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The Laws of Thermodynamics : Definition, Examples

Jul 16, 2022

The laws of Thermodynamics


William Thomson first used the term thermodynamics in 1749. The science of thermodynamics examines the ideas of temperature, heat, and the interactions between heat and other energy sources. The evolution of these values is governed by the four laws of thermodynamics, which also provide a numerical representation. 

What is Thermodynamics?

Physics’ branch of thermodynamics examines how temperature, work, and heat are related to radiation, energy, and the physicochemical properties of matter. It specifically discusses how thermal energy is changed into or out of other types of energy and how this cycle affects matter. The energy that heat produces is known as thermal energy. The movement of microscopic particles inside an object produces this heat; the quicker these particles travel, will produce more heat. 


It is also known as macroscopic science and doesn’t give any significant importance to the mechanism and speed of these energy exchanges. It depends on the starting and ending conditions of the transformation. It indicates that it works with the system to support rather than the way matter is made up of molecules.

The laws of thermodynamics use a collection of basic physical quantities to describe how a thermodynamic system evolves.


The Law of Thermodynamics

The law of thermodynamics elaborates on the physical quantities like radiation, temperature, and entropy governed by the thermodynamic system at thermal equilibrium. These thermodynamic principles describe how these quantities act in different situations.

Thermodynamics laws were introduced before the full acceptance of the atomic theory. Therefore it makes sense that they do not specifically address the why and how of heat transport. They don’t consider the particular characteristics of heat transference at the molecular or atomic level; instead, they deal with the whole of energy and heat changes within a structure. 


The Zeroth Law of Thermodynamics

This law explains that when two objects are in thermal equilibrium with a third object, they are likewise in thermodynamic equilibrium.

It indicates that if systems A and C are in equilibrium temperature and systems B and C are likewise in equilibrium temperature, systems A and B must also be in thermodynamic equilibrium.


The following law can best explain the zeroth law:

Think about two glasses A and B filled with hot water. A thermometer is put in glass A, where the water is warmed until it reaches 100 °C. We can now say that the thermometer in glass A has reached its equilibrium level when it displays 100 °C.


At the same moment, when we transfer the thermometer to glass B, it continues to display 100 °C. Additionally, the thermometer and glass B are in equilibrium. We can infer from the zeroth law of thermodynamics that glasses A and B are in a state of equilibrium.

The First Law of Thermodynamics

Although it may seem tricky, the concept is very straightforward. There are only two possible outcomes when one adds heat to a system: either it changes the system’s internal energy or makes the system work (or the combination of two). These tasks require a lot of heat energy.


Many people consider the first law to be the cornerstone of the idea of energy conservation. It essentially states that energy introduced into a system can never be vanished in the process but must be put to use in some way, such as changing internal energy or producing work.

Examples of the first law

  • Through photosynthesis, plants transform the radiated energy of sunlight into electrical and chemical energy. Humans eat these plants and transform their chemical energy into kinetic energy. Similarly, humans breathe, walk, swim, and walk. All these are examples of the conversion of chemical energy into kinetic energy.
  • Even while turning on light may produce energy, electrical energy is processed in this case; therefore, it is an example.

The Second Law of Thermodynamics

The second law is phrased in various methods, as will be discussed subsequently. Still, it is essentially a law that, unlike most other laws in physics, deals exclusively with restricting what may be done rather than how to accomplish anything.

Did You Know?

The concept of the second law was originally bought by the French physicist and engineer Sadi Carnot when he created the Carnot cycle engine in 1824. German physicist Rudolf Clausius later codified it as a rule of thermodynamics.

It states that nature forbids humans from achieving particular results without investing much effort. As such, it is strongly related to energy conservation, just like the first law. This law states that no heat engine or comparable device subject to the laws of thermodynamics is ever, even in theory, completely efficient.

According to the second law, entropy always rises in an isolated system. Any isolated system will naturally progress towards thermal equilibrium, the condition in which the system’s entropy is greatest.

The entropy of a system (universe) never decreases; it only grows. This statement has a significant influence and has consequences that are often overlooked and taken for granted.

A closed system can never entirely return to the same state after undergoing a thermodynamic process. As per the second law, the universe’s entropy will always rise with time; hence this is one definition of the flow of time.

For instance, if a house is not kept tidy or clean, it will grow more disorganised over time. The entropy of the house decreases when it is finished cleaning, but because of the maintenance effort, entropy outside the house has increased, surpassing the lost entropy.

The Third Law of Thermodynamics

As per the third law, it is possible to establish an absolute temperature scale, with absolute zero being the location where a solid’s internal energy equals exactly 0.

The third law can be expressed in the following three possible ways, according to a variety of sources:

  • A finite number of operations cannot reduce any system to absolute zero.
  • As the temperature gets closer to absolute zero, the entropy of a crystalline sample of a component at its most stable position tends to zero.
  • The entropy of a system meets its constant as the temperature gets closer to absolute zero.

To gradually comprehend the third law, let’s use steam as an example:

  • Its molecules are very entropic and freely mobile.
  • Lowering the temperature below 100 °C causes the steam to turn into water, where the mobility of molecules is constrained, and the entropy of water is reduced.
  • Water progressively transforms into solid ice when it cools below 0 °C. The mobility of molecules is more constrained in this condition, and the system’s entropy decreases even more.
  • The movement of the molecules within the ice is increasingly constrained as the temperature of the substance drops and the entropy of the object continues to decrease.
  • Theoretically, the entropy must be zero when the ice reaches absolute zero. However, it is not feasible to completely chill any object.

Examples of Thermodynamics in Everyday Life

The use of thermodynamics is ubiquitous, whether a person is standing in an air-conditioned hotel or driving anywhere. A couple of these implementations are listed below:

  • The second rule of thermodynamics is the foundation upon which various forms of vehicles, including ships, trucks, and aeroplanes, operate.
  • It serves as the foundation for the three types of heat transport. Coolers, radiators and heaters frequently use the notions of heat transfer.
  • Different kinds of power plants, including nuclear and thermal plants, are studied.


Thermodynamics is governed by four laws, some of which are most significant in physics. These laws are.

  • As per the zeroth law, two thermodynamic systems are in thermodynamic equilibrium with one another if they both are in thermodynamic equilibrium with a third system.
  • According to the first law, energy can neither be generated nor destroyed. It can only alter its form. The universe’s total energy does not change during any process. In a thermodynamic cycle, the network performed by the system and the net heat it receives are equal.
  • As per the second law, the entropy of an enclosed system that is not in equilibrium will tend to rise with time, reaching its peak at equilibrium. 
  • As per the third law, as a system’s entropy decreases, it will eventually reach a stable minimum.

Frequently Asked Questions

1. Why are the laws of thermodynamics important?

Ans. The laws of thermodynamics elaborate on the physical quantities such as energy, temperature, and entropy that specify thermodynamic structures at thermal equilibrium.

2. Do the rules of thermodynamics apply to the human body?

Ans. The human body does indeed abide by the law of thermodynamics. Humans feel warm and sweaty when in a busy room with other individuals. It is how the body cools itself. Sweat absorbs body heat and radiates it. The sweat absorbs more heat as it evaporates off the body, becoming disorganised and adding heat to the atmosphere, raising the area’s temperature.

3. Describe entropy.

Ans. Entropy has a crucial role in the field of thermodynamics. The second and third laws’ central concept, which appears frequently, is represented by this law. Entropy is essentially a measurement of a system’s instability and unpredictability.

Laws of Thermodynamics


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