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. 🙂
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
I can’t understand how my flask
knows when to keep something hot and when to keep something cold. I can’t find a switch on it anywhere.
Heat leaks out the least insulated portion… the top. A captain and coke in a yeti rambler will still have ice in it in the morning with no lid.
All the more technical answers are totally true and good, but I would like to offer a simpler to understand alternative:
Heat = energy. There is no such thing as cold energy that makes something cold, there is simply the absence of energy. So, when you have a hot drink you have a cup full of energy that radiates outward to its surroundings where there is less energy. The vacuum seal of the thermos attempts to keep it all locked in, but energy radiates, man. It’s what it does.
On the other hand, if you have a cold drink in a thermos you have a low-energy zone in your hand, which radiates nothing (or comparatively little). Instead, it is the outside environment that has the energy and wants to get in, but the energy difference isn’t that big so the vacuum is more effective at keeping the low-energy zone from the higher-energy environment.
As many have already mentioned, heat transfer rates are directly tied to delta t, but there’s also another phenomenon that helps, the elusive and unexplainable mpemba effect. Hot water freezes faster than cold water. Sounds like it breaks physics to me, but damn if it hasn’t actually been proven thousands of times in cold climates
Cold or hot doesnt exists, its relative to another measure. So its all about temperature gradient. If its winter, it can stay cold for a month. So cold is like 5C in a 25C environment where hot is 100C in the same environment. Gradient is larger, so its cooling faster. The larger the gradient, the faster the exchange.
Think of heat as energy. Hot has a lot of energy, that energy wants to escape, whereas with a cold drink you have very little energy, and energy has to come in for it to heat up
How does it know the difference?
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