Energy exchange diagram

As with mechanics problems, every thermodynamics problem can be represented with a bubble diagram: one bubble for each type of energy, with the initial (state A) and final (state B) values written inside, and arrows indicating the transfers.

Bubble = type of energy. Arrow = transfer.

In the thermodynamic case the typical bubbles are: the internal energy of the gas $E_\text{int,gas} = \frac{f}{2}nRT$, the thermal energy of any second body or thermostat $E_\text{term}$, the energy of the atmosphere $E_\text{atm}$ and the mechanical energy $E_\text{mecc}$ of weights or machines. The arrows are the three exchanges of the first law: heat $Q$ to a thermostat, work $L_\text{gas}$ to mechanical parts, work $L_\text{atm}$ to the atmosphere.

Exchange diagram for a thermodynamics problem. The gas exchanges heat $Q$ with the thermostat, work $L_\text{gas}$ with mechanical parts (weights, machines) and work $L_\text{atm}$ with the atmosphere. The total energy of the system is conserved.

The advantage of the diagram is the same as in mechanics: it makes visible at a glance which exchanges are present and which are zero. In the three typical problems that follow, the strategy is always to zero out the arrows that are absent (for example $L_\text{gas}=0$ at constant volume, $Q=0$ in an insulated container) and keep only the relevant terms.

Connections

Topics: Termodinamica Concepts: Primo principio della termodinamica · Energia interna Skills: Bilancio energetico a bolle Methods: Diagramma a bolle di scambio energetico Objects: Gas ideale

Related exercises: Nitrogen heated in a rigid cylinder · Heating under the piston · Heat as a substance?