Eli5: Why can a thermal flask keep items cold for 24 hours, but only hot for 12 hours.
In: Physics
A factor that doesn’t seem to have been mentioned is the latent heat of fusion of ice, which is important if your “cold” flask is fully/partially frozen.
It takes about 4 Joules to heat 1 gram of water one degree Celsius, but about 334 Joules to melt 1 gram of ice.
Randall Munroe’s answer on his What If? blog to “Would a toaster still work in a freezer?” is a good read, and has some relevant info: [https://what-if.xkcd.com/155/](https://what-if.xkcd.com/155/)
Me no sciency, but I’m guessing that the difference between room temp and 0 Celsius (frozen) is only like 20-25 degrees, where as boiling temp is 100 degrees (~75 degrees away from room temp).
Also, anything under room temp general feels ‘cold’ where as a few degrees under boiling things start to go from hot to ‘warm’ fast.
I wondered that as well recently when I bought a thermal flask. My (semi-educated) guess would be that, in this context, what we consider “hot” (ca. 80-90 °C) is generally further away from room temperature (ca. 25 °C) than what we consider “cold” (ca. 4-10 °C).
Thermal flasks slow down the rate of equilibration between the flask contents and the environment (in either direction) – hot things lose heat to the environment, cold things gain heat from the environment – and I’d assume the rate of that heat transfer is fairly independent of the direction but depends mainly on the temperature difference between the two objects. That is, the “hot” contents will drop to a temperature we no longer consider sufficiently hot (let’s say 50-60 °C) faster than the “cold” contents will heat up to a temperature we no longer consider cold (let’s say 15 °C).
As such, I think it’s more of an issue of definitions than anything else. I suspect that if you had a hot object 10 °C hotter than the surroundings and a cold object 10 °C colder than the surroundings, the time taken to reach the same temperature would be roughly the same (the comparison here could be a bit out – it might depend on the ratio of the two rather than the absolute temperature difference, not sure).
Edit: took so long writing this in bed that others have already answered to say some of the same stuff. 🙂
Hot is farther from room temperature than cold is. Typical cold beverage might be 35 which is 35 degrees below a room temp of 70. By contrast, a typical hot coffee might be 180, which is 110 degrees above room temp.
The bigger the difference in temp between inside and outside the thermos, the faster the temp will drop or rise.
The rate of temperature exchange is proportionate to the gradient. A “Cold” drink on a hot summer day is going to be around 5° when ambient temperatures are 25-30°. A “Hot” drink could be as hot as 100° if it’s freshly brewed tea. So there’s a bigger temperature gradient and thus faster temperature exchange.
Edit: before you whine about “my five year old doesn’t know those words,” save your time and mine and just read one of the 50 such replies already posted
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