Not 100% sure, but the water around the cup will never get above 212F (boiling point of water). Maybe they are saying that the water in the cup can shed it's heat out the top of the cup faster than heat from the boiling water can enter through the non-zero resistance of the walls of the cup?
Not sure if thats true but as soon as water reaches the boiling point(depending on airpressure) at 100°C, it wont become hotter. it will stay at 100°C. The cup keeps the temperature of the water in it always slightly below 100°C.
>>7761707 /thread >>7761743 No, he didn't. Let me try to explain it. First, consider these three facts; 1) water boils when it reaches 100° C and then you add additional heat to it 2) while it's boiling it keeps the 100° C temperature 3) between two bodies at different temperatures in contact there is a heat transfer from the warmer body to the colder body 4) the heat transfer rate is proportional to the temperature difference (and other factors that are of no interest to us now) Knowing this, the water in the cup will receive heat from the surrounding water and its temperature will become asymptotically close to 100° C while never reaching it. Even if we wait an infinite amount of time for the water in the cup to actually reach 100° C, it won't start boiling since at that point it would need other heat for the phase transition, but being in thermodynamic equilibrium with the surrounding water no heat exchange happens.
Of course, this is all in theory, in reality there would be interactions with steam bubbles and the surrounding air that could be well over 100° C in temperature.
>>7761695 Depends on how insulative the cup is and the real temperature of the hot plate, the temperature profile from the plate to the cup could have a cup water temperature below boiling if the heat transfer coefficient is low enough.
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