What is thermodynamics?

Thermodynamics is a field of research, which needs the creation of absolute and ideal states and logical and practical examination of the actual conditions. By considering all the laws and theories of thermodynamics, the chemical reactions, the occurrence of processes, and physiological functions can be predicted with this field.

Thermodynamics can be described as the field investigating, energy concerns, and changes related to the matter and its modifications. There are specific basic equations, which oversee the processes of any system concerning its surroundings. Here, the importance is given to the conservation of mass and conservation of energy.

Overview of thermodynamics

The physics department involves the transfer of different kinds of energy and the analysis linked with viewing the relations between energy and heat. The investigation of the impacts of heat, energy, and work on a structure is thermodynamics. Thermodynamics is only correlated with the changes and measurements of large-scale.

Thermodynamics is the field of physics that deals with the conditions that deal with material systems and the changes in circumstances that can happen either instantaneously or due to the communication between systems.

The system can distinguish the path the work followed and different states of work and heat. It also depends on the temperature, internal energy, or entropy. The basis of thermodynamics is established by four laws, although the first and second laws are deemed far more significant.

Thermodynamics system, surroundings, and types

A system is usually described as the matter or group of matter in an enclosed area that is under consideration, while the surroundings are those that cover the rest of the environment or universe, excluding the system under consideration. The boundaries that divide a system from its surroundings can be real or imaginary as well as fixed or movable. When analyzing the boundaries, the boundaries have no thickness, no weight, and no volume.

Systems can be divided into the following various types based upon their boundaries:

  • Closed system or control mass
  • Open system or control volume
  • Isolated system
  • Rigid system
  • Adiabatic system

Closed system or control mass

In this thermodynamic system, the mass is conserved. No matter can enter or leave the system; that is, there will be no exchange of matter between the system and its surroundings. Nevertheless, the energy can move through the boundaries and can even be transformed to different forms favorable. Here, the volume of the system does not have to be fixed. It can change respectively.

Open system or control volume

Open systems are those areas, which are accurately recognized or selected in any space. Here, as compared to closed systems, both energy and mass can move back and forth via the boundary.

Isolated system

In this system, neither energy nor mass can be sent or received back and forth; that is, there is no communication between the system and surroundings.

Rigid system

A rigid system can be described as a kind of system that can interact with the surroundings of the pre-elected system only by energy.

Adiabatic system

This type of system can be the one that can be open or closed but doesn't transfer energy in any form.

What are the laws of thermodynamics?

The laws of thermodynamics are as follows:

  • Zeroth law of thermodynamics
  • First law of thermodynamics
  • Second law of thermodynamics
  • Third law of thermodynamics

Zeroth law of thermodynamics

The zeroth law is one of the four laws of thermodynamics. Ralph H. Fowler established this law. Although it is named the zeroth law, it came into existence much later than the other three laws of thermodynamics. The importance of the zeroth law is: it shows the significance of temperature as an indicator for thermal equilibrium. Thermodynamics Zeroth Law asserts that “If each of the two objects is in thermal equilibrium with a third object, then all objects are in equilibrium with one another.” Thermal equilibrium assures that there will be no heat shift from one to the other as two objects come in contact with one another and are isolated by a heat permeable membrane.

Three systems A, B, and C are in thermal equilibrium.
Zeroth law

First law of thermodynamics

The first law is one of the most fundamental laws applied in physics. The first law asserts that energy can be transformed from one form to another. It can neither be produced nor can be destroyed. Thermodynamics deals with the exchange of heat, work, or matter between the system and the surroundings.

A system with surrounding in which both matter and energy are exchanged from system to surrounding or surrounding to the system.
The first law of thermodynamics

The correlation between heat and work and the principle of internal energy must be understood. Internal energy is the energy connected with the system's molecules, which include kinetic energy and potential energy which is the thermodynamic property of the system. It is accompanied by several energy transitions and conversions if a system undergoes some changes due to the interaction of heat, function, and internal energy. There is no net shift in the overall energy during these transitions, however. The first law is also known as the conservation of energy principle.

The equation for the first law is as follows:


Here, ΔU is the change in the internal energy, Q is the heat added, and W is the work done by the system.

Second law of thermodynamics

One of the common fundamental laws of nature is the second law of thermodynamics, having ardent implications. In brief, the second law tells that the disorder level of the universe is regularly growing or an isolated system's entropy can never decline over time. Systems are prone to show more random behavior from ordered behavior.

One indication of the second law is that heat spontaneously moves from a hotter area to a cooler area but does not move spontaneously vice versa. It applies to anything that flows, which means that flow is naturally downwards rather than upwards.

Heat flows from the hot part of an object to the cold part.
The second law of thermodynamics

Entropy is the degree of disorder of a system. If the number of possible microstates for a system is higher, the system will be more disordered, and thus, the system's entropy will also be higher. The standard international unit of entropy is J/K. An increase in entropy is given as ΔS>0, and a decrease in entropy is given as ΔS<0. Thus, the change in entropy is the difference between the initial and final states of entropies. The number of microscopic states and microstates correlated with a system determine the magnitude of its entropy. Larger the number of microstates, higher is the entropy. The expression for the difference in entropy is given as:


Here, ΔS is the difference in the entropy, Sf is the entropy in the final state, and Si is the entropy in the initial state.

Third law of thermodynamics

As the temperature reaches absolute zero, the third law of thermodynamics is concerned with the limiting performance of processes. Most determinations in thermodynamics only use entropy differences, so the entropy scale's zero points are often not significant. However, for completeness, the third law is considered since it defines the zero entropy condition. The following phenomenon in a closed system can be found at a temperature of zero Kelvin:

  • No heat is stored in the system.
  • The atoms and molecules observed in the structure are at their lowest energy points. Therefore there is only one open microstate for a device at absolute zero; it is the ground state. The entropy of such a system is precisely zero, as per the third law of thermodynamics.

Common Mistakes

Students get confused between the system and surroundings. A system can be described as an amount of matter or an area in space taken for research, while the surrounding is the mass or the area outside the system.

Students get confused between internal energy and enthalpy. Enthalpy is considered as the heat energy that is consumed or evolved during the progress of a reaction, while internal energy is the addition of kinetic energy and potential energy of the system.

Context and Applications

The topic of the laws of thermodynamics is very much significant in the several professional exams and courses for undergraduate, diploma level, graduate, and postgraduate. For example:

  • Bachelors of Technology
  • Bachelors in Science
  • Masters of Technology
  • Masters in Science
  • Internal energy of ideal gas
  • Thermodynamics processes
  • Principle of conservation of energy
  • Forms of energy
  • Maxwell velocity distribution

Practice Problems

Q1: The S.I. unit of temperature is:

(a) Kelvin

(b) Rankine

(c) Centigrade

(d) Celsius

Correct option: (a)

Explanation: The international standard unit used for the measurement of temperature is Kelvin. Temperature is also measured in degree centigrade and Rankine.

Q2: Among the four options, which of the following law declares that the internal energy of a gas is a function of temperature?

(a) Boyle’s law

(b) Charles law

(c) Joule’s law

(d) None of these

Correct option: (c)

Explanation: Joule's law states that the specific internal energy of the gas is a function of temperature only. The magnitude of the internal energy of a gas is independent of its pressure and volume.

Q3: The first law of thermodynamic is defined about:

(a) Pressure

(b) Volume

(c) Temperature

(d) Internal energy

Correct option: (d)

Explanation: The first law of thermodynamics states the law of conservation of energy. It describes that the system's internal energy is equal to the difference between heat supplied and work done by the system.

Q4: The second law of thermodynamics is related to:

(a) Internal energy

(b) Enthalpy

(c) Entropy

(d) Pressure

Correct option: (c)

Explanation: The second law of thermodynamics states that the entire entropy of the isolated system will not decrease over time. It also states that change in entropy in the universe is always positive.

Q5: The difference between the heat added and work done for a closed system is known as:

(a) Enthalpy

(b) Entropy

(c) Pressure

(d) Internal energy

Correct option: (d)

Explanation: The internal energy for a closed system is equal to the difference of heat supplied to the system and work done by the system. It is formulated as: Q=dU+W.

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