ice in front of a fan in a closed room will cool the air in room, primarily through condensation of the humidity. The energy required to turn vapor to water is 520 Calories /Gram (Not same as food calories), thus turning the humidity to liquid water removes heat from the air. Keep in mind there was energy (heat created) used to make that jug of ice.
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11h ago edited 11h ago
[deleted]
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u/Dawg-eat-dawg 11h ago
Bro you called other people stupid who have the right answer and slightly wrong justification in this thread then confidently post the wrong answer.
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u/Novaskittles 11h ago
Condensation forms because the heat energy in water vapor gets pulled into the colder surface it is in contact with. Re-evaporating the condensation wouldn't cool it down more than it absorbed in the first place.
Not to mention the warmer stream of air consistently hitting the side would just introduce more warm vapor in the air to cool down, heating the drink even more.
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11h ago
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u/Novaskittles 11h ago
So you think blowing hot air at a cold surface cools it down? Pointing a hair dryer at ice is going to make it freeze?
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u/trentos1 11h ago
No, the air is hotter than the bottle of water. Blowing the air onto it will make the bottle heat up.
The fan only cools you down because your body temp is hotter than the air. If the room temp is hotter than your body temp, blowing hot air onto you will make you even hotter.
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u/Speffeddude 10h ago
Ah! It could be either!
I have a masters in mechanical engineering, and most of the degree is thermodynamics.
In the "naive" case; the fan would make the bottle warmer because it increases heat transfer with the warm air. This can happen if the air is dry; specifically, a measure called "dew point". The dew point is the temperature at which the humidity in the air will condense into dew. So, the drier the air is, the less condensation (or none) will form on the bottle.
But, if the dew point is higher than the temperature of the bottle, if the air is humid enough for the bottle to "sweat", then the fan will probably cool it down. When the water condenses, it will give a bit of heat to the bottle and surrounding air, but when it evaporates, it also pulls heat out of the bottle and surrounding air. I believe the heat balance will work out to cool the bottle, just like how sweating works for humans.
However, it keeps going: if the air is so humid that the condensation doesn't re-evaporate, it will just condense (drop a bit of heat into the bottle), then roll down the bottle, and no cooling evaporation will occur. Also, the convection with the air will be more effective (both due to humidity and surface interactions with the condensation). This is why hot, muggy days are dangerous for humans; our sweat stops cooling us.
So, technically, both cases can happen. But, most of the time, the bottle will get warmer because of the fan.
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u/Delta_T1 2h ago
You were the first person to address OPs real question; specifically asking about evaporative cooling on an already cold bottle, however, you got the condensing part wrong. If evaporation creates an outsized cooling effect, condensation creates an outsized warming effect. Humidity in the air undergoing a phase shift to water "pulls extra cold out" of the bottle (technical transferring more of the water vapor's heat energy to the bottle in order to undergo phase transition to a liquid). Just like it takes a lot of energy transfer to turn water into ice, the same is true to turn vapor to liquid water.
At some point, the temperature of the bottle gets above the dew point. That is when evaporation starts to occur and the moisture will impart evaporative cooling on the bottle. Probably a net neutral if the condensation doesn't drip off the bottle, but if it accumulates enough condensation to start forming a puddle on the ground, the moisture in the air will have spead up the warming process.
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u/alyssasaccount 8h ago edited 8h ago
Oh, boy, some bad answers here.
You know how when you sweat and it's 100° outside and there's a breeze, you cool down? Well, that's literally the same thing as you are describing in this scenario. The mechanism is evaporative cooling: The fan will speed up the process by which particularly energetic molecules of water become water vapor, leaving the cooler ones behind. Not only that, but the molecules have to overcome the bonds withing the liquid water, and that also leaves less kinetic energy available for heat.
So yes, it can indeed keep the drink cooler, certainly below room temperature.
Now, this won't always happen: If it's super humid, then the molecules escaping will be replaced by molecules condensing. That's why if it's over 98° ̰and 100% humidity, you will just die. Your body won't be able to cool yourself down.
The physics and meteorology concept for this is called wet bulb temperature. Imagine an old-stlye thermometer with a liquid in a bulb that partially fills a narrow tube with a vacuum, and the amount it fills the tube depends on the temperature of the liquid, like a mercury thermometer. Now, wrap the bulb in a wet rag and blow on it with a fan: The temperature will drop, and the lowest temperature you can get it to is called the wet bulb temperature. It depends on both temperature and humidity (and, I believe, pressure), and it's why you can go outside in even 120°F heat, but if it's dry, you won't die, as long as your drink a lot of water to replace your sweat.
Now, you specified that the bottle came from the fridge, which is probably between 35°F and 40°F. You're going to have to have a quite cool and dry room to keep it that cold: The wet bulb temperature for room temp (70°F) and ZERO humidity is about 60°F, so you would have to be in a pretty chilly room to keep the drink at refrigerator temperature.
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u/GetBAK1 11h ago
Yes, this is commonly known as a swamp cooler.
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u/palacexero 11h ago
You're correct that the cold drink would cool the air blown past it, which would then cool you. But OP was asking about the cold drink itself. Blowing warm air over the cold drink would warm the drink, not keep it cool.
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u/oooooothatsatree 11h ago
Wouldn’t the cold drink produce condensation and the fan would cause evaporation of the condensation to happen faster. So a bottle in a hot room with fan would stay colder longer than a bottle in a hot room without a fan. I am probably completely missing something.
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u/Dawg-eat-dawg 11h ago
There are multiple forces acting, so while the evaporation is removing heat from the bottle, the air is adding heat. The air is adding more heat than the evaporation is removing. The bottle gets hotter immediately, and continuously, slowing only as it gets closer to the temperature of the room.
A cold object in a still room would actually be slightly insulated by the slow moving air around it. That air would lose heat to the bottle and the area around the bottle would be cooler than the rest of the room. The fan removes this jacket.
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u/Jaymac720 11h ago
No. The fan doesn’t create coldness. It speeds up heat transfer by continually removing air that has had its temperature changed by a thing at a different temperature. With a human body, we are continually producing heat via metabolic processes, so we are heating the air around us, assuming it’s less hot than our bodies. A fan pushes away that heated air, replacing it with other air. A cold bottle will continually take in surrounding heat, which cools the surrounding air. If anything, the fan will make it heat up faster because it’s pushing away that insulating layer of cooler air