Radiation in the coffee cup

It is commonly assumed that radiative heat transfer only plays a relevant role in situations involving temperatures of several hundred degrees Celsius and above. Other common misconceptions relate to the cooling of metallic vs non-metallic objects, and the effect of their colour. A series of alternative demonstrations that counteract these beliefs and show the role played by radiation in the cooling of common tableware containing hot liquids are described here. The experiments require basic equipment, easily available even at home. The cooling curves obtained can be modelled with Newton's law of cooling and the corresponding heat transfer coefficients estimated. The results elucidate the influence of surface emissivity on radiative heat transfer, and that this mechanism has a non-minor share on cooling processes even at temperatures below the normal boiling point of water. A final demonstration concerns heating of the same systems and corroborates the previous conclusions.