Vacuum between the inner and outer walls. Meaning, your coffee can’t cool down via conduction(like when you touch something hot) or convection(like when the blow dryer heats up your hair). Only method of heat transfer left is radiation , and your coffee is not the temp of a neutron star, so the coffee’s radiation and therefore it’s overall heat transfer, happen much more slowly. This keep the coffee hot.
Heat transfer is my jam.
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