Transformation of energy

Thermal energy and combustion.
Effects of thermodynamics


Second Law Of Thermodynamics

Second law of thermodynamics

According to the first law of thermodynamics, every process that occurs in a given system must satisfy the principle of conservation of energy, including the flow of heat.

Equation: Increase in entropy minus work equals heat

states, in other words, that any process whose sole purpose is to create or destroy energy, is impossible, that is, it denies the existence of a first-class perpetual motion machine.

However, the first law does not tell us anything about the direction in which a process can occur in a System. Thus within the context of said law there is no limitation to transform energy from one form to another. For example, heat in work or vice versa. The transformation of work into heat is a process that can occur virtually without any limitation: for example by friction between two surfaces, by the passage of electric current, etc. But experience tells us that the first alternative is only feasible under very severe limitations.

This restriction in the direction, in which a process may or may not occur in nature, manifests itself in all spontaneous or natural processes. In fact, we always observe that a compressed gas tends to expand, that heat flows from hot bodies to cold bodies, etc., but we never observe that these processes occur spontaneously in the opposite direction. Through the second law of thermodynamics, which constitutes the generalization of these observations, we can understand these phenomena.

Entropy in the second law of thermodynamics

The second law of thermodynamics requires that, in general, the total entropy of any system can not decrease more than by increasing the entropy of some other system. Therefore, in a system isolated from its surroundings, the entropy of that system tends not to diminish. It follows that heat can not flow from a colder body to a warmer body without the application of work (the imposition of order) to the colder body.

Secondly, it is impossible for a device that works in a cycle to produce networking from a single temperature reservoir; The production of networked work requires heat flow from a warmer tank to a colder tank, or a single expanding tank subjected to adiabatic cooling, which performs adiabatic work. As a result, there is no possibility of a perpetual motion system.

It follows that a reduction in the entropy increase in a specific process, such as a chemical reaction, means that it is energetically more efficient.

From the second law of thermodynamics it follows that the entropy of a system that is not isolated can decrease. An air conditioner, for example, can cool the air in a room, thus reducing the entropy of the air in that system. The heat expelled from the room (the system), which the air conditioner transports and discharges to the outside air, always contributes more to the entropy of the environment than the decrease in the entropy of the air of that system. Therefore, the total entropy of the room plus the entropy of the environment increases, according to the second law of thermodynamics.

Another example can be seen in a solar thermal water sanitation installation. We define the circuit fluid as a system. At the moment when the liquid stops for the solar collector and receives solar radiation, its thermal energy increases and, therefore, its entropy. The fluid continues circulating through the circuit passing through the radiators and it cools down. When cooled, it reduces its thermal energy and, therefore, its entropy.

In mechanics, the second of the thermodynamic laws together with the fundamental thermodynamic relationship puts limits on the ability of a system to do useful work. The entropy change of a system at temperature T absorbing an infinitesimal amount of heat dQ in a reversible manner is given by dQ / T.

The applicability of a second law of thermodynamics is limited to systems that are near or in a state of equilibrium. At the same time, the laws that govern systems that are far from equilibrium are still debatable. One of the guiding principles for such systems is the principle of maximum entropy production. He states that unbalanced systems evolve in a way that maximizes their entropy production.

Thermal Machines

One of the most important applications of the first law of Thermodynamics is the Carnot cycle that underlies the operation of thermal machines, and in fact, in the formulation most related to the engineering of the second law of Thermodynamics.

Definition of thermal machines

A motor or thermal machine whose objective is to continuously provide work to the outside, transforming into work the maximum possible heat absorbed, consists of a device through which a cycle is made to cycle through a system, in the sense that it absorbs heat while the temperature it is high, yields a smaller quantity at a lower temperature and performs a net work on the outside.

An example is in the solar collectors. The thermal energy obtained from the solar radiation that hits the solar panel will always be greater than the energy that is finally obtained from the system (electrical energy, heat or mechanical energy).

If we imagine a cycle carried out in the opposite direction to that of an engine, the final result will be the absorption of heat at a low temperature, the expulsion of a greater quantity at a higher temperature, and finally, the realization of a net amount of work on the system. This is the simplest concept of a refrigerator and, in effect, this is a device that cycles in this direction and is called a refrigerator. The system constitutes a refrigerant.

Development and efficiency of thermal machines

The French engineer N.Sadi Carnot (1796-1832) was the first to consider the operation of thermal machines. He published in 1824 his famous memory "Reflections on the motor power of heat and on the appropriate machines to develop this power" where he devoted himself to reasoning on the general question of how to produce mechanical work (motive power), from sources that produce hot.

Carnot found, that the key point in his study was to recognize that a thermal machine requires a temperature difference in order to operate. That is, when a machine operates between two bodies and extracts heat from the hottest one, it transfers a quantity of heat to the coldest body to equal the temperatures of both, that is until the thermal equilibrium is restored. This is the Carnot principle, but Carnot never demonstrated the conjecture that the efficiency of such a machine depends only on the temperature of the vessels between which it operates.

Regarding the dependence of machines on the temperature, it occurs to Carnot to think that an efficient thermal machine must be designed in such a way that there are no wasteful heat fluxes during its operation. To do this, I think of a cyclic process in which only the thermal source appears from which the machine extracts heat to operate and the cold source to which the non-usable heat is supplied. This operation minimizes heat losses due to spurious temperature differences and, moreover, as at the end of the cycle Uf = Ui, the internal energy of the operating substance is the same as at the beginning. Therefore the net work done in the cycle is the heat absorbed from the hot body minus the heat given to the cold body.

The second important result that emerges from Carnot's ideas was to demonstrate that no machine operating between two bodies at different temperatures can be more efficient than the machine conceived by him, through:

Carnot's theorem: "No thermal machine operating in cycles between two given thermal vessels, has an efficiency greater than that of a reversible machine (of Carnot) operating between the same vessels"

And further: "All reversible machines (Carnot machines with different operating substances) operating between two thermal vessels at given temperatures, have the same efficiency."

The demonstration, is due to W. Thomson, (Lord Kelvin). Additionally, Kelvin Planck's Theorem is found: "Any cyclic transformation, whose only end result is that of absorbing heat from a body or thermal source at a given temperature and converting it entirely into work, is impossible."


Published: August 17, 2016
Last review: April 12, 2018