In my previous post [Traffic hold-ups, 13th March 2013] the application of Kirchhoff’s Law to the flow of electrons, water and traffic was discussed. In this context, electrical current or electrons were conceived as flowing. Instead, electrical current can be considered as electrical energy being transferred across a potential difference, or voltage. When this terminology is used, then it is only a short step to extend the use of Kirchhoff’s law to consider the combined effect of multiple resistance to other forms of energy transfer, such as heat transfer. Heat transfer occurs across a temperature difference, from hot to cold, and some materials offer more resistance than others, e.g. wood compared to glass. Kirchhoff’s law can be used to calculate the total resistance to heat transfer of complex structure such as a house wall that some components in series, e.g. layers of brick, insulation and plasterboard, and some in parallel, e.g. doors and windows. This information is important in designing a house to achieve minimum energy consumption and to specify the heating and cooling systems required. Note that the inverse form of Kirchhoff’s Law means that the low resistance to heat transfer of a door or window dominates the heat transfer characteristics of a well-insulated structure. Of course, the extreme case is when you leave the door open and on a cold day someone shouts at you: ‘Were you born in a barn?’.