Heat capacity of air

The heat capacity as a physical quantity shows the amount of thermal energy that is necessary to change the temperature of the working fluid, in this case, air, by one degree. The specific heat of the air depends directly on temperature and pressure. Different 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 usually called specific. The molar heat capacity of air is the heat capacity of one mole of the substance. The heat capacity is J / K. The molar heat capacity is, respectively, J / (mol * K).

The heat capacity can be considered a 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 quantities. In this case, the temperature changes at 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 they are considered in thermodynamics, the following assumptions are accepted. Each gas in the mixture should be evenly distributed throughout the volumes. Thus, the volume of the gas is equal to the volume of the whole mixture. Each gas in the mixture has its 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 whole mixture. The sum of the partial pressures of all components is equal to the mixture pressure. Calculation of the heat capacity of air is performed on the basis of data on the composition of the gas mixture and the heat capacity of individual components.

The heat capacity characterizes the substance ambiguously. From the first law of thermodynamics we can conclude that the internal energy of the body changes not only depending on the amount of heat received, but also on the body's perfect work. Under different conditions of the heat transfer process, the body's work can differ. Thus, the same amount of heat reported to the body can cause different in magnitude changes in temperature and internal energy of the body. This feature is characteristic only of gaseous substances. Unlike solid and liquid bodies, gaseous substances can greatly change the volume and perform work. That is why the heat capacity of air determines the nature of the thermodynamic process itself.

However, with a constant volume, air does not work. Therefore, the change in internal energy is proportional to the change in its temperature. The ratio of the heat capacity in a process with constant pressure, to the heat capacity in a process with a constant volume, is part of the adiabatic process formula . It is denoted by the Greek grammar.

From the history

The terms "heat capacity" and "heat quantity" do not describe their essence very well. This is due to the fact that they came to modern science from the theory of heat, which was popular in the eighteenth century. The followers of this theory regarded heat as some weightless substance that is contained in bodies. This substance can not be destroyed or created. Cooling and heating of the bodies were explained by a decrease or an increase in the content of heat, respectively. Over time, this theory was declared invalid. She could not explain why the same change in the internal energy of any body is obtained by transferring to him a different amount of heat, and also depends on the work done by the body.