Eli5: Why can a thermal flask keep items cold for 24 hours, but only hot for 12 hours.
In: Physics
I fill my new thermal flask up with water before bed, as I’m filling it up I remember little info graphic that was on the packaging about the 24/12 hour thing and I start thinking about it for a while as I get ready for bed and come to the loose conclusion that it must be something to do with hot things being really hot and cold things being slightly cool… then something else fills my mind and the chances I’ll look it up dwindle towards zero. I get into bed and do my usual browse through Reddit, and I stumble upon this question and the correct answer. What a universe.
The hot beverages give off more heat and steam that’s lost at the top. The coldest parts of the beverage stay at the bottom.
There are two factors here.
1. The temperature difference between hot drinks and the outside could be lower than the temperature difference between a cold drink and the outside.
2. Cold drinks often have ice in them. Ice functions as a ‘cold reservoir’. As long as there is a little ice, the drink will stay close to the freezing point of water. When combined with thermal insulation, ice is exceptionally good at keeping your drink the perfect temperature.
Hot things not close to how hot it is outside bottle. Cold things closer to how cold it is outside bottle.
Thermodynamics & environmental conditions.
Heat flows from hot to cold.
21°C (room temp) is closer to 0°C (cold items) than 70-80°C (hot items) which slows down the rate of energy loss.
Because hot items are twice as hot as cold items are cold.
There’s too much jargon in most of these responses for ELI5.
How does it know the difference?
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
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.
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
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