The activity of almost any heat engine is based on such a thermodynamic phenomenon as the work done by a gas during expansion or compression. Here it is worth remembering that in physics, work is understood as a quantitative measure that characterizes the action of a certain force on a body. In accordance with this, the work of a gas, the necessary condition for which is a change in its volume, is nothing more than the product of pressure and this change in volume.
The work of a gas with a change in its volume can be both isobaric and isothermal. In addition, the expansion process itself can also be arbitrary. The work done by a gas during isobaric expansion can be found using the following formula:
A=pΔV, in which p is a quantitative characteristic of gas pressure, and ΔV is the difference between the initial and final volume.
The process of arbitrary gas expansion in physics is usually represented as a sequence of separate isobaric and isochoric processes. The latter are characterized by the fact that the work of the gas, as well as its quantitative indicators, is equal to zero, because the piston does not move in the cylinder. Atunder such conditions, it turns out that the work 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 a gas during expansion and compression, then it can be noted that during expansion, the direction of the piston displacement vector coincides with the vector of the pressure force of this gas itself, therefore, in scalar calculus, the work of the gas is positive, and external forces are negative. When gas is compressed, the vector of external forces already coincides with the general direction of movement of the cylinder, so their work is positive, and the work of the gas is negative.
Consideration of the concept of "work done by a gas" will be incomplete if we do not also touch upon adiabatic processes. In thermodynamics, such a phenomenon is understood as a process when there is no heat exchange with any external bodies.
This is possible, for example, in the case when a vessel with a working piston is provided with good thermal insulation. In addition, the processes of compression or expansion of a gas can be equated to adiabatic if the time of change in the gas volume is much less than the time interval for which thermal equilibrium between the surrounding bodies and the gas sets in.
The most common 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 gaswill be quantitatively equal to the work of forces directed from outside. This work is positive in its direction, and therefore the internal energy of the gas will increase, and its temperature will rise. Under such initial conditions, it is clear that during adiabatic expansion, the work of the gas will occur due to a decrease in its internal energy, respectively, the temperature in this process will decrease.
