Temperature, Heat and Work

When you touch a hot object, you feel a sensation related to its temperature. But what really is temperature? It is a macroscopic quantity that reflects the microscopic agitation of particles: the faster the molecules jiggle, the higher the temperature. It is measured in kelvins (K) for physics calculations, with the relation T(K) = T(°C) + 273.15.

Heat, denoted Q, is a transfer of energy that occurs spontaneously between two bodies at different temperatures, always from the hot body to the cold body, never the reverse without outside intervention. This transfer stops when thermal equilibrium is reached, that is, when the two temperatures become equal.

Work, denoted W, is another mode of energy transfer, but an organized one: it corresponds to a mechanical action, such as the compression of a gas by a piston. Unlike heat, work does not depend on a temperature difference; it comes from a force that moves an object or changes a volume.

A common confusion is to believe that heat is a substance contained in a body. In reality, neither Q nor W is a state function: we cannot say that a system "contains" heat or work, only that it has exchanged some during a transformation. What the system possesses is internal energy, a notion you will explore in more depth in the next lesson.

Remember the spontaneous direction of heat exchanges: with no machine and no outside input, energy always flows from the hot reservoir to the cold reservoir, as shown in the diagram below. This preferred direction is the first trace of what will later become the second law of thermodynamics.

 (hot reservoir, Tc)             (cold reservoir, Tf)
      +--------+                     +--------+
      |   Tc   |          Q          |   Tf   |
      |        | ------------------> |        |
      +--------+       (heat)        +--------+
                     Tc greater than Tf

 Spontaneous direction: always from Tc to Tf (never the reverse on its own)