Thermodynamics is an important branch of physics that studies and describes the thermodynamic systems that are in equilibrium or tend to it. In order to describe the transition from a certain initial state to a final state using the equations of thermodynamics, it is necessary to make an approximation of the quasistatic process. What is this approximation, and what types of these processes are, we will consider in this article.
What is understood by a quasi-static process?
As is known, thermodynamics uses a set of macroscopic characteristics that can be measured experimentally to describe the state of a system. These include pressure P, volume V, and absolute temperature T. If all three values ββare currently known for the system under study, then it is said that its state is determined.
The concept of a quasistatic process implies a transition between two states. In the process of such a transition, naturally, the thermodynamic characteristics of the system change. If at every moment in time during which the transition continues, T, P and V are known for the system, and it is not far from its equilibrium state, then it is said that a quasistatic process is occurring. In other words, this process is a sequential transition between many equilibrium states. He suggests that the external impact on the system is negligible, so that it has time to quickly come to equilibrium.
Real processes are not quasi-static, so the concept under consideration will be idealized. For example, during expansion or contraction of a gas, there are turbulent changes and wave processes in it, which suggest some time for their attenuation. Nevertheless, in a number of practical cases for gases in which particles move at high speeds, equilibrium sets in quickly, therefore, various transitions between states in them can be considered quasistatic with high accuracy.
Equation of state and types of processes in gases
Gas is a convenient aggregate state of a substance for its study in thermodynamics. This is due to the fact that for its description there is a simple equation connecting all three of the above thermodynamic quantities. This equation is called the Clapeyron-Mendeleev law. It has the following form:
P * V = n * R * T
Using this equation, all types of isoprocesses and the adiabatic transition are studied and plots of isobars, isotherms, isochores and adiabats are constructed. In the equality n is the amount of substance in the system, R is the constant for all gases. Below we consider all the noted types of quasistatic processes.
Isothermal Transition
It was first studied at the end of the 17th century using various gases as an example. Corresponding experiments were performed by Robert Boyle and Edm Mariott. Scientists have come to the following result:
P * V = const at T = const
If you increase the pressure in the system, then its volume will decrease in proportion to this increase, if the temperature is kept constant in the system. It is easy to get this law from the equation of state yourself.
The isotherm on the graph is a hyperbola that approaches the axes P and V.
Isobaric and isochoric transitions
Isobaric (at constant pressure) and isochoric (at constant volume) transitions in gases were studied at the beginning of the 19th century. Great merits in their study and discovery of the relevant laws belong to the French Jacques Charles and Gay-Lussac. Both processes are mathematically represented as follows:
V / T = const at P = const;
P / T = const at V = const
Both expressions follow from the equation of state if we put the corresponding parameter constant.
We combined these transitions in one paragraph of the article because they have the same graphical representation. In contrast to the isotherm, isobar, and isochore, these are straight lines that show direct proportionality between volume and temperature and pressure and temperature, respectively.
Adiabatic process
It differs from the described isoprocesses in that it proceeds in complete thermal isolation from the environment. As a result of the adiabatic transition, the gas expands or contracts without exchanging heat with the environment. In this case, a corresponding change in its internal energy occurs, that is:
dU = - P * dV
To describe the adiabatic quasistatic process, it is important to know two quantities: isobaric C P and isochoric C V specific heat. The value of C P indicates how much heat should be reported to the system so that it increases its temperature by 1 K during isobaric expansion. The value of C V means the same, only for heating at a constant volume.
The equation of this process for an ideal gas is called the Poisson equation. It is written in the parameters P and V as follows:
P * V Ξ³ = const
Here, the parameter Ξ³ is called the adiabatic exponent. It is equal to the ratio of C P and C V. For a monatomic gas, Ξ³ = 1.67, for a diatomic gas, 1.4; if the gas is formed by more complex molecules, then Ξ³ = 1.33.
Since the adiabatic process occurs solely due to its own internal energy resources, the adiabatic graph in the PV axes behaves more sharply than the isotherm (hyperbola) graph.