Can a Thermos Truly Keep Water Hot Forever?

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Boiling water can remain hot in a thermos for 3-4 days, but the idea of a thermos that prevents all heat escape raises questions about thermodynamics. According to the first law of thermodynamics, in a perfectly insulated system with rigid walls, temperature would remain constant. However, perfect insulation does not exist in reality, making it impossible to create a thermos that completely prevents heat loss. Heat transfer occurs through conduction, convection, and radiation, and while devices like the Devar flask minimize two of these methods, they cannot eliminate all heat transfer. Thus, even in an ideal scenario, water would eventually cool down over time.
Edward Wij
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If you keep boling water in thermos It remains hot up to 3-4 days. Let's say you can create a thermo where no heat can escape outside. Does the water remain hot for decades? Or does it get cold even if heat does not escape? And why? What is the corresponding laws of thermodynamics about thermal energy can't be created nor destroyed. And are there no such thermos where no heat can escape?
 
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In an idealized case where the walls of the container are rigid (so no work can be done) and perfect insulators, then the first law of thermodynamics tells us that the temperature would not change. Unfortunately, perfectly adiabatic systems do not exist as far as I know and are useful as simplifying assumptions to do calculations.
 
In other words: there is no such thing as a perfect insulator. Heat can be transferred in three ways, you can't block them all. In Devar flask two of them are minimalized.
 

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