Thermodynamics Notes: 13 Facts You Should Know

Thermodynamics Notes

Thermodynamics: The branch of physics and science that deals with the correlation between heat and other forms of energy that can be transferred from one form and place to another can be defined as thermodynamics. Certain terms to know about when examining thermodynamics can be better understood by following term.


Heat is a form of energy, the transfer of energy from one body to other happens due to temperature difference and heat-energy flows from a hot body to a cold body, to make it thermal equilibrium and plays a very critical role in the principle of thermodynamics.


An external force applied in the direction of displacement which enables the object to move a particular distance undergoes a certain energy transfer which can be defined as work in the books of physics or science. In mathematical terms, work can be described as the force applied multiplied by the distance covered. If the displacement is involved at an angle Θ when force is exerted, then the equation can be:

W = fs

W = fscosӨ


 f= force applied

s= distance covered

Ө= displacement angle

Thermodynamics is a very vital aspect of our daily life. They follow a set of laws to abide by when applied in terms of physics.

Laws of thermodynamics

The Universe, though it is defined by many laws, only very few are mighty. The laws of thermodynamics as a discipline were formulated and opened ways to numerous other phenomena varying from refrigerators, to chemistry and way beyond life processes.

The four basic laws of thermodynamics consider empirical facts and construe physical quantities, like temperature, heat, thermodynamic work, and entropy, that defines thermodynamic operations and systems in thermodynamic equilibrium. They explain the links between these quantities. Besides their application in thermodynamics, the laws have integrative applications in other branches of science. In thermodynamics, a ‘System’ can be a metal block or a container with water, or even our human body, and everything else is called ‘Surroundings’.

The zeroth law of thermodynamics obeys the transitive property of basic mathematics that if a two systems are in thermal equilibrium with a 3rd system, then these are in thermal equilibrium state with each other too.

The basic concepts that need to be covered to comprehend the laws of thermodynamics are system and surroundings.

System and Surroundings

The collection of a particular set of items we define or include (something as small as an atom to something as big as the solar system) can be called a system whereas everything that does not fall under the system can be considered as the surroundings and these two concepts are separated by a boundary.

For example, coffee in a flask is considered as a system and surroundings with a boundary.

Essentially, a system consists of three types namely, opened, closed, and isolated.

thermodynamics note
Figure: System and Surroundings in thermodynamics

Thermodynamics equations

The equations formed in thermodynamics are a mathematical representation of the thermodynamic principle subjected to mechanical work in the form of equational expressions.

The various equations that are formed in the thermodynamic laws and functions are as follows:

●      ΔU = q + w (first law of TD)

●      ΔU = Uf – Ui (internal energy)

●      q   =   m Cs ΔT (heat/g)

●      w   =   -PextΔV (work)

●      H = U + PV



ΔU = ΔH – ΔnRT ( enthalpy to internal energy)

●      S  =  k ln Ω (second law in Boltzman formula)

●      ΔSrxn° = ΣnS° (products) – ΣnS° (reactants) (third law)

●      ΔG = ΔH – TΔS (free energy)

First law of thermodynamics

The 1st law of thermodynamics elaborate that when energy (as work, heat, or matter) carries in or out of a system, the system’s internal energy will change according to the law of conservation of energy (which means that energy can neither be created nor destroyed and can only be transferred or converted from one form to another), i.e., perpetual motion machine of the 1st kind ( a machine which actually works without energy i/p) are un-attainable.

For example, lighting a bulb is a law of electrical energy being converted light energy which actually illuminates and some part will be lost as heat energy.

 ΔU= q + w

  • ΔU  is the total internal energy change of a system.
  • q  is the heat transfer between a system and its surroundings.
  • w  is the work done by the system.
Thermodynamics notes : First law of Thermodynamics

Second law of thermodynamics

The second law of thermodynamics defines an important property of a system called entropy. The entropy of the universe is always increasing and mathematically represented as ΔSuniv > 0 where ΔSuniv is the change in the entropy of the universe.


Entropy is the measurement of the system’s randomness or it is the measure of energy or chaos with in an isolate system, this can be contemplated as a quantitative index that described the classification of energy.

The second law also gives the upper limit of efficiency of systems and the direction of the process. It is a basic concept that heat does not flow from an object of lesser temperature to an object of greater temperature. For that to happen, and external work input is to be supplied to the system. This is an explanation for one of the fundamentals of the second law of thermodynamics called “Clausius statement of second law “. It states that “It is impossible to transfer heat in a cyclic process from low temperature to high temperature without work from an external source”.

2nd law 1
Figure: Second law of thermodynamics Image source : NASA

 A real-life example of this statement is refrigerators and heat pumps. It is also known that a machine that can’t convert all of the energy supplied to a system cannot be converted to work with an efficiency of 100 percent. This then guides us to the following statement called the “Kelvin-Planck statement of second law”. The statement is as follows “It is impossible to construct a device (engine) operating in a cycle that will produce no effect other than extraction of heat from a single reservoir and convert all of it into work”.

Mathematically, the Kelvin-Planck statement can be written as: Wcycle ≤ 0 (for a single reservoir) A machine that can produce work continuously by taking heat from a single heat reservoir and converting all of it into work is called a perpetual motion machine of the second kind. This machine directly violates the Kelvin-Planck statement. So, to put it in simple terms, for a system to produce to work in a cycle it has to interact with two thermal reservoirs at different temperatures.

Thus, in layman’s term the 2nd law of thermodynamics elaborates, when energy conversion happens from one to other state, entropy will not decreases but always increases regardless within a closed-system.

Third law of thermodynamics

In layman’s terms, the third law states that the entropy of an object approaches zero as the absolute temperature approaches zero (0K). This law assists to find an absolute credential point to obtain the entropy. The 3rd law of thermodynamics has 2 significant characteristics as follows.

The sign of the entropy of any particular substance at any temperature above 0K is recognized as positive sign, and it gives a fixed reference-point to identify the absolute-entropy of any specific substance at any temp.

Figure: TS diagram Image source: Wikipedia commons

Different measures of energy


Energy is defined as the capacity to do work. It is a scalar quantity. It is measured in KJ in SI units and Kcal in MKS units. Energy can have many forms.


 Energy can exist in numerous forms such as

  • 1. Internal energy
  • 2. Thermal energy
  • 3. Electrical energy
  • 4. Mechanical energy
  • 5. Kinetic energy
  • 6. Potential energy
  • 7. Wind energy and
  • 8. Nuclear energy

This further categorized in

(a) Stored energy and (b) Transit energy.

Stored Energy

The stored form of energy can be either of the following two types.

  • Macroscopic forms of energy: Potential energy and kinetic energy.
  • Microscopic forms of energy: Internal energy.

Transit Energy

Transit energy means energy in transition, basically represented by the energy possessed by a system that is capable of crossing the boundaries


 It is a transfer form of energy that flows between two systems under the temperature difference between them.

(a) Calorie (cal) It is the heat needed to raise the temperature of 1 g of H2O by 1 deg C

(b) British thermal unit (BTU) It is the heat needed to raise the temperature of 1 lb of H2O by 1 deg F


An energy interaction between a system and its surroundings during a process can be regarded as work transfer.


Enthalpy (H) defined as the summation of the system’s internal energies and the product of it’s pressure and volume and enthalpy is a state function used in the field of, physical, mechanical, and chemical systems at a constant pressure, represented in Joules (J) in SI units.

Relationship between the units of measurement of energy (with respect to Joules, J)

UnitEquivalent to
1eV1.1602 x 10-19 J
1 cal4.184 J
1 BTU1.055 kJ
1 W1 J/sec

Table: Relation table 

Maxwell’s Relations

The four most traditional Maxwell relations are the equalities of the second derivatives of every one of the four thermodynamic perspectives, concerning their mechanical variables such as Pressure (P) and Volume (V) plus their thermal variables such as Temperature (T) and Entropy (S).

lN3F6ycg qCMH2raNFdQs NNj chwCPKYnquRMUnVAI5hPc9NkCiQ7yz mfLNADWrE

Equation: common Maxwell’s Relations


This article on Thermodynamics gives you a glimpse of the fundamental laws, definitions, equations relations, and its few applications, although the content is short, it can be used to quantify many unknowns. Thermodynamics finds its use in various fields as some quantities are easier to measure than others, though this topic is profound by itself, thermodynamics is fundamental, and its fascinating phenomena gives us a deep understanding of the role of energy in this universe

Some questions related to the field of Thermodynamics

What are the applications of thermodynamics in engineering?

There are several applications of thermodynamics in our daily lives as well as in the domain of engineering. The laws of thermodynamics are intrinsically used in the automobile and the aeronautical sector of engineering such as in IC engines and gas turbines in the respective departments. It is also applied in heat engines, heat pumps, refrigerators, power plants, air conditioning, and more following the principles of thermodynamics.

Why is thermodynamics important?

There are various contributions of thermodynamics in our daily life as well as in the engineering sector. The processes that occur naturally in our daily life fall under the guidance of thermodynamic laws. The concepts of heat transfer and the thermal systems in the environment are explained by the thermodynamic fundamental which is why the subject is very important to us.

How long does it take a bottle of water to freeze while at a temperature of 32˚F?

 In terms of a conceptual solution to the given question, the amount of time taken to freeze a bottle of water at a temperature of 32F will be depending upon the nucleation point of the water which can be defined as the point where the molecules in the liquid are gathered to turn into a crystal structure of solid where pure water will freeze at -39C.

Other factors into consideration are the latent heat of fusion of water which is the amount of energy required to change its state, essentially liquid to solid or solid to liquid. The latent heat of water at 0C for fusion is 334 joules per gram.

What is cut-off ratio and how does it affect the thermal efficiency of a diesel engine?

Cut off ratio is inversely proportional to the diesel cycle as there is an increase in efficiency of the cut-off ratio, there is a decrease or reduction in the efficiency of a diesel engine. The cut-off ratio is based on its equation where the correspondence of the cylinder volume before and after combustion is in proportion to each other.

It goes as follows:

Mj Ib6Mi8W4mf6Q6fJ8ghe0KVGXrgdTjXc6BkTr4nqx6mg 1BqNHIA UVHBQP jCYZ9LoCkxatHjmCa7Fa4Vk8mcDqD0wyMx2QcazAtmsG u WIpjaYZCOGPOj92cjITeSjaEqQ
 Equation 1: Cut-Off Ratio

What is a steady-state in thermodynamics?

The current state of a system that contains a flow through it over time and the variables of that particular process remains constant, then that state can be defined as a steady-state system in the subject of thermodynamics.

What are the examples of fixed boundary and movable boundary in the case of thermodynamics?

A moveable boundary or in other terms, control mass is a certain class of system where matter cannot move across the boundary of the system while the boundary itself acts as a flexible character that can expand or contract without allowing any mass to flow in or out of it. A simple example of a moveable boundary system in basic thermodynamics would be a piston in an IC engine where the boundary expands as the piston is displaced while the mass of the gas in the cylinder remains constant allowing work to be done.

5ormIhuE6JPsN Og1pk2xsKt3x3WtRRbpVzne2HBNSTyDGjszLIgEXkLr3BhdWzARk1f2lTSwv XjZHGsH2A79In7X8qqL1csLigCTBzqv3inA NMtg91TiUb0KPO XYD6OUM2qI
Figure: Piston movement

Whereas in the case of a fixed boundary, there is no work being permitted as they keep volume constant while the mass is allowed to flow in and out freely in the system. It can also be called a control volume process. Example: gas flowing out of a household cylinder connected to a stove while the volume is fixed.

 What are the similarities and dissimilarities of heat and work in thermodynamics?


  • ●  Both these energies are considered as path functions or process quantities.
  • ●  They are also inexact differentials.
  • ●  Both the form of energies are not stored and can be transferred in and out of the system following the transient phenomenon.


  • ●  Heat flow in a system is always associated with the entropy function whereas there is no entropy transfer along with the work system.
  • ●  Heat cannot be converted a hundred percent into work, while work can be converted into heat a 100%.
  • ●  Heat is considered as a low-grade energy meaning, it is easy to convert heat into other forms while work is high-grade energy.

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