Thermodynamics is an important branch of physics that studies and describes thermodynamic systems in equilibrium or tending to it. In order to be able to describe the transition from some initial state to a final state using the equations of thermodynamics, it is necessary to make an approximation of a quasi-static process. What is this approximation, and what types of these processes are, we will consider in this article.
What is meant by a quasi-static process?
As you know, thermodynamics to describe the state of the system uses a set of macroscopic characteristics that can be measured experimentally. These include pressure P, volume V, and absolute temperature T. If all three quantities are known for the system under study at a given moment, then they say that its state has been determined.
The concept of a quasi-static process implies a transition between two states. During this transition,Naturally, the thermodynamic characteristics of the system change. If at each moment of 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 we say that a quasi-static process occurs. In other words, this process is a sequential transition between a set of equilibrium states. He assumes that the external influence on the system is insignificant 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, when expanding or compressing a gas, there are turbulent changes and wave processes in it, which require 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, so various transitions between states in them can be considered quasi-static with high accuracy.
Equation of state and types of processes in gases
Gas is a convenient aggregate state of matter for its study in thermodynamics. This is due to the fact that for its description there is a simple equation that connects all three of the above thermodynamic quantities. This equation is called the Clapeyron-Mendeleev law. It looks like this:
PV=nRT
Using this equation, all kinds of isoprocesses and adiabatic transition andgraphs of the isobar, isotherm, isochore and adiabat are constructed. In equality, n is the amount of substance in the system, R is a constant for all gases. Below we consider all the noted types of quasi-static processes.
Isothermal transition
It was first studied at the end of the 17th century using various gases as an example. The corresponding experiments were carried out by Robert Boyle and Edm Mariotte. Scientists came up with the following result:
PV=const when T=const
If you increase the pressure in the system, then its volume will decrease in proportion to this increase, if the system maintains a constant temperature. It is easy to derive this law from the equation of state yourself.
The isotherm on the graph is a hyperbola that approaches the P and V axes.
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 merit in their study and discovery of the relevant laws belongs to the French Jacques Charles and Gay-Lussac. Both processes are mathematically represented as follows:
V/T=const when P=const;
P/T=const when V=const
Both expressions follow from the equation of state if we set the corresponding parameter constant.
We have combined these transitions under one paragraph of the article because they have the same graphical representation. Unlike the isotherm, the isobar and isochore are straight lines thatshow 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 heat exchange with the environment. In this case, a corresponding change in its internal energy occurs, that is:
dU=- PdV
To describe an adiabatic quasi-static process, it is important to know two quantities: isobaric CP and isochoric CVheat capacity. The value CP tells how much heat must be imparted to the system so that it increases its temperature by 1 K during isobaric expansion. The value CV means the same, only for constant volume heating.
The equation for this process for an ideal gas is called the Poisson equation. It is written in parameters P and V as follows:
PVγ=const
Here the parameter γ is called the adiabatic exponent. It is equal to the ratio of CP and CV. For a monatomic gas γ=1.67, for a diatomic gas - 1.4, if the gas is formed by more complex molecules, then γ=1.33.
Since the adiabatic process occurs solely due to its own internal energy resources, the adiabatic graph in the P-V axes behaves more sharply than the isotherm graph(hyperbole).
