The Laws of Thermodynamics

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The chemical energy stored in molecules can be released as heat in chemical reactions that occur when the fuel methane, coal or cooking gas burns in the air. Chemical energy could also be utilized to carry out mechanical work when fuel is burned in an engine or to generate electric energy via an electrolytic cell similar to a dry cells. So, the various kinds of energy are interconnected and, under certain conditions, they may transform into a different form. The study of these transformations is the focus of thermodynamics. The thermodynamic laws cover energy changes in macroscopic systems with a vast amount of molecules instead of microscopic systems that contain a handful of molecules. Thermodynamics doesn’t care the manner in which and how fast the energy transformations performed, but rather is based on the initial and final conditions of the system that is undergoing a changes. The laws of thermodynamics only apply for systems that are at equilibrium or is moving between equilibrium and a different equilibrium. Properties of the macroscopic kind like temperature and pressure don’t change over time when an equilibrium state system.

Thermodynamics is concerned with the notions of temperature and heat, as well as the inter-conversion of heat as well as other types of energy. The laws that govern thermodynamics regulate the behavior of these variables and give a detailed description. William Thomson, in 1749 was the first to coin the term “thermodynamics”.

What is Thermodynamics?

Thermodynamics in Physics is a field that examines heat, work and temperature, as well as their connection to radiation, energy, and the physical property of matter.


To be more specific the process of how thermal energy is transformed into or transferred to various forms of energy, and how matter is affected through this process. Thermal energy is energy generated by heat. This heat is created through the movement of tiny particles inside an object. The more quickly these particles move, the greater the amount of heat generated.

Thermodynamics doesn’t care about the manner in which and how fast the energy transformations performed. This is because it is founded on original and final state that undergoes the transformation. It should be noted that Thermodynamics is an atomic science. It concentrates on the bulk system, but does not address the molecular nature of matter.

Different Branches of Thermodynamics

Thermodynamics can be classified into four distinct branches:

  1. Classical Thermodynamics: During classical thermodynamics, the nature of matter is studied using an approach that is macroscopic. Temperature and pressure are considered to help analyze other properties and determine the nature of the matter that is being studied.
  2. Statistical Thermodynamics: In statistics thermodynamics, all molecules are in the spotlight, i.e. the characteristics of each molecule and the way they interact are considered to determine the behavior of a particular group of molecules.
  3. Chemical Thermodynamics: Chemical thermodynamics refers to an investigation into how heat and heat are related to one another in chemical reactions as well as changes of states.
  4. Equilibrium Thermodynamics: Equilibrium thermodynamics studies the transformations of matter and energy as they move towards the equilibrium state.

Thermodynamic Terms


A thermodynamic system is a particular area of matter that has defined boundaries on which our concentration is. The boundary of the system could be imaginary or real as well as deformable or fixed. There are three kinds of systems:

  1. Open system: An open system is one in which there is an exchange of matter and energy between the system and its surroundings. The presence of reactants within an unclosed beaker can be an example of an open system. The boundary in this case represents an imaginary surface which surrounds the beaker as well as reactants.
  2. Closed System: A closed system, there isn’t any exchange of matter, however the exchange of energy can be made between the system and its surroundings. The presence of reactants in closed vessels that are comprised of conductors e.g. copper, copper or steel is an example of a closed system.
  3. Isolated System: When a system is isolated, it does not allow the exchange of any matter or energy between the surrounding and system. The presence of reactants within thermos flasks or other vessel that is closed and insulated is an example of an isolation system.
System | Source:
Type of systemMass flowWorkHeat
Isolated System
Open System
Closed System
Surrounding and system
Surrounding and system


Anything outside of the system that has direct influence on the actions that the system performs is referred to as surrounding.

The environment encompasses all things other than the system. The system and the surrounding area together form the universe . The universe = The system + The surroundings. The entire universe, excluding the system isn’t directly affected by changes that take place within the system. So in all senses the environment is the part of the universe that can interact to the systems. The most common definition is the area of space is the system’s neighborhood is the system’s surroundings.

In the example above, if you are studying the reaction of two different substances A and B kept in a glass beaker the beaker that contains reactant mixtures is considered to be the system. Likewise, the space in which the beaker is kept is its surrounding. It is important to note that the system could exist as a physical boundary such as the beaker and test tubes or it could be defined by the set of Cartesian coordinates that specify an particular volume in space. It is essential to consider the system as separate from the surrounding area by a type of wall, which could be imaginary or real. The wall that divides the system from its surroundings is known as a boundary. It is designed to enable us to monitor and monitor the movements of energy and matter within and outside the systems.

Thermodynamic Process

A system goes through a thermodynamic process whenever there is a change in energy within the system caused by changes in volume, pressure or internal energies.

If the condition of a system alters, then it undergoes the course of a process. The sequence of states through which the system travels determines the path through which the procedure proceeds. When, at the end of the process properties are back to their initial values The system has gone through a cyclic process, or cycle. It is important to note that even if the system is back to its initial state and completed a cycle, the status of the surrounding environment may have changed.

There are four kinds of thermodynamic processes that possess distinct properties. they are:

  • Adiabatic process – A procedure that does not allow heat to flow into and out of the system is observed.
  • Isochoric Process – A procedure that does not result in any change in volume, and the system performs nothing.
  • Isobaric Process – A method where no pressure change occurs.
  • Isothermal Process – A method that does not see changes in temperature occur.

A thermodynamic cycle is a procedure or combination of actions that are conducted in a way that the beginning and end conditions of the system are identical. A thermodynamic cycle may also be called a cyclic process or cycles.

Thermodynamic Equilibrium

In a certain state, the properties of a system are fixed values. Therefore if the value of any properties changes, then the state of the system changes to a different one. If a system is in equilibrium, there are no change in property values will occur when it is separated from the environment around it.

  • When the temperature remains identical throughout the entire system, we can consider it to be operating in equilibrium thermally.
  • If there is no changes in the pressure at any part of the system, then we can consider it to be operating in equilibrium.
  • If the chemical composition of an entire system doesn’t change with time, then we can say that the system is considered to be in equilibrium chemically.
  • Phase equilibrium in a two-phase system is when the weight of each phase is at its equilibrium.

A thermodynamic structure is considered to be at thermodynamic equilibrium when it is in equilibrium with chemical as well as mechanical equilibrium and thermal equilibrium, and when the key parameters no longer fluctuate in the passage of time.

Thermodynamic Properties

Thermodynamic properties are defined as the most significant characteristics of a system which are capable of describing the state of the system. These thermodynamic characteristics of a substance can be classified in a variety of ways.

  • Measured properties: The properties of the system that are directly accessible from the laboratory are called measured properties. Examples are temperature, volume and pressure.
  • Fundamental properties: The properties of the system directly linked to the thermodynamic laws that are fundamental to the system are called fundamental properties. Examples include internal energy and Entropy
  • Derived properties: Property of the system that exhibit particular relations and consist of the combination with measured as well as derived properties are referred to as derived properties. Examples include: Gibbs, Enthalpy Free energy.

System’s thermodynamic properties can be classified into two classes:

  • Extensive property: The value of an extensive property’s value is determined by the amount or size of the matter within the system. However, the large variables can help define the system under study. Examples: volume, mass in-built energy and enthalpy heat capacity and entropy, the Gibbs Free Energy.
  • Intensive property: The ones which do not depend on the size or quantity of matter are referred to as properties that are intensive. The intensity property can vary between different locations in the entire system at any time. Examples of this are: Density, Pressure temperature, specific volume specific entropy, thermal Conductivity Thermal Expansion, Compressibility and numerous other.

Thermodynamic state

The thermodynamic condition of a system is determined by defining the values of a set specific properties that can be measured to establish all other aspects. Fluid systems the most common properties are volume, pressure and temperature. Complex systems might necessitate the specification of unique characteristics. For instance the condition that an electrical battery is in demands to be specified in terms of quantity of charge it has.

Properties can be extensive or intense. The properties that are extensive are also additive. Therefore, if the system is broken down into several sub-systems, then the worth of the property across the whole system is the value of all the components. Volume is an expansive property. Intense properties don’t depend on the amount of matter that is present. Pressure and temperature are both intensive properties.

Specific properties are properties that are complex per unit mass . They are indicated by lower cases letters. Examples:

specific volume = V/m = v

Specific properties are in-depth because they don’t depend on the size of the system.

The basic properties of a system are the same across the entire system. However, in general the characteristics of a system could differ from one point to the next. It is common to study a system’s general properties by subdividing the entire system (either conceptually or actually) into several simple systems, in each of which the properties are believed to be the same.

It is crucial to remember that properties are only used to describe states at the point of equilibrium.

Laws of Thermodynamics

Thermodynamics laws describe the essential physical quantities like energy temperature, and entropy that define thermodynamic systems in equilibrium temperature. The thermodynamics laws define the way these quantities work in various conditions.

There are four thermodynamic laws and are listed in the following:

  1. Zeroth law of thermodynamics
  2. First law of thermodynamics
  3. Second law of thermodynamics
  4. Third law of thermodynamics

Zeroth law of thermodynamics

The Zeroth law of thermodynamics states that if two bodies are individually in equilibrium with a separate third body, then the first two bodies are also in thermal equilibrium with each other.

This means that if A is in equilibrium with C, as well as system B in equilibrium with C, then systems A as well as B are in equilibrium with thermal energy.

An example of Zeroth Law

Think about 2 cups B and A filled with boiling water. If a thermometer is put inside cup A will be heated through the water to read 100°C. If it reads 100 degrees Celsius then we can say that thermometer was in equilibrium cup A. If we move the thermometer into cup B in order to measure the temperature it will continue to read 100 degrees Celsius. It is in equilibrium also with the cup. If we keep in mind the law that is zero in thermodynamics, one can say that cups A and B both are both in equilibrium with one another.

Zeroth law of thermodynamics
Zeroth law of thermodynamics

A thermodynamic law that is zeroth in its application permits us to make use of thermometers to determine the temperature of two objects we would like.

First law of thermodynamics

The First Law of Thermodynamics states that heat is a form of energy, and thermodynamic processes are therefore subject to the principle of conservation of energy. This means that heat energy cannot be created or destroyed. It can, however, be transferred from one location to another and converted to and from other forms of energy.

First Law Of Thermodynamics
First Law Of Thermodynamics

As per this law, some of the heat that is supplied to the system is utilized to alter the energy within the system, while the remainder is utilized in performing the work of the system.

It is mathematically represented as

Δ Q = Δ U + W


  • ΔQ is the heat given or lost
  • ΔU is the change in internal energy
  • W is the work done

We can also represent the above equation as follows,

Δ U = Δ Q W

Therefore, we can deduce from the equation above that the amount (ΔQ – W)  is not dependent on the route taken to alter the state. Furthermore, we can conclude that the internal energy will increase when heat is transferred to the system, and the reverse is true.

Sign Conventions

The table below illustrates the sign conventions that are appropriate for the three quantities in different circumstances:

ΔU (change in internal energy)Q (heat)W (work done on the gas)
is “+” if temperature increasesis “+” if heat enters gasis “+” if gas is compressed
is “-” if temperature decreasesis “-” if heat exist gasis “-” if gas expands
is “0” if temperature is constantis “0” if no heat is exchangedis “0” if volume is constant

Examples of First Law Of Thermodynamics

  • Plants convert the energy from sunlight into chemical energy by photosynthesis. We eat plants , and transform this chemical energy to kinetic energy when we walk, swim in the air, breathe, and browse this page.
  • Turning on the lights may appear to generate energy, but it’s the electrical energy that is converted.

Second Law of Thermodynamics

Second law of thermodynamics states that the entropy in an isolated system always increases. Any isolated system spontaneously evolves towards thermal equilibrium—the state of maximum entropy of the system.

Second law of thermodynamics stipulates that any event that happens spontaneously is always accompanied by an increase in the amount of entropy (S) in the world. Simply put this law states that the entropy of a system isolated is never going to decrease as time passes by.

However, in certain situations where the system is at equilibrium in thermodynamics or is going through an irreversible process, the total entropy of the system and its surrounding remains in a constant state. Second law called”the Law of Increased Entropy.

This second rule clearly shows that it is not possible in converting heat into mechanical energy with 100 % effectiveness. As an example, if we take a look at the piston of an engine it is heated in order to increase the pressure and power a piston. But, as the piston is moving there’s always left-over heat which is not used for doing any other task. It is wasted heat and is discarded. In this scenario this is accomplished by moving it to a sink, or in the case of an engine in a car it is removed by releasing the gas and the air mix into the air. In addition, heat produced by friction, which is usually not usable, should be removed off the engine.

Second Law of Thermodynamics Equation

Mathematically, the second law of thermodynamics is represented as;

ΔSuniv > 0

where ΔSuniv is the change in the entropy of the universe.

Entropy is a measurement of the randomness of the system , or it’s the measure the amount of energy, or even chaos in the system. It can be viewed as an index of quantitative value that defines the energy quality.

In addition, there are several causes that result in an increase in the entropy of an enclosed system. In closed systems, although the mass stays constant, there is a exchange of heat between the surrounding. This shift in the temperature content causes disturbances within the system, thereby increasing the energy content of the system.

Additionally, internal changes can be observed in the movement of the molecules within the system. This can cause disturbances that create irreversible effects within the system, leading to the increase in its the entropy.

Different Statements of The Law

Two statements are made regarding the second law of thermodynamics that are

1. Kelvin- Plank Statement

  • It is not possible for the heat engine to build the network over a full cycle if it is able to exchange heat only with other bodies with a single temperature.
  • Exceptions: If Q2 =0 (i.e., Wnet = Q1, or efficiency=1.00), The heat engine creates work throughout the entire cycle by exchanging heat just one reservoir, thereby in violation of the Kelvin-Planck assertion.

2. Clausius Statement

It is difficult to build devices operating in a circular cycle that could transfer the warmth of a colder object to a warmer one , without needing any effort. Furthermore, energy won’t be transferred from a low temperature object to a high-temperature object. It is crucial to remember that we’re referring to the transfer of energy. The transfer of energy can occur between a cold object and hot objects through the energy particles, and electromagnetic radiation. But the net transfer of energy will happen between the warm and cold one in any process that is spontaneous. Some form of work is required to transfer the heat to the cold object. Also, if the compressor is controlled from an external power source, the refrigerator will not be able operate. The heat pump and the refrigerator are based on Clausius’s assertion.

Clausius Statement
Clausius Statement

Both statements of Kelvin and Clausius are identical i.e devices that violate Clausius’s assertion will also violate Kelvin’s claim and vice the reverse.

Clausius Statement
Clausius Statement

Alongside these claims in addition to these statements, the French scientist by the name of Nicolas Leonard Sadi Carnot also called”the “father of thermodynamics,” was the one who invented his Second Law of Thermodynamics. But, according to his assertion, he stressed the necessity of using caloric theories in the formulation of the law. Caloric (self repellent liquid) refers to heat. Carnot noticed that some caloric is removed during the motion.

Perpetual Motion Machine of the Second Kind (PMM2)

The device that generates work in conjunction with a single reservoir of heat is called one of the perpetual motion machines of second variety (PMM2). Also, a machine that does not conform to two laws of thermodynamics can be described as a permanent movement machine second variety.

Perpetual Motion Machine of the Second Kind (PMM2)
Perpetual Motion Machine of the Second Kind (PMM2)

So, a heating engine must interact with at minimum two thermal reservoirs of different temperatures to generate work during the course of a cycle. In the event of an inconsistency in temperature, motivation power (i.e. work) is produced. If the bodies that exchange heat have finite capacities for heat it will produce work by the heat engine up to the temperature of both bodies is equalized.

Perpetual Motion Machine of the Second Kind (PMM2)
Perpetual Motion Machine of the Second Kind (PMM2)

Examples of the second law of thermodynamics

If a space isn’t cleaned or tidied then it will become messy and chaotic with time. If the room is clean the entropy of the room decreases however, the effort required to clean has led to an increase in the entropy of the space, which is greater than the amount of entropy lost.

Third Law Of Thermodynamics

Third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero.

Entropy, as indicated by the letter ‘S’, is a measure for disorder or randomness of an enclosed system. It is directly proportional to the number of microstates (a permanent microscopic state that is utilized by an entire system) that the system is able to access, i.e. the more microstates a closed system can be able to occupy, the higher its energy consumption. The microstate at which the system’s energy is at its lowest is called the ground state the system.

At temperatures of zero Kelvin the following phenomena are observed in a closed system

  • The system doesn’t contain any heating.
  • All the molecules and atoms within the entire system remain at their lowest energie levels.

Thus, a system that is that is at zero absolute has one microstate that is accessible – its the ground state. In accordance with the thermodynamic third law the entropy of the system is absolutely zero. The law was developed by the German scientist Walther Nernst between the years 1906 between 1912 and 1906.

Alternate Statements of the 3rd Law of Thermodynamics

The Nernst statement of the third law of thermodynamics implies that it is not possible for a process to bring the entropy of a given system to zero in a finite number of operations.

The American physical chemical chemists Merle Randall as well as Gilbert Lewis stated this law differently according to the following formula: when the entropy value of every element (in their perfectly crystallized states) is taken to be 0 when the temperature is absolute and the entropy for every substance has to be positive and finite value. However, the entropy of a substance at absolute zero could be zero, which happens when a perfectly formed crystal is taken into consideration.

The Nernst-Simon formulation in the third law of thermodynamics could be written in the form the following: for a condensed system undergoing an isothermal process that is reversible in nature, the associated entropy change approaches zero as the associated temperature approaches zero.

A further implication that the law 3 has the fact that The exchange of energy among two thermodynamics systems (whose combination forms one isolated unit) is restricted.

Mathematical Explanation of the Third Law

According to statistics the entropy in an entire system can be calculated using the following equation:

S – S0 = 𝑘B ln𝛀


  • S is the entropy of the system.
  • S0 is the initial entropy.
  • 𝑘B denotes the Boltzmann constant.
  • 𝛀 refers to the total number of microstates that are consistent with the system’s macroscopic configuration.

Now, for a perfect crystal that has exactly one unique ground state, 𝛀 = 1. Therefore, the equation can be rewritten as follows:

S – S0 = 𝑘B ln(1) = 0 [because ln(1) = 0]

When the initial entropy of the system is selected as zero, the following value of ‘S’ can be obtained:

S – 0 = 0 ⇒ S = 0

Thus, the entropy of a perfect crystal at absolute zero is zero.

What is Enthalpy?

Enthalpy is the measure of heat in the thermodynamic process. The amount of enthalpy corresponds to the total amount of heat contained in an entire system, or the internal energy of the system, as well as the product of pressure and volume.

Mathematically, the enthalpy, H, equals the sum of the internal energy, E, and the product of the pressure, P, and volume, V, of the system.

H = E + PV

What is Entropy?

The thermodynamic entropy is a quantity that’s value is contingent on the physical condition or state of an entire system. It is a thermodynamic parameter used to determine the amount of irregularity or disorder.

For instance the entropy of solids, in which particles aren’t free to move, is lower than the entropy of gas, in which particles are free to overflow the container.

Thermodynamics Examples in Daily Life

No matter if we’re in a cool room or in a vehicle The application of thermodynamics is all around us. Here are some of these applications below:

  • The various kinds of vehicles like trucks, planes and ships are based on the 2nd law of thermodynamics.
  • The three types of heat transfer operate in accordance with thermodynamics. The theories of heat transfer are used extensively in heaters, radiators, and coolers.
  • Thermodynamics plays a role with the research of various types of power plant, including thermal power plants, nuclear power plants and power plants and others.




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