Work in a thermodynamic system

Work in a thermodynamic system

In everyday life, when we think of “work”, we might imagine physical efforts, projects or activities. However, in a thermodynamic context , “ work ” takes on a particular meaning. It represents a means by which energy is transferred between a system and its environment , often leading to observable changes that can manifest in various forms, such as motion or temperature changes.

Thermodynamic work can be thought of as an energy bridge. Imagine compressing a spring: the effort you exert is converted into potential energy in the spring. Similarly, when a gas expands in a cylinder by pushing a piston, the gas does “work” on the piston, transferring energy in the form of motion. These transformations of energy, from one form to another, are at the heart of the notion of work in thermodynamics.

Applications of this concept are everywhere around us. When we pump air into a bicycle, when an internal combustion engine moves an automobile, or when a steam engine powers a generator, we are experiencing, in various forms, thermodynamic work in action. Let’s find out more about thermodynamic work together !

Thermodynamic work in an isobaric transformation

 

 

To understand how work varies in an isobaric transformation, let’s take our cylinder once again (containing perfect gas, heated by a flame placed underneath, and with a piston that closes it).

We slowly heat the gas contained in the cylinder so that it expands at constant pressure and let the volume of the gas increase in a quasi-static manner. As the piston rises, the system does positive work.

Just think: you could use this mechanism to lift an object via a pulley.

From a quantitative point of view, the work W that the system performs is equal to the product of the force , which pushes the piston upwards, and the displacement of the piston: W = F h.From this consideration, it can be deduced that, in any transformation, the work is equal to the area of ​​the rectangle between the volume axis and the graph of the transformation in question.

Thermodynamic work in an isochoric transformation

 

 

Now that you know the characteristics of an isochoric transformation , calculating the work in a transformation of this type will be very easy. In fact, since the work is equal to the area underneath the transformation graph , and since the graph in question is a vertical segment , it is intuitive that in isochore transformations :

Thermodynamic work in a cyclic transformation

Expansion

Compression

Cyclic transformation

When a gas expands, during an expansion, the volume changes Δ�is positive, therefore the work it’s positive. During compression, however, the volume changes Δ�is negative, and consequently, work it is negative .

During a cyclic transformation of the type shown in the image, there is an expansion and a compression phase. During the expansion phase the system performs positive work equal to the area of ​​the part of the plane shaded in red. During the compression phase the system performs negative work, the absolute value of which is given by the gray shaded area . The total work done is equal to the algebraic sum of the two works.

Work in thermodynamics is not a state function

Example – Transformation 1

Example – Transformation 2

 

Let us consider two quasistatic transformations that cause a system to pass from the same initial state TO  the same final state following two different paths.

The work 1accomplished in the first transformation is different from work TOaccomplished in the second. Therefore the work done in a transformation does not only depend on the initial and final states , but also on the particular transformation followed in passing fromTOto.

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