Cooling Coffee

How do you get your coffee cool enough to drink in the shortest possible time?

Leave it black for a while, then add cream.

The heat loss rate by conduction, convection, and radiation all depend upon the difference in temperature between the hot object and its surroundings. Therefore the cooling rate is greater before you add cream. Also, the rate of evaporation from the coffee is greater if you leave it hot for a time, and evaporation is a potent cooling mechanism since the evaporating water must extract its large heat of vaporization from the remaining liquid. All these mechanisms cool the coffee faster before you add cream.

An often cited but bogus reasoning in this example is to say that black objects radiate better than white ones. This is true for visible light, but this radiation is in the infrared where the visible color is not expected to be a significant factor.
Heat Questions

Heat concepts

Heat transfer concepts
HyperPhysics***** Thermodynamics R Nave
Go Back