Eli5: Why can a thermal flask keep items cold for 24 hours, but only hot for 12 hours.
In: Physics
The difference between cold (0c) and not cold (25c) smaller than the different between hot (80c) and not hot (25c).
Which is why it’s harder to keep hot drinks hot compared to cold drinks cold.
TIL physicists have a particularly hard time at Eli5. Impossible to resist throwing in an equation or two
The science behind this is called thermodynamics.
When things have high energy they contain high heat which makes them hot. When they have low energy they are cold.
Energy like this always wants to flow from hot to cold. From high energy to low energy.
When something is already cold in the flask, for it to get warm, energy must flow from the flask into the cold item inside the flask. Flasks insulate from the outside so the heat flows very slowly in that direction.
When something is hot inside the flask, the energy flows must faster trying to leave the flask since the energy difference is greater.
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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