Sign conventions for heat and work (contd.)

  • The first is still the most common in the English-speaking world and probably dates back to the introduction of steam engines. In that era, engineers were particularly interested in heat engines for pumping water or driving machinery. The desired output was work and the required input was heat. It made sense to describe both as positive quantities.
  • The second sign convention, which I favour, is the most common in the non-English-speaking world. According to this convention heat and work are both taken as positive when their direction is into the system of interest.
  • In my opinion the second convention is the most consistent, as all transfers or interactions at the boundary can be treated in the same way: they are taken as positive when inwards and negative when outwards. We will use the second convention.