Thermodynamics Class 11 Notes Physics Chapter 12 - CBSE

Chapter : 12

What Are Thermodynamics ?


The study of the flow of mass, heat and energy is the study of thermodynamics.

Thermodynamic Terminology


A notable part of the universe that is kept under observation is known as the system.


The remaining part of the universe except for the system which isn’t kept under observation is known as surroundings.

Types Of System

Open System

The system in which heat energy and mass energy both circulate.

Eg: Human body.

Closed System

A system with constant mass that experiences heat energy flow.

Eg: Pressure cooker.

Isolated System

The system where none of the flow takes place.

Eg: Thermoflask.

Thermodynamic Equilibrium

Mechanical Equilibrium

Mechanical equilibrium is brought about by the absence of mechanical motion, constant
pressure, and volume.

Thermal Equilibrium

Thermal equilibrium is created by the constant heat and temperature w.r.t. time.

Chemical Equilibrium

The chemical equilibrium is reached when the rate of forward reaction equals the rate of backward reaction.


Temperature is a measure of the average kinetic energy of the particles in an object. When the temperature increases, the motion of these particles also increases. Temperature is measured with a thermometer or a

Zeroth Law Of Thermodynamics

The zeroth law may be stated as follows: If a body C, be in thermal equilibrium with two other bodies, A and B, then A and B are in thermal equilibrium with one another.

Internal Energy

Internal energy is the totality of the elements of the energy impacted by the system's internal forces. It is frequently represented by the letters U or E.

Modes of Energy Transport


Heat (Q) is the energy that is transmitted as a result of temperature variations between a system and its surroundings.


Work (W) is the energy expended to overcome the outside forces operating on the system.

Laws Of Thermodynamics

First Law of Thermodynamics

According to the first law of thermodynamics, energy cannot be generated or destroyed.

∆U = Q + W

Second Law Of Thermodynamics

The state of entropy of the entire universe, as an isolated system will always increase over time, is the standard statement of the second law of thermodynamics.


The measure of randomness or disorder in the process of a body is known as its entropy. It is a state function and is represented as S.

$$\Delta S =\frac{Q_{rev}}{\text{T}}$$

Boyle’s Law

At constant temperature, the volume of a given mass of gas is inversely proportional to its pressure.


Charles’ Law

At constant pressure, the volume of a given mass of a gas is directly proportional to its absolute temperature.

V ∝ T

Gay Lussac’s Law

At constant volume, pressure of a given mass of a gas is directly proportional to the temperature.

P ∝ T

Avogadro’s Law

Avogadro’s law states that equal volumes of all gases under the same conditions of temperature and pressure contain equal number of molecules.

Isothermal Process

The constant temperature process is known as the isothermal process. Here, ∆U=0 and ∆H=0.

$$\text{W} =-2.303n\text{RT}.log\frac{\text{V}_{2}}{\text{V}_{1}}\\\text{Q} = 2.303\text{nRT}.\text{log}\frac{\text{V}_{2}}{\text{V}_{1}}$$


Enthalpy is the measurement of energy in a thermodynamic system. The quantity of enthalpy equals to the total content of heat of a system, equivalent to addition of the system's internal energy and the product of volume and pressure.

Adiabatic Process

When the heat exchanged with the surrounding is zero, such a process is known as adiabatic process.

TVγ−1 = Constant,


Q = 0 ⇒ W = ∆U

∆H = n CP ∆T

Isochoric Process

Constant volume process is known as isochoric process. Here, W = 0, ∆H = nCp ∆T and ∆U = nCv ∆T = Qv

Isobaric Process

Constant pressure process is known as isobaric process. Here, W = −P∆V = −n R∆T

∆H = np ∆T = Qp and ∆U = n CV∆T


Reversible Process

A reversible process is where the participants go back to its initial form by inculcating minor or negligible changes in their surroundings.

Irreversible Process

An irreversible process is a naturally occurring phenomenon, which does not go back to its original state.

Cyclic Process

A process that comes back to its original and initial state is known as a cyclic process. A closed graph determines this process and here, ∆U = 0 and Qnet = −Wnet.