The subject of 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 the conversion of heat into mechanical work and vice versa. Gradually, concepts that characterize thermal processes appeared: heat of fusion, heat capacity and, finally, the unit of measurement of the amount of heat is 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 , some conventions have been adopted that make it possible to single out the subject under study and specify its properties that are to be studied. The objects under investigation are represented by closed systems of a huge number of particles. If the system can determine the boundaries of a certain volume, then it is called the body. And so the main participant in the thermodynamic action appeared: a system of particles enclosed in a certain volume - 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 we take temperature T, volume V, and pressure P as parameters, then they are enough to describe any thermodynamic process. All systems are considered only for equilibrium states. Establishing equilibrium, for example, thermal, is the process of heat transfer - something cools, and something heats up. At the same time, the amounts “given away - received”, as the first law of thermodynamics claims, will be the same. And here lies the main task that scientists have been solving for centuries: searching for participants in energy metabolism and determining their role in the process.
The basis of the theoretical apparatus of thermodynamics are 3 laws. It is believed that the body can absorb energy, increasing its internal (for example, when heated) and / or due to its internal energy to do work to overcome 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 external forces A. Mathematically, this is expressed through infinitesimal changes as follows:
dU = dQ + dA (1)
In fact, this is the law of conservation of energy, one might say, the law of being.
The features of thermodynamic processes are usually considered on a model where an ideal gas is taken with a working fluid, which can be heated and / or mechanically performed on it by external forces (compression - expansion) using a piston, and one of the parameters is pressure P, volume V or temperature T - equal to a constant. The application of the first law of thermodynamics to isoprocesses makes it possible to determine sources of energy for specific conditions.
The isochoric process means that V = const. The consequence of the fact that mechanical work is absent, because the volume does not change, due to heating only the internal energy changes, and then: dA = pdV = 0, which means dU = dQ and it can be determined from the relation:
dQ = (m / M) * CV * dT (2)
Thus, the isochoric process is due to an increase in temperature.
The isobaric process assumes p = const, and this condition is satisfied if the working fluid performs mechanical work during heating, for example, moving the piston. If we alternately apply the expressions for the heating energy and the Mendeleev-Klaiperon equation, then 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 volume of gas in an amount of one mole if the temperature changes by one degree Kelvin. Conclusion: during the isobaric process, the gas is replenished with heating energy (2) and spends part of the increased internal energy for expansion (3).
The process in which T = const is called isothermal in thermodynamics. Its essence is that the internal energy obtained through heating is completely expended in the work of overcoming external forces. The first law of thermodynamics for isoprocesses suggests that in order to maintain a constant body temperature, its internal energy replenishes the cost of performing mechanical work and depends on pressure changes. You can calculate these energy costs from the expression:
Q = A = (m / M) * R * T * (ln (p1 / p2)).