When in gas there are any processes, all three of its parameters usually change: p, V, T. It is natural that such processes which proceed at change only of two parameters are simplest, and the third remains to constants.
The increase in pressure of gas in any capacity or in an electric bulb when heating is izokhorny process. Izokhorny process is used in gas thermometers of constant volume. Also izokhorny process can be carried out when heating air at the constant volume.
The processes proceeding at invariable value of one of parameters are called isoprocesses. However, actually any process cannot proceed at strictly fixed value of any parameter. Always there are these or those influences breaking constancy of temperature, pressure or volume. Only in vitro it is possible to maintain constancy of this or that parameter with a good accuracy, but in the operating technical devices and in the nature it is almost impracticable. Isoprocess is an idealized model of real process which only approximately reflects reality.
It is necessary for maintenance of temperature of gas of a constant that it could exchange warmth with big system – the thermostat. As the thermostat atmospheric air if its temperature considerably does not change throughout all process can serve.
To different constant temperatures there correspond various isotherms. At temperature increase pressure according to the state equation (increases if V=const. Therefore the isotherm corresponding to more high temperature lies above the isotherm corresponding to lower temperature.
As at adiabatic compression temperature of gas increases, gas pressure with reduction of volume grows quicker, than at isothermal process. Fall of temperature of gas at adiabatic expansion leads to that pressure of gas decreases quicker, than at isothermal expansion.
Adiabatic cooling of gases at their expansion is used in cars for liquefaction of gases. Cooling of gas at adiabatic expansion happens in grandiose scales in the atmosphere of Earth. Heated air rises up and extends as atmospheric pressure falls with increase in height. This expansion is followed by considerable cooling. As a result water vapor is condensed and forms clouds.