[Encyclios]

Energy transfers: heat and work

The energy that, during any transformation, crosses the surfaces of the system is given the name of heat or work: we speak of heat when the energy is transferred as a consequence of a temperature difference existing between the system and the environment; otherwise, if the energy flow does not derive from a temperature difference, this is called work.

According to these last definitions, it is possible to talk about heat and work only in presence of an energy flow between the system and the environment; it doesn’t make sense to talk about heat and work for a system that is in a defined thermodynamic state (i.e. are wrong expressions like heat of a system or work of a system). In other words, heat and work are not state properties of the system.

A type of work that is often encountered is related to the displacement of one or more walls of the system as a result of an alteration of the mechanical equilibrium: in this case we speak of (mechanical) work of volume variation and it is obviously equal to the work done by the external forces acting on the walls that move. Let’s take a concrete example: we consider a system consisting of a fluid contained in a cylinder on which a piston is weighted; initially, we assume that the fluid pressure balances the pressure of the external forces (weight force of the piston), so the system is in thermodynamic equilibrium. Suppose now to increase the external weight force by adding weights on the piston: the initial equilibrium undergoes a perturbation and the system undergoes a transformation that brings it in a new state of equilibrium.

During the transformation, there is energy that is transferred from the environment to the system: this energy, assuming no friction, corresponds to the work of the external forces (since there is no difference in temperature, the energy can only be work according to the definitions given above), work that produces a decrease in fluid volume. If, on the contrary, there had been an increase in fluid volume, the work of volume change would have involved a transfer of energy from the system to the environment.

Another important type of work is the one related to the rotation of one or more walls of the system, due to an alteration of the mechanical equilibrium: this type of work is called mechanical work of propeller; it is obtained when, for example, inside a cylinder containing fluid there is a rotating propeller. During the transformation, due to friction between the rotating surfaces and the fluid, there is energy that is transmitted from the environment to the system: this energy, in the absence of mechanical friction, corresponds exactly to the energy used to move the propeller.

An important thing to note is that the propeller work, for a closed system, can involve energy transfer in only one direction, that is from the environment to the system, and never vice versa (it is good to remember that this is valid only for a closed system as we will see that there are special open systems – such as turbines – in which a fluid in motion is used to make the propellers rotate and therefore produce mechanical energy to be converted, subsequently, in other forms of energy). The two examples just examined (work of volume variation and work of propeller) therefore involve an alteration of the mechanical equilibrium of the system. In other cases, the balance that is altered can be, for example, electrical or magnetic: in these cases, in the presence of energy transfer between the environment and the system, we will speak, respectively, of electrical work or magnetic work. An example of electric work is a system consisting of a container containing fluid and a resistance immersed inside, connected with an external electric circuit. When the electric circuit is activated, the current flows through the resistor, which by heating alters the electrical balance, transferring energy from the environment to the system: in fact the resistor heats up, by Joule effect, and gives heat to the fluid (by convection).

It should be noted that, if we consider only the fluid as the system of interest, we cannot speak of work but of heat, because the energy is transmitted, in this case, by temperature difference between the resistor (which now is the environment) and the system and not by electric potential difference. Another observation concerns the fact that the energy transfer, also in this example, can never happen from the system to the environment but only from the environment to the system, since the system contains only purely passive elements. Different would be the case if the system includes a capacitor, which can receive energy when charging and give it when discharging.