Why do some things that can get very hot cool much faster than others?
September 23, 2015 8:29 AM Subscribe
It makes sense that things being very hot take a long time to cool off, it makes sense that some materials don't get too hot and others hold heat for a very long time, but how is it possible that some material can both get extremely hot while also cooling off very quickly?
I'm sure this is basic for someone with a physics background but googling around I couldn't find something that really got to the essence of what I'm thinking of.
On my recent European trip, several showers of apartments we stayed in had no place to put soap and had exposed partial pipes near the faucets. The hot water pipe was EXTREMELY hot (as in, you could burn yourself). Anyway, I got used to putting my soap on top of the pipes. I noticed that when I took the soap off the pipe after a couple minutes, it was EXTREMELY hot -- barely touchable -- but returned to normal temperature almost instantly, within about 1 second. I don't think I've ever experienced something like that and it struck me that I had no idea what caused it.
I'm sure this is basic for someone with a physics background but googling around I couldn't find something that really got to the essence of what I'm thinking of.
On my recent European trip, several showers of apartments we stayed in had no place to put soap and had exposed partial pipes near the faucets. The hot water pipe was EXTREMELY hot (as in, you could burn yourself). Anyway, I got used to putting my soap on top of the pipes. I noticed that when I took the soap off the pipe after a couple minutes, it was EXTREMELY hot -- barely touchable -- but returned to normal temperature almost instantly, within about 1 second. I don't think I've ever experienced something like that and it struck me that I had no idea what caused it.
"Specific heat" is the phrase to look up here, along with the difference between heat and temperature. What's actually being transferred is heat, not temperature--some substances require less heat energy to increase their temperature. (Conduction of heat within the substance/object can also make a difference--if heat can spread out, the object won't be so hot in one particular place.)
posted by cogitron at 8:39 AM on September 23, 2015
posted by cogitron at 8:39 AM on September 23, 2015
how is it possible that some material can both get extremely hot while also cooling off very quickly?
Some things get heated up better than others. The easier it is to heat something up, the easier it is to cool down. For example, water takes a long time to heat up, and then it takes a long time to cool down. On the other hand, copper heats up and cools down very quickly. As noted by other answers, this is because of the varying heat capacity between the various substances.
posted by Tanizaki at 8:41 AM on September 23, 2015 [2 favorites]
Some things get heated up better than others. The easier it is to heat something up, the easier it is to cool down. For example, water takes a long time to heat up, and then it takes a long time to cool down. On the other hand, copper heats up and cools down very quickly. As noted by other answers, this is because of the varying heat capacity between the various substances.
posted by Tanizaki at 8:41 AM on September 23, 2015 [2 favorites]
Best answer: In case of soap, I'd guess it's more because of its low heat conductivity - its specific heat is relatively high at 0.58 kcal/kg, in comparison with other solids, higher than bricks or stone.
The heat you feel is the thermal energy that passes from soap to your hand. The places you touch cool quickly and since soap is a poor heat conductor, the inside heats them back slowly. Low thermal conductivity combined with low specific heat allowed one to safely hold a piece of Space Shuttle insulation heated to 2200 F in a bare hand, as explained (and shown) here.
posted by hat_eater at 9:04 AM on September 23, 2015 [8 favorites]
The heat you feel is the thermal energy that passes from soap to your hand. The places you touch cool quickly and since soap is a poor heat conductor, the inside heats them back slowly. Low thermal conductivity combined with low specific heat allowed one to safely hold a piece of Space Shuttle insulation heated to 2200 F in a bare hand, as explained (and shown) here.
posted by hat_eater at 9:04 AM on September 23, 2015 [8 favorites]
there are two different things, which are not the same... one is energy, and the other is temperature.
when you place something over a flame, say, or pass electricity through a resistor (like in a kettle's heating element) you are putting (thermal) energy into the thing over the flame, or the water in the kettle.
as a general rule, the temperature of something increases when you put (thermal) energy into it. so the water in the kettle gets hotter, for example.
but temperature doesn't change as fast for everything. obviously, the more stuff you have, the slower temperature goes up. so it takes longer to boil a full kettle than a half empty one (twice as long, in fact). but also, different substances change differently. so water heats up faster than oil, for example.
the measurement of how much the temperature goes up, for a given weight and amount of (thermal) energy is the specific heat capacity. so something with a low specific heat capacity gets hot quickly (with little energy needed), while something with a high specific heat capacity takes longer.
and the reverse happens on cooling. if you have two things at the same temperature, but one has a higher heat capacity, then the one with the higher heat capacity took longer (or more energy) to get that hot. in other words, it has more energy "stored up" inside. and so it cools more slowly (because cooling is typically losing energy at a certain rate - so the one with more energy "goes down" more slowly).
as to why things have different heat capacities. well, it's to do with how they can store energy internally. simple things can only store (thermal) energy by vibrating more. but more complex materials can do other things like rotating (we're talking about at the molecular level here). if a material has more ways of storing energy then it has a higher heat capacity - it can take in more energy without increasing its temperature so much.
posted by andrewcooke at 9:10 AM on September 23, 2015
when you place something over a flame, say, or pass electricity through a resistor (like in a kettle's heating element) you are putting (thermal) energy into the thing over the flame, or the water in the kettle.
as a general rule, the temperature of something increases when you put (thermal) energy into it. so the water in the kettle gets hotter, for example.
but temperature doesn't change as fast for everything. obviously, the more stuff you have, the slower temperature goes up. so it takes longer to boil a full kettle than a half empty one (twice as long, in fact). but also, different substances change differently. so water heats up faster than oil, for example.
the measurement of how much the temperature goes up, for a given weight and amount of (thermal) energy is the specific heat capacity. so something with a low specific heat capacity gets hot quickly (with little energy needed), while something with a high specific heat capacity takes longer.
and the reverse happens on cooling. if you have two things at the same temperature, but one has a higher heat capacity, then the one with the higher heat capacity took longer (or more energy) to get that hot. in other words, it has more energy "stored up" inside. and so it cools more slowly (because cooling is typically losing energy at a certain rate - so the one with more energy "goes down" more slowly).
as to why things have different heat capacities. well, it's to do with how they can store energy internally. simple things can only store (thermal) energy by vibrating more. but more complex materials can do other things like rotating (we're talking about at the molecular level here). if a material has more ways of storing energy then it has a higher heat capacity - it can take in more energy without increasing its temperature so much.
posted by andrewcooke at 9:10 AM on September 23, 2015
Response by poster: The 2200F space shuttle tiles being touchable almost instantly after removal from the oven is what I was getting at.. A more extreme example.. Amazing!
posted by basehead at 9:15 AM on September 23, 2015
posted by basehead at 9:15 AM on September 23, 2015
Aluminum foil illustrates another example. Foil can be hot from the oven, but safe to touch because it is so light per sq inch. With only a little mass, it can't hold much heat, and the small amount it does hold is not enough to heat your skin to a dangerous temperature.
posted by SemiSalt at 3:50 PM on September 23, 2015 [1 favorite]
posted by SemiSalt at 3:50 PM on September 23, 2015 [1 favorite]
Specific heat and heat capacity are both referenced in the responses, often used interchangeably. Plain old heat capacity, more than specific heat capacity, is likely the dominant factor in soap. Specific heat is the material-dependent coefficient multiplying the weight of a substance to get its heat capacity, aka heat capacity per unit mass. Soap's specific heat is quite high; approximately half that of water. I'd suspect primarily the outermost layer is heated significantly by your shower, and thus mostly heat from the outermost part of the soap has to exit. This makes the mass component of the heat capacity small, and thus the soap has little thermal energy to transfer despite high temperature.
The key to the puzzle involves thermal conductivity. It was previously stated by hat_eater that low thermal conductivity played a significant role by not allowing stored heat to be transferred to the outside edges quickly after they cooled off. I would add that low thermal conductivity also plays the role of preventing heating of the interior in the first place. This is the reasoning behind my assumption in the first paragraph that the relevant heated mass, and thus heat capacity, would be small. It could just as easily be said that the relevant heat capacity was small because only the exterior will transfer its energy, while the interior takes too long to get its energy to the outside. An easy test of which of these phenomena is dominant is to keep the soap under the shower longer, and see if this increases the length of time it takes the soap to cool. Regardless, your soap cools quickly because it does not have much thermal energy to give, as it can only transmit heat from its surface.
posted by Foibuls at 2:44 AM on September 24, 2015
The key to the puzzle involves thermal conductivity. It was previously stated by hat_eater that low thermal conductivity played a significant role by not allowing stored heat to be transferred to the outside edges quickly after they cooled off. I would add that low thermal conductivity also plays the role of preventing heating of the interior in the first place. This is the reasoning behind my assumption in the first paragraph that the relevant heated mass, and thus heat capacity, would be small. It could just as easily be said that the relevant heat capacity was small because only the exterior will transfer its energy, while the interior takes too long to get its energy to the outside. An easy test of which of these phenomena is dominant is to keep the soap under the shower longer, and see if this increases the length of time it takes the soap to cool. Regardless, your soap cools quickly because it does not have much thermal energy to give, as it can only transmit heat from its surface.
posted by Foibuls at 2:44 AM on September 24, 2015
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posted by Comrade_robot at 8:38 AM on September 23, 2015 [1 favorite]