As you know, any body has its own unique structure, which is determined by its chemical composition and structure. Moreover, the particles that make up this structure are mobile, they interact with each other, and, therefore, have a certain amount of internal energy. In solids, the bonds of the particles that make up the structure of the body are strong, so their interaction with the particles that make up the structure of other bodies is difficult.
It looks completely different in liquids or gases, where molecular bonds are weak, and therefore molecules can move fairly freely and interact with particles of other substances. In this, for example, the solubility property is manifested.
This means that the internal energy of the gas is a parameter that determines the state of the gas itself, that is, the energy of the thermal motion of its microparticles, which are molecules, atoms, nuclei, etc. In addition, this concept also characterizes the energy of their interaction.
When a molecule passes from one state to another, the internal energy of the gas, the formula of which - WU = dQ - dA - shows only the process of changing this internal energy. Precisely because it is actually evident from the formula, it is always characterized by the difference between its values at the beginning and end of the transition of a molecule from one state to another. In this case, the path of the transition itself, that is, its size, does not play any role. This argument leads to the most basic conclusion that characterizes this phenomenon - the internal energy of a gas is determined solely by the temperature of the gas and is completely independent of the value of its volume. For mathematical analysis, this conclusion is important in the sense that it is not possible to directly measure the amount of internal energy; only its change can be determined and represented by mathematical means (this is emphasized by the presence of the symbol W in the formula).
For physical bodies, their internal energy is subject to dynamics (change) only if these bodies interact with other bodies. At the same time, there are two main ways of this change: work (performed during friction, impact, compression, etc.) and heat transfer. The latter method - heat transfer - reflects the dynamics of changes in internal energy in cases where work is not completed, and energy is transmitted, for example, from bodies with a higher temperature to bodies with a lower value.
In this case, there are such types of heat transfer as:
- thermal conductivity (direct exchange of energy by particles performing random motion);
- convection (the internal energy of the gas is carried by their flows);
- radiation (energy is carried by electromagnetic waves).
All these processes are reflected by the law of conservation of energy. If this law is considered in relation to thermodynamic processes occurring in gases, then it can be formulated as follows: the internal energy of a real gas, or rather, its change, is the total amount of heat that was transferred to it from external sources, and from the work that was committed over this gas.
If we consider the effect of this law (the first law of thermodynamics) in relation to an ideal gas, then we can see the following laws. In a process whose temperature remains unchanged (isothermal process), the internal energy will also always be a constant value.
In the framework of the isobaric process, which is characterized by a change in the temperature of the gas, its increase or decrease, respectively, leads to an increase or decrease in the internal energy and the work performed by the gas. This phenomenon, for example, clearly demonstrates the expansion of a gas when heated and the ability of such a gas to propel steam units.
When considering the isochoric process, in which the parameter of its volume remains unchanged, the internal energy of the gas changes only under the influence of the amount of transferred heat.
There is also an adiabatic process, which is characterized by the absence of heat exchange of gas with external sources. In this case, the value of its internal energy decreases, therefore, the gas cools.