Gas operation under isobaric, isothermal and adiabatic processes

At the heart of almost any thermal machine's operation is a thermodynamic phenomenon, such as the work done by the gas during expansion or compression. Here it is worth remembering that in physics, a quantitative measure is defined as work, characterizing the action of a certain force on the body. In accordance with this, the work of gas, a necessary condition for the fulfillment of which is a change in its volume, is nothing more than the product of pressure on this change in volume.

The work of a gas with a change in its volume can be both isobaric and isothermal. In addition, the actual expansion process can be of an arbitrary nature. The work of gas, which takes place under isobar expansion, can be found by the following formula:

A = pΔV,

In which p is the quantitative characteristic of the gas pressure, and ΔV is the difference between the initial and final volumes.

The process of arbitrary gas expansion in physics is usually represented as a sequence of individual isobaric and isochoric processes. The latter are characterized by the fact that the work of the gas, like its quantitative indicators, is zero, because the piston does not move in the cylinder. Under such conditions, it turns out that the operation of the gas in an arbitrary process will change in direct proportion to the increase in the volume of the vessel in which the piston moves.

If we compare the work done by the gas during expansion and compression, then it can be noted that when expanding the direction of the displacement vector of the piston coincides with the vector of the pressure force of this gas itself, therefore in scalar calculus the work of the gas is positive, and the external force is negative. When the gas is compressed with the general direction of the displacement of the cylinder, the vector of external forces coincides, and therefore their work is positive, and the work of the gas is negative.

Consideration of the concept of "work done by gas" will be incomplete, if not to affect also the adiabatic processes. This phenomenon in thermodynamics is understood as a process where there is no heat exchange with any external bodies. This is possible, for example, in the case when the vessel with the working piston is provided with good thermal insulation. In addition, the processes of compression or expansion of the gas can be equated to adiabatic if the time of the change in the volume of the gas is much less than the time interval during which the thermal equilibrium between the surrounding bodies and the gas occurs.

The most frequent adiabatic process in everyday life can be considered the work of a piston in an internal combustion engine. The essence of this process is as follows: as is known from the first law of thermodynamics, the change in the internal energy of the gas will be in quantitative terms equal to the work of forces directed from the outside. This work in its direction is positive, and therefore the internal energy of the gas will increase, and its temperature will rise. Under such initial conditions, it is clear that under adiabatic expansion, the gas will work by reducing its internal energy, respectively, the temperature within this process will decrease.