The first law of thermodynamics is the beginning of everything that exists

The subject of studying thermodynamics is energy in all its manifestations and, most importantly, energy transitions from one species to another. It so happened that the term itself arose at the dawn of scientific research in the field of energy, and at that time the list of various types of energy was still small - mechanical and thermal. Therefore, the name "thermodynamics" most accurately reflected the essence of the subject - movement (transfer) and conversion of heat into mechanical work and vice versa. Gradually there were concepts that characterize thermal processes: heat of fusion, heat capacity and, finally, a unit for measuring the amount of heat - calorie (1772, M. Wilke). A lot of time will pass and the first law of thermodynamics will be formulated, but each step was the result of the painstaking work of many researchers.

To study the laws of thermodynamics , certain conventions have been adopted that make it possible to isolate the object under study and to specify its properties to be studied. The objects under investigation are represented as closed systems from a huge number of particles. If in the system it is possible to determine the boundaries of a certain volume, then it is called the body. This is how the main participant of the thermodynamic action appeared: the particle system, enclosed in a certain volume, is an ideal gas. In the process of energy transformations, the thermodynamic system changes its state, and these changes are described by a set of concepts - process parameters. If the temperature T, volume V and pressure P are taken as parameters, then they are sufficient to describe any thermodynamic process. All systems are considered only for equilibrium states. Establishing an equilibrium, for example, heat, is a process of heat transfer - something cools down, and something is heated up. At the same time, the quantities "gave-received", as the first law of thermodynamics states, will be the same. And here lies the main task that scientists solve for centuries: the search for participants in the energy exchange and the definition of their role in the process.

The basis of the theoretical apparatus of thermodynamics is 3 laws. It is assumed that the body can absorb energy by increasing its internal (for example, heating) and / or due to its internal energy to work on overcoming external forces (for example, pushing the piston). Proceeding from this, the first law of thermodynamics is interpreted as follows: the change in the internal energy of the body U is the sum of the energy Q absorbed by it and the energy of the external forces A. Mathematically, this is expressed in terms of infinitesimal changes as follows:

DU = dQ + dA (1)

In fact, this is the law of conservation of energy, we can say, the law of being.

The peculiarities of thermodynamic processes are usually considered in the model where an ideal gas is taken by the working body, which can be heated and / or mechanically operated by external forces (compression-expansion) by means of a piston, and one of the parameters - pressure P, volume V or temperature T Is equal to a constant. The application of the first law of thermodynamics to isoprocesses makes it possible to determine sources of energy receivers for specific conditions.

The isochoric process means that V = const. The consequence is that mechanical work is not available, because The volume does not change, due to heating only the internal energy is changed, and then: dA = pdV = 0, and hence dU = dQ and it can be determined from the relation:

DQ = (m / M) * CV * dT (2)

Thus, the isochoric process is due to a temperature increase.

The isobaric process assumes p = const, and this condition is met if the working medium performs mechanical work on heating, for example, moving the piston. If we alternately apply the expressions for the heating energy and the Mendeleev-Klaiperon equation, we can easily obtain an expression for calculating the mechanical work of the gas :

A = (m / M) * R * (T2-T1) (3)

R is the gas constant, and means work to increase the gas volume in the amount of one mole, if the temperature changes by one degree Kelvin. Conclusion: in the isobaric process, the gas is replenished by the heating energy (2) and consumes part of the increased internal energy by expansion (3).

The process in which T = const, in thermodynamics is called isothermal. Its essence lies in the fact that the internal energy received due to heating is completely spent for work on overcoming external forces. The first law of thermodynamics for isoprocesses suggests that in order to maintain a constant body temperature, its internal energy makes up for the cost of doing mechanical work and depends on the change in pressure. Calculate these energy costs can be from the expression:

Q = A = (m / M) * R * T * (ln (p1 / p2)).