The gas state of the matter around us is one of the three common forms of matter. The indicated fluid state of aggregation in physics is considered to be considered in the ideal gas approximation. Using this approximation, we describe in the article possible isoprocesses in gases.
The ideal gas and the universal equation for its description
An ideal gas is a gas whose particles do not have dimensions and do not interact with each other. Obviously, there is not a single gas that exactly satisfies these conditions, since even the smallest atom - hydrogen, has certain dimensions. Moreover, even between the neutral atoms of noble gases, there is a weak van der Waals interaction. Then the question arises: in what cases can the size of gas particles and the interaction between them be neglected? The answer to this question will be the observance of the following physical and chemical conditions:
- low pressure (about 1 atmosphere and below);
- high temperatures (near room temperature and above);
- chemical inertness of molecules and gas atoms.
If at least one of the conditions is not met, then the gas should be considered real and described by the special van der Waals equation.
The Mendeleev-Clapeyron equation must be considered before studying isoprocesses. The ideal gas equation is its second name. It has the following notation:
P * V = n * R * T
That is, it connects three thermodynamic parameters: pressure P, temperature T and volume V, as well as the amount n of the substance. The symbol R here denotes a gas constant, it is 8.314 J / (K * mol).
What are isoprocesses in gases?
By these processes we mean the transitions between two different states of the gas (initial and final), as a result of which some quantities are saved and others change. There are three types of isotropes in gases:
- isothermal;
- isobaric;
- isochoric.
It is important to note that all of them were experimentally studied and described in the period from the second half of the 17th century to the 30s of the 19th century. Based on these experimental results, Emil Clapeyron in 1834 obtained an equation universal for gases. This article is built on the contrary - using the equation of state, we obtain formulas for isoprocesses in ideal gases.
Constant temperature transition
It is called the isothermal process. It follows from the equation of state of an ideal gas that, at a constant absolute temperature in a closed system, the product of volume and pressure should remain constant, that is:
P * V = const
This dependence was indeed observed by Robert Boyle and Edm Marriott in the second half of the 17th century, therefore, the currently recorded equality bears their names.
The functional dependences P (V) or V (P), expressed graphically, have the form of hyperbolas. The higher the temperature at which the isothermal experiment is conducted, the greater the product P * V.
In an isothermal process, the gas expands or contracts, performing work and not changing its internal energy.
Constant pressure transition
Now we will study the isobaric process, during which the pressure is kept constant. An example of such a transition is the heating of a gas under a piston. As a result of heating, the kinetic energy of the particles increases, they begin to strike the piston more often and with greater force, as a result of which the gas expands. In the process of expansion, the gas does some work, the efficiency of which is 40% (for monatomic gas).
For this isoprocess, the equation of state of an ideal gas suggests that the following relationship should be satisfied:
V / T = const
It is easy to obtain it if the constant pressure is transferred to the right side of the Clapeyron equation, and the temperature to the left. This equality is called Charles's law.
Equality indicates that the functions V (T) and T (V) have the form of straight lines on the graphs. The slope of the V (T) line relative to the abscissa axis will be the smaller, the greater the pressure P.
Transition at a constant volume
The last isoprocess in gases, which we will consider in the article, is an isochoric transition. Using the universal Clapeyron equation, it is easy to obtain the following equality for this transition:
P / T = const
The isochoric transition is described by the Gay - Lussac law. It is seen that graphically the functions P (T) and T (P) will be straight lines. Among all three isoprocesses, isochoric is the most effective if it is necessary to increase the temperature of the system due to the supply of external heat. During this process, the gas does not perform work, that is, all the heat will be directed to increase the internal energy of the system.