Eli5: Why can a thermal flask keep items cold for 24 hours, but only hot for 12 hours.
In: Physics
Because hot items are twice as hot as cold items are cold.
There’s too much jargon in most of these responses for ELI5.
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.
Hot things not close to how hot it is outside bottle. Cold things closer to how cold it is outside bottle.
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.
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.
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.
Because the temperature difference between room temperature (20C) and a cold drink (~5C) is less than the temperature difference between room temperature and a hot drink (~60C)
When talking about a vacuum sealed tumbler, you also have to factor in the heat loss from heat rising through the hole in the lid, where as the cold wants to stay in due to the cold air being heavier..
i’d guess “cold” is closer to ambient temperature than “hot”. example:
cold = 5°C
ambient = 25°C
hot = 70°C
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.
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