Scientists began to study the behavior of gases under various external conditions from the second half of the 17th century. For almost two centuries, the basic gas laws in physics were experimentally discovered, analyzing which Emil Clapeyron wrote the universal equation of state in 1834. Let us consider these laws in more detail in the article.
Ideal gases
Before proceeding to the description of the laws of gases in physics, we give a description of the model on which they rely. This is an ideal gas model. According to her, the considered aggregate state of matter consists of particles (atoms and molecules) that do not interact with each other, which are considered dimensionless. Particles move at speeds of several hundred meters per second in different directions. Their average kinetic energy characterizes the temperature of the entire system, and collisions with the walls of the vessel lead to the appearance of pressure in it.
The behavior of real gases, which are at not too high pressures and temperatures of several hundred kelvin, is in good agreement with the predictions of the described model. For example, terrestrial air can be considered an ideal mixture of gases.
Boyle-Marriott Law
Considering gases from the point of view of thermodynamics, the gas law of Robert Boyle and Edm Mariott should be called first of all, since it was experimentally discovered before all other laws (second half of the 17th century). It states that at a constant temperature in a closed gas system, any change in volume leads to an inversely proportional change in pressure. Mathematically, the law is written in the following two forms:
P * V = const at T = const;
P 1 * V 1 = P 2 * V 2 .
The graph of the Boyle-Mariotte law is called the isotherm. It is a hyperbolic dependence of the functions P (V) or V (P).
The laws of Charles and Gay-Lussac
Both laws of gases in physics were discovered at about the same time (late XVIII - early XIX centuries) by the French Charles and Gay-Lussac.
Charles's law describes the behavior of a gas system during an isobaric process, that is, a transition between equilibrium states at constant pressure. The law states that as the temperature rises under the indicated conditions, the volume grows in direct proportion. Mathematically, it is written as follows:
V / T = const at P = const;
V 1 / T 1 = V 2 / T 2 .
Charles's law can be obtained experimentally if a certain volume of gas is pumped into the cylinder under a movable piston, and then the system is heated and the temperature and volume are fixed in it.
The third main gas law bears the name of Gay-Lussac. It describes the behavior of the system during the isochoric process, that is, with a constant volume. If in the previous case the gas was pumped into a cylinder with a movable piston, then in the Gay-Lussac law the cylinder volume is fixed. Only pressure and temperature change. Mathematically it is expressed as follows:
P / T = const at V = const;
P 1 / T 1 = P 2 / T 2 .
It can be seen that this dependence is no different from the previous one. Only in this case, instead of V is P.
The graphs of isobars (P = const) and isochores (V = const) are straight lines with a positive slope, since an increase in volume (pressure) always leads to an increase in absolute temperature. It is important to remember that the formulas given in this paragraph are valid only if they use the temperature in absolute units, that is, in kelvins.
Avogadro's Law or Principle
At the beginning of the same XIX century, the Italian Amedeo Avogadro, exploring the behavior of different gases taken in different quantities, came to an amazing conclusion. The scientist showed experimentally that different gases at the same pressures and temperatures occupy a volume proportional to the amount of substance in moles. In the form of a formula, this principle is represented as follows:
n / V = const at T, P = const;
n 1 / V 1 = n 2 / V 2 .
An important conclusion that can be drawn from this law of gases in physics is that it does not depend on the chemical composition. Note that the principle of partial pressures for a mixture of ideal gases, which bears the name of Dalton, is based on the Avogadro principle.
Clapeyron-Mendeleev Law
It is also called the law of state of gases and summarizes the experimental experience of all previous laws. The corresponding equation was first written down by Emil Clapeyron in 1834, and Mendeleev introduced a constant universal for gases R. The Clapeyron-Mendeleev equation has the form:
P * V = n * R * T.
Each student can get from him any of the above laws of gases in physics, if he fixes two corresponding variables.