Eli5: Why can a thermal flask keep items cold for 24 hours, but only hot for 12 hours.
In: Physics
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. 🙂
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