Knowledge of definitions in physics is a key factor for the successful solution of various physical problems. In the article, we consider what is meant by isobaric, isochoric, isothermal, and adiabatic processes for an ideal gas system.
The ideal gas and its equation
Before proceeding to the description of isobaric, isochoric and isothermal processes, we consider what an ideal gas is. Under this definition in physics we mean a system consisting of a huge number of dimensionless and non-interacting particles that move at high speeds in all directions. In fact, we are talking about the gas state of aggregation of a substance in which the distances between atoms and molecules are much larger than their sizes and in which the potential energy of interaction of particles is neglected due to its smallness, compared with the kinetic energy.
The state of an ideal gas is the set of its thermodynamic parameters. The main ones are temperature, volume and pressure. Denote them by the letters T, V, and P, respectively. In the 30s of the XIX century, Clapeyron (a French scientist) first wrote down an equation that combines the indicated thermodynamic parameters in the framework of a single equality. It has the form:
P * V = n * R * T,
where n and R are the substances quantity and gas constant, respectively.
What are isoprocesses in gases?
As many have noticed, isobaric, isochoric and isothermal processes use the same prefix "iso" in their name. It means the equality of one thermodynamic parameter during the passage of the entire process, while the remaining parameters change. For example, an isothermal process suggests that, as a result, the absolute temperature of the system is kept constant, and the isochoric process indicates a constant volume.
It is convenient to study isoprocesses, since fixing one of the thermodynamic parameters leads to a simplification of the general gas equation of state. It is important to note that gas laws for all the named isoprocesses were discovered experimentally. Their analysis allowed Clapeyron to obtain the given universal equation.
Isobaric, isochoric and isothermal processes
The first law was discovered for an isothermal process in an ideal gas. Now it is called the Boyle-Marriott law. Since T does not change, the equation of state implies the equality:
P * V = const.
In other words, any change in pressure in the system leads to an inversely proportional change in its volume, if the gas temperature is kept constant. The graph of the function P (V) is a hyperbola.
An isobaric process is a change in the state of the system in which the pressure remains constant. Having fixed the value of P in the Clapeyron equation, we obtain the following law:
V / T = const.
This equality bears the name of the French physicist Jacques Charles, who received it at the end of the 18th century. Isobar (graphic representation of the function V (T)) looks like a straight line. The greater the pressure in the system, the faster this line increases.
The isobaric process is easy to carry out if gas is heated under the piston. The molecules of the latter increase their speed (kinetic energy), create a higher pressure on the piston, which leads to expansion of the gas and maintaining a constant value of P.
Finally, the third isoprocess is isochoric. It runs with a constant volume. From the equation of state we obtain the corresponding equality:
P / T = const.
It is known among physicists as the Gay-Lussac law. Direct proportionality between pressure and absolute temperature indicates that the graph of the isochoric process, like the graph of the isobaric process, is a straight line with a positive slope coefficient.
It is important to understand that all isoprocesses occur in closed systems, that is, in their course, the value of n is preserved.
Adiabatic process
This process does not belong to the βisoβ category, since all three thermodynamic parameters change during its passage. Adiabatic is the transition between two states of a system in which it does not exchange heat with the environment. So, the expansion of the system is carried out due to its internal energy reserves, which leads to a significant drop in pressure and absolute temperature in it.
The adiabatic process for an ideal gas is described by the Poisson equations. One of them is given below:
P * V Ξ³ = const,
where Ξ³ is the ratio of specific heat at constant pressure and constant volume.
The adiabatic plot differs from the plot of the isochoric process and from the plot of the isobaric one, however, it looks like a hyperbola (isotherm). The adiabat in the PV axes behaves more sharply than the isotherm.