where Q represents heat, in denotes input to an object, out for output from an object, and h for the hot thermal reservoir. If heat Q h out flows from the hot reservoir then it has the sign of + while if Q h in flows from the hot reservoir then it has the sign of -. This expression can be easily derived by using the definition of the efficiency of a heat engine, η η = W / Q h in , where work and heat in this expression are net quantities per engine cycle, and the conservation of energy for each engine as shown below. The sign convention of work W , with which the sign of + for work done by an engine to its surroundings, is employed.