Heat capacity as a physical quantity shows the amount of thermal energy that is needed to change the temperature of the working fluid, in this case, air, by one degree. The heat capacity of air directly depends on temperature and pressure. At the same time, various methods can be used to study different types of heat capacity.
Mathematically, the heat capacity of air is expressed as the ratio of the amount of heat to the increment of its temperature. The heat capacity of a body having a mass of 1 kg is commonly called specific. Molar heat capacity of air - the heat capacity of one mole of a substance. The heat capacity is indicated - J / K. The molar heat capacity, respectively, J / (mol * K).
The heat capacity can be considered the physical characteristic of a substance, in this case air, if the measurement is carried out under constant conditions. Most often, such measurements are carried out at constant pressure. This determines the isobaric heat capacity of air. It increases with increasing temperature and pressure, and is also a linear function of these values. In this case, a temperature change occurs at a constant pressure. To calculate the isobaric heat capacity, it is necessary to determine the pseudocritical temperature and pressure. It is determined using reference data.
Heat capacity of air. Features
Air is a gas mixture. When considering them in thermodynamics, the following assumptions are made. Each gas in the mixture should be evenly distributed throughout the volumes. Thus, the volume of gas is equal to the volume of the entire mixture. Each gas in the mixture has its own partial pressure, which it exerts on the walls of the vessel. Each of the components of the gas mixture must have a temperature equal to the temperature of the entire mixture. In this case, the sum of the partial pressures of all components is equal to the pressure of the mixture. Calculation of the heat capacity of the air is based on data on the composition of the gas mixture and the heat capacity of the individual components.
Heat capacity ambiguously characterizes a substance. From the first law of thermodynamics, we can conclude that the internal energy of the body varies not only depending on the amount of heat received, but also on the work done by the body. Under different conditions of the process of heat transfer, the work of the body may vary. Thus, the same amount of heat communicated to the body can cause changes in temperature and internal energy of the body that are different in value. This feature is characteristic only for gaseous substances. Unlike solids and liquids, gaseous substances can greatly change the volume and do the job. That is why the heat capacity of air determines the nature of the thermodynamic process itself.
However, with a constant volume, air does not do the job. Therefore, a change in internal energy is proportional to a change in its temperature. The ratio of heat capacity in a process with constant pressure to heat capacity in a process with constant volume is part of the formula for the adiabatic process. It is indicated by the Greek letter gamma.
From the history
The terms “heat capacity” and “amount of heat” do not very well describe their essence. This is due to the fact that they came to modern science from the theory of calorific, which was popular in the eighteenth century. The followers of this theory considered heat as a kind of weightless substance that is contained in bodies. This substance can neither be destroyed nor created. The cooling and heating of the bodies was explained by a decrease or increase in the calorific value, respectively. Over time, this theory was found to be bankrupt. She could not explain why the same change in the internal energy of any body is obtained when a different amount of heat is transferred to it, and also depends on the work performed by the body.