What is Thermodynamics? Definition, Working Principle, Formula, Applications & Class 11 Notes

Updated on May 21, 2025 16:24 IST

By Jaya Sharma, Assistant Manager - Content

Thermodynamics is not just a theory in a textbook; it is the science behind everyday things like melting ice, running engines, and even how your fridge works. In simple terms, thermodynamics looks at how heat, energy, and work move and change. Be it CBSE board exam or be it JEE Mains exam, this chapter has approximately 6.6% weightage which is equivalent to 2 questions. This overview walks you through all the important parts, including the first law of thermodynamics, the second law of thermodynamics, and others, which have been mentioned below:

Helmholtz Free Energy
First Law of Thermodynamics
Second Law of Thermodynamics
Entropy
Joule–Thomson Effect
Enthalpy
Equilibrium Thermodynamics
Kelvin–Planck Statement
Carnot Engine
Carnot Theorem
Clausius Statement
Law of Equipartition of Energy
Chemical Thermodynamics
Maxwell Relations
What Is Internal Energy
Helmholtz Equation
Boltzmann Equation
Third Law of Thermodynamics
Van der Waals Equation
Law of Conservation of Energy
Law of Conservation of Mass
Specific Heat
Specific Heat Capacity
Uses of Calorimeter

Table of content

Helmholtz Free Energy
First Law of Thermodynamics
Second Law of Thermodynamics
Entropy
Joule–Thomson Effect
Enthalpy
Equilibrium Thermodynamics
Kelvin–Planck Statement
Carnot Engine
Carnot Theorem
Clausius Statement
Law of Equipartition of Energy
Chemical Thermodynamics
Maxwell Relations
What Is Internal Energy
Helmholtz Equation
Boltzmann Equation
Third Law of Thermodynamics
Van der Waals Equation
Law of Conservation of Energy
Law of Conservation of Mass
Specific Heat
Specific Heat Capacity
Uses of Calorimeter

Helmholtz Free Energy

When you are dealing with constant volume and temperature, not all the energy in a system is available to do work. That's where Helmholtz's free energy comes in. It is the energy you can actually use.

A = U - TS

Here, A is Helmholtz free energy, U is internal energy, T is temperature, and S is entropy. The term TS is energy "lost" to randomness. So what’s left is what can do something useful like move a piston or drive a reaction.

First Law of Thermodynamics

The first law of thermodynamics, also known as the law of conservation of energy, is pretty simple: energy cannot just vanish or appear out of nowhere.

ΔU = Q - W

This means the change in internal energy (ΔU) equals the heat added (Q) minus the work done (W) by the system. If you heat up a gas in a piston, some energy stays inside (as heat), and some pushes the piston (as work). Whether it is a steam engine or a tea kettle, this law helps you figure out where energy goes.

Please note that Thermodynamics is an important chapter for NEET exam as well.

Second Law of Thermodynamics

The second law of thermodynamics is about direction. It says that entropy (disorder) in an isolated system always increases. Heat flows from hot to cold, never the other way around. You can force it (like in a fridge), but nature does not do that on its own. That’s why you cannot build a machine that just takes in heat and spits out 100% work. This law is a reality check for every power plant, engine, or chemical reaction.

Physics Thermodynamics

Try these practice questions

Q1:

Given below are two statements:

Statement – I : When $μ$ moment of an ideal gas undergoes adiabatic change from state $(P_{1}, V_{1}, T_{1})$ to state $(P_{2}, V_{2}, T_{2})$ then work done is w = $\frac{μ R (T_{2} - T_{1})}{1 - γ},$ where $γ = \frac{C_{p}}{C_{v}} a n d$ R = universal gas constant.

Statement – II :

View Full Question

Both statement – I and statement – II are true.

Both statement – I and statement – II are false.

Statement – I is true but statement – II is false.

Statement – I is false but statement – II is true.

Q2:

A gas has n degrees of freedom. The ratio of specific heat of gas at constant volume to the specific heat of gas at constant pressure will be:

$\frac{n}{n + 2}$

$\frac{n + 2}{n}$

$\frac{n}{2 n + 2}$

$\frac{n}{n - 2}$

Entropy

Entropy sounds abstract, but it really just measures how spread out energy is. High entropy means more randomness. Low entropy means things are more ordered. Think of a perfume bottle in a room. Open it, and the gas spreads, entropy increases. Close it, and you cannot "unspread" the gas without work. That’s how nature prefers it.

Joule–Thomson Effect

The Joule–Thomson effect is what happens when a gas expands without exchanging heat and cools down even though no energy seems to be lost. This principle is used in refrigeration and liquefaction of gases. It does not show up with ideal gases but matters a lot with real ones like nitrogen or CO₂.

Enthalpy

Enthalpy (H) is another way to measure energy, especially when pressure stays the same.

H = U + PV

It adds the internal energy to the work needed to "make space" in the environment. So it is useful for things like chemical reactions or phase changes where pressure stays constant

Equilibrium Thermodynamics

Once a system has no more energy flowing around, and everything’s balanced you are in equilibrium. No more change. No more work done. Equilibrium thermodynamics helps describe systems at rest, using tools like the ideal gas law and thermodynamic potentials. It is the calm after the storm.

Kelvin–Planck Statement

This version of the second law of thermodynamics says no engine can turn all heat into work some always goes to waste. You cannot build a machine that just pulls in heat and keeps spinning forever. Nature always demands a "leftover" that you cannot use.

Carnot Engine

The Carnot engine is a model of the perfect engine 100% ideal, no friction, no losses. It works between two thermal reservoirs and shows the upper limit of efficiency.

η = 1 - (T_cold / T_hot)

Real engines cannot beat that number. The bigger the temperature gap, the more work you can get but you’ll always lose some heat.

Carnot Theorem

The Carnot theorem says that no real engine can be more efficient than a Carnot engine working between the same two temperatures. It is a benchmark one every engineer aims for but cannot reach.

Clausius Statement

This version of the second law of thermodynamics says heat does not move from cold to hot unless you do work. Your refrigerator works this way it uses energy to pump heat from a cold inside to a warm kitchen.

Law of Equipartition of Energy

In classical thermodynamics, this law says energy gets evenly split among all available degrees of freedom. Each degree of freedom (like movement in the x, y, or z direction) gets (½)kT of energy. This idea links molecular motion to temperature and helps explain specific heat.

Chemical Thermodynamics

This part of thermodynamics looks at how energy and entropy drive chemical reactions. It predicts whether a reaction will happen, how much heat it gives off, and where the equilibrium lies. It is critical in understanding batteries, combustion, and even reactions in biology.

Maxwell Relations

These are equations that tie different thermodynamic properties together. They come from partial derivatives of energy functions like enthalpy or Gibbs free energy. They help you figure out hard-to-measure stuff like entropy changes from things you can measure, like pressure or volume.

What Is Internal Energy

Internal energy is all the energy inside a system the movement of molecules, their vibrations, and the forces between them. It changes when you heat something up or compress it. It is the energy content you don’t see, but it is always there.

Helmholtz Equation

The Helmholtz equation links Helmholtz free energy to entropy, temperature, and other properties. It helps figure out if reactions or changes will occur under fixed volume and temperature.

Boltzmann Equation

The Boltzmann equation describes how molecules are distributed in a system. It is part of statistical mechanics and connects particle-level behavior with large-scale thermodynamic effects like pressure and temperature.

Third Law of Thermodynamics

As you approach absolute zero, entropy gets closer to zero too assuming a perfectly ordered system. The third law of thermodynamics tells us we cannot actually reach absolute zero in a finite number of steps. It is a limit built into nature.

Van der Waals Equation

Real gases aren’t perfect; they take up space and attract each other. The Van der Waals equation adds corrections to the ideal gas law to reflect real behaviour, especially under high pressure or low temperature.

Law of Conservation of Energy

This principle says the total energy in a closed system stays constant. It might change form from heat to kinetic energy, for example, but the total never changes. It underpins the first law of thermodynamics.

Law of Conservation of Mass

In any chemical reaction, the total mass stays the same. Atoms just rearrange, they don’t disappear. That’s why balanced chemical equations are so important.

Specific Heat

Specific heat tells you how much energy is needed to raise the temperature of 1 gram of a substance by 1°C. Water, for example, has a high specific heat, which is why it takes longer to boil.

Specific Heat Capacity

This expands on specific heat, it depends on the substance and how much of it there is. It is used in designing heating systems, choosing insulation, and managing thermal energy in buildings and electronics.

Uses of Calorimeter

A calorimeter measures the heat involved in physical changes or chemical reactions. It is how you calculate things like enthalpy changes, specific heat, or combustion energy. This simple idea is very useful in science labs. It is important to go through Thermodynamics class 11 NCERT solutions for better performance.

Physics Thermodynamics Exam

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Which is the best Mechanical Engineering book to understand the basics of thermodynamics for beginners?

Answered 2 months ago

One of the most important Mechanical Engineering subjects is Thermodynamics, which deals with the concepts of heat energy and temperature, along with its conversion between different forms. You would come across a variety of books that will give you a basic understanding of the concept, but here are

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