Thermodynamic Principles

To understand, in an easy way, the wide and complex world of Thermodynamics, it is recommended to go step by step starting with a review of basic terms, an introduction to thermodynamic principles, and then studying in more depth the thermodynamic laws, how they are expressed mathematically. and its applications.

With the four laws of thermodynamics (zero law, first law, second law and third law), it is described how the transfers and transformations of energy between different systems work; being the basis for understanding many physicochemical phenomena of nature.

Review of basic concepts

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Forms of energy

Energy, the property of bodies to transform themselves by modifying their situation or state, occurs in many forms, such as kinetic energy, potential energy and internal energy of bodies. See figure 1.

Work

It is the product of a force and displacement, both measured in the same direction. To calculate the work, the component of the force that is parallel to the displacement of the object is used. Work is measured in Nm, Joule (J), ft.lb-f, or BTU. See figure 2.

Heat (Q)

Transfer of thermal energy between two bodies that are at different temperatures, and it only occurs in the sense that the temperature decreases. Heat is measured in Joule, BTU, pound-feet, or in calories. See figure 3.

Thermodynamic Principles

Zero Law - Zero Principle

The zero law of thermodynamics states that if two objects, A and B, are in thermal equilibrium with each other, and object A is in equilibrium with a third object C, then object B is in thermal equilibrium with object C. The Thermal equilibrium occurs when two or more bodies are at the same temperature. See figure 4.

This law is considered a basic law of thermodynamics. It was postulated as "Zero Law" in 1935, since it was postulated after the first and second laws of thermodynamics were made.

1st Law of Thermodynamics (Principle of conservation of energy)

Statement of the First Law of Thermodynamics:

The first law of thermodynamics, also known as the principle of conservation of energy, states that energy is not created or destroyed, it is only transformed into another type of energy, or it is transferred from one object to another. Thus the total amount of energy in the universe does not change.

The first law is fulfilled in “everything”, energy is transferred and transformed continuously, for example, in some electrical devices, such as mixers and blenders, electrical energy is transformed into mechanical and thermal energy, in the human body they are transformed the chemical energy of food that is ingested into kinetic energy when the body is in motion, or other examples such as those shown in figure 5.

Equation of the First Law of Thermodynamics:

The equation of the first law within the thermodynamic principles expresses the balance that must exist between the different types of energy in a given process. Since, in closed systems [1], the energy exchanges can be given only by the transfer of heat, or by the work done (by or on the system), it is established that the energy variation of a system is equal to the sum of energy transfers through heat and through work. See figure 6.

Considering that the energies considered in this energy balance are kinetic energy, potential energy and internal energy [1], the energy balance for closed systems remains as shown in figure 7.

• (ec) Kinetic energy , due to the movement of a body;
• (ep) Potential Energy, due to the position of a body in a gravitational field;
• (U) Internal energy , due to the microscopic contributions of the kinetic and potential energy of the internal molecules of a body.

Exercise 1.

A sealed container contains a substance, with an initial energy of 10 kJ. The substance is stirred with a propeller that does 500 J work, while a heat source transfers 20 kJ of heat to the substance. In addition, 3kJ of heat is released into the air during the process. Determine the final energy of the substance. See figure 8.

Solution:

In figure 9 we can see the heat added by the heat source, which is considered "positive" since it increases the energy of the substance, the heat that is released into the air, negative since it decreases the energy of the substance, and the work of the propeller, which increased the energy took a positive sign.

In figure 10 the energy balance is presented, according to the first law of thermodynamics and the final energy of the substance is obtained.

Second law of thermodynamics

There are several statements of the second law of thermodynamics: Statement of Planck-Kelvin, Clausius, Carnot. Each of them shows a different aspect of the second law. In general the second law of thermodynamics postulates:

• The direction of thermodynamic processes, irreversibility of physical phenomena.
• The efficiency of thermal machines.
• Enter the property "entropy".

Direction of thermodynamic processes:

Spontaneously in nature, energy flows or is transferred from the highest energy state to the lowest energy state. Heat flows from hot bodies to cold bodies and not vice versa. See figure 11.

Efficiency or thermal performance:

According to the first law of thermodynamics, energy is neither created nor destroyed, but it can be transformed or transferred. But in all transfers or transformations of energy an amount of it is not useful to do work. As energy is transferred or transformed, part of the initial energy is released as thermal energy: energy degrades, loses quality.

In any energy transformation, the amount of energy obtained is always less than the energy supplied. Thermal efficiency is the amount of heat from the source that is converted into work, the ratio between the useful energy obtained and the energy supplied in a transformation. See figure 12.

Thermal Machine or Heat Machine:

The heat engine is a device that partially converts heat into work or mechanical energy, for which it requires a source that supplies heat at a high temperature.

In thermal machines a substance such as water vapor, air or fuel is used. The substance undergoes a series of thermodynamic transformations in a cyclical way, so that the machine can work continuously.

Exercise 2.

The engine of a cargo vehicle produces heat in combustion by burning gasoline. For each cycle of the engine, the heat of 5 kJ is converted into 1kJ of mechanical work. What is the efficiency of the motor? How much heat is released for each cycle of the engine? See figure 13

Solution:

To determine the heat released, it is assumed that in thermal machines the net work is equal to the net heat transfer to the system. See figure 14.

Entropy:

Entropy is the degree of randomness or disorder in a system. Entropy makes it possible to quantify the part of the energy that cannot be used to produce work, that is, it allows to quantify the irreversibility of a thermodynamic process.

Each energy transfer that occurs increases the entropy of the universe and reduces the amount of usable energy available to do work. Any thermodynamic process will proceed in a direction that increases the total entropy of the universe. See figure 15.

3rd Law of Thermodynamics

Third Law of Thermodynamics or Nerst Postulate

The third law of thermodynamics is related to temperature and cooling. It states that the entropy of a system at absolute zero is a definite constant. See figure 16.

Absolute zero is the lowest temperature below which there is no longer a lower measure, it is the coldest that a body can be. Absolute zero is 0 K, equivalent to -273,15 ºC.

Conclusion

There are four thermodynamic principles. In the zero principle it is established that thermal equilibrium occurs when two or more bodies are at the same temperature.

The first law of thermodynamics deals with the conservation of energy between processes, while the second law of thermodynamics deals with the directionality from lowest to highest entropy, and the efficiency or performance of heat engines that convert heat into work.

The third law of thermodynamics is related to temperature and cooling, it states that the entropy of a system at absolute zero is a definite constant.