What Is the First Law of Thermodynamics?

The first law of thermodynamics is the law of conservation of different forms of energy in the process of transfer and conversion. The expression is U = Q + W. Expression: Heat can be transferred from one object to another, and can also be converted into mechanical energy or other energy, but the total value of energy remains unchanged during the conversion process. Its extension and essence is the famous law of conservation of energy.

The increase in the internal energy of an object is equal to the sum of the heat absorbed by the object and the work done on the object ^[1]

At the beginning of the 19th century, due to the further development of the steam engine, it was urgent to study the relationship between heat and work, and to make a theoretical analysis of the "output" of the steam engine. Therefore, the mutual conversion of heat and mechanical work was widely studied ^.

Expression : Thermal energy can be transferred from one object to another, and can also be converted with mechanical energy or other energy. During the process of transfer and conversion, the total value of energy does not change.

Within the scope of engineering thermodynamics, the first law of thermodynamics can be expressed as: when the thermal or mechanical energy is transferred or converted, the total amount of energy must be conserved.

Basic content: heat can be transformed into work, and work can also be transformed into heat; when a certain amount of work is consumed, a certain amount of heat is generated, and when a certain amount of heat disappears, a certain amount of work is also generated.

Another formulation of the first law of thermodynamics is:

The system must be a closed system, and an open system with material exchange is not considered by the first law of thermodynamics. That is, the basic definition cannot be used. See the previous paragraph for specific statements. ^[5]

Heat capacity and enthalpy

Application of the first law of thermodynamics in pV systems.

For gas, liquid, and isotropic solids, their state can be described using any two of the three quantities p, V, and T as state parameters, regardless of surface tension and no external force field. The object system is a pV system ^[3]

The first law of thermodynamics is the basis of thermodynamics, and has a wide range of applications in energy. Energy is the material basis of human social activities. The development of society is inseparable from the emergence of high-quality energy and the use of advanced energy technologies. The scope of energy resources follows The development of science and technology has expanded, so the broad development prospects of the first law of thermodynamics will become increasingly bright. ^[5]