A termodinâmica é uma área da Física que estuda as transferências de energia. Busca compreender as relações entre calor, energia e trabalho, analisando quantidades de calor trocadas e os trabalhos realizados em um processo físico.

Thermodynamic science was initially developed by researchers looking for a way to improve machines during the Industrial Revolution period, improving their efficiency.

This knowledge currently applies in various situations of our daily lives. For example: thermal machines and refrigerators, car engines and ore and petroleum processing processes.

The fundamental laws of thermodynamics govern the way heat becomes work and vice versa.

First Law of Thermodynamics

The First Law of Thermodynamics relates to the principle of energy conservation. This means that the energy in a system cannot be destroyed or created, only transformed.

When a person uses a bomb to fill an inflatable object, he is using force to put air into the object. This means that the kinetic energy causes the piston to lower. However, part of this energy turns into heat, which is lost to the environment.

The formula that represents the first law of thermodynamics is as follows:


Hess's Law is a particular case of the principle of energy conservation. Know more!


Heat transfers always occur from the warmer to the colder body, this happens spontaneously, but not the other way around. This means that thermal energy transfer processes are irreversible.

Thus, by the Second Law of Thermodynamics, it is not possible for heat to become entirely another form of energy. For this reason, heat is considered a degraded form of energy.

Also read:

  • Entropia
  • Carnot Cycle
  • Thermal Dilatation

Zero Law of Thermodynamics

The Zero Law of Thermodynamics deals with the conditions for obtaining thermal equilibrium . Among these conditions we can mention the influence of the materials that make the thermal conductivity higher or lower.

According to this law,

  1. if body A is in thermal equilibrium in contact with body B and
  2. if that body A is in thermal equilibrium in contact with body C, then
  3. B is in thermal equilibrium in contact with C.

When two bodies with different temperatures are brought into contact, the one that is warmer will transfer heat to the one that is colder. This makes the temperatures even out reaching the thermal equilibrium.

It is called the zero law because its understanding proved necessary for the first two laws that already existed, the first and second laws of thermodynamics.

Third Law of Thermodynamics

The Third Law of Thermodynamics arises as an attempt to establish an absolute reference point that determines entropy. Entropy is actually the basis of the Second Law of Thermodynamics.

Nernst, the physicist who proposed it, concluded that it was not possible for a pure zero-temperature substance to have entropy at approximately zero.

For this reason, it is a controversial law, considered by many physicists as a rule rather than a law.

Thermodynamic systems

In a thermodynamic system there may be one or several related bodies. The environment that surrounds it and the universe represent the environment external to the system. The system can be defined as: open, closed or isolated.


When the system is opened, there is mass and energy transfer between the system and the external environment. In the closed system there is only energy transfer (heat), and when it is isolated there is no exchange.

Behavior of gases

The microscopic behavior of gases is described and interpreted more easily than in other physical states (liquid and solid). This is why gases are most commonly used in these studies.

In thermodynamic studies ideal or perfect gases are used. It is a model in which particles move chaotically and interact only in collisions. Moreover, these collisions between the particles, and of them with the container walls, are considered to be elastic and last for a very short time.

In a closed system, the ideal gas presupposes a behavior that involves the following physical quantities: pressure, volume and temperature. These variables define the thermodynamic state of a gas.


Pressure (p) is produced by the movement of gas particles within the container. The space occupied by the gas inside the container is volume (v). And the temperature (t) is related to the average kinetic energy of the moving gas particles.

Also read Gas Law and Avogadro Law.

Internal Energy

The internal energy of a system is a physical quantity that helps measure how the transformations a gas goes through. This quantity is related to the variation of the temperature and the kinetic energy of the particles.

An ideal gas, formed by only one type of atom, has internal energy directly proportional to the gas temperature. This is represented by the following formula:


Exercises done

1 - A movable piston cylinder contains a gas at a pressure of 4.0.10 4 N / m 2 . When 6 kJ of heat is supplied to the system at constant pressure, the gas volume expands by 1.0.10 -1 m 3 . Determine the work performed and the internal energy variation in this situation.

Dados: P = 4,0.104 N/m2Q = 6KJ ou 6000 J ΔV = 1,0.10-1 m3 T = ? ΔU = ?

Step 1: Calculate the work with the problem data.

T = P. ΔV T = 4,0.104. 1,0.10-1 T = 4000 J

Step 2: Calculate the variation of the internal energy with the new data.

Q = T + ΔU ΔU = Q - T ΔU = 6000 - 4000 ΔU = 2000 J

Therefore, the work done is 4000 J and the internal energy variation is 2000 J.

2 - (Adapted from ENEM 2011) One engine only You can do work if you receive an amount of power from another system. In this case, the energy stored in the fuel is partly released during combustion so that the appliance can function. When the engine is running, part of the energy converted or transformed into combustion may not be used to perform work. This means that energy is leaking in another form.

According to the text, the power transformations that occur during engine operation are due to:

a) heat release inside the engine is impossible.
b) performing work by the engine being unmanageable.
c) integral conversion of heat into work is impossible.
d) transformation of thermal energy into kinetics be impossible.
e) use of potential fuel energy to be uncontrollable.

Alternativa c: conversão integral de calor em trabalho ser impossível.

As seen earlier, heat cannot be fully converted into work. During engine operation part of the thermal energy is lost and transferred to the external environment.