The activity of almost any heat engine is based on such a thermodynamic phenomenon as the work performed by a gas during expansion or contraction. It is worth recalling that in physics, work is understood as a quantitative measure characterizing the action of a certain force on the body. In accordance with this, the work of gas, the necessary condition for which is to change its volume, is nothing more than the product of pressure on this change in volume.
The work of gas when changing its volume can be both isobaric and isothermal. In addition, the expansion process itself can also be arbitrary. The work of gas, which occurs during isobaric 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 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, like its quantitative indicators, is zero, because the piston does not move in the cylinder. Under such conditions, it turns out that the gas operation during 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 performed by a gas during expansion and contraction, it can be noted that during expansion the direction of the piston displacement vector coincides with the pressure force vector of this gas itself, therefore, in the scalar calculus, the gas work is positive, and the external forces are negative. When gas is compressed, the vector of external forces already coincides with the general direction of movement of the cylinder; therefore, their work is positive and the work of gas is negative.
Consideration of the concept of “work done by gas” will be incomplete if we do not touch on adiabatic processes as well. 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 gas can be equated to adiabatic if the time of the change in gas volume is much shorter than the time interval for which thermal equilibrium sets in between the surrounding bodies and the gas.
The most common adiabatic process in everyday life is piston operation in an internal combustion engine. The essence of this process is as follows: as is known from the first law of thermodynamics, a change in the internal energy of a gas will be quantitatively equal to the work of forces directed from outside. This work is positive in its direction, 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 gas will work due to a decrease in its internal energy, and accordingly, the temperature will decrease as part of this process.