NCERT Class 11 Physics Chapter 12 Notes Thermodynamics - Download PDF

Consider a hot coffee cup that has been placed on a table. It eventually cools down to room temperature. Thermodynamics is the branch of physics that studies the concepts of heat, temperature, energy, and work; and is directly applied in this everyday observation.

These basic concepts are explored in depth in Chapter 11 of Class 11 Physics, which offers an organized method for understanding thermodynamics. The chapter's NCERT notes provide a clear and useful overview of important subjects like, the idea of thermal equilibrium, different kinds of equilibrium, systems and their environments, the basic laws of thermodynamics, and applications like refrigerators and heat engines.

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This Story also Contains

1. Mechanics vs. Thermodynamics
2. Thermal Equilibrium
3. System and Surroundings
4. Thermodynamics' Zeroth Law
5. Heat, Work and Internal Energy
6. Thermodynamics First Law
7. Specific Heat Capacity
8. Isothermal Processes
9. Adiabatic Processes
10. Isochoric Processes
11. Isobaric Processes
12. Cyclic Processes
13. Heat Engines
14. Thermodynamics' Second Law
15. Reversible and Irreversible Process
16. Carnot Engine
17. Importance of NCERT Class 11 Physics Chapter 12 Notes
18. NCERT Class 11 Notes Chapter-Wise

Also, students can refer,

• NCERT Solutions for Class 11 Physics Chapter 11 Thermodynamics
• NCERT Exemplar for Class 11 Physics Solutions Chapter 12 Thermodynamics

Mechanics vs. Thermodynamics

In thermodynamics, we focus exclusively on the state of the object, evaluating macroscopic variables such as pressure, volume, and temperature. whereas, mechanics is concerned with the motion of an object, taking into account parameters such as velocity and acceleration.

Thermal Equilibrium

When two systems have identical temperatures, we say they are in thermal equilibrium. In mechanics, a system is considered equilibrium when its net force is zero. Equilibrium in thermodynamics refers to the stability of all macroscopic variables over time, including pressure, temperature, and volume.

Different Types of Equilibrium

Various types of equilibrium characterise different aspects of physical systems:

1. Thermal equilibrium: Thermal equilibrium occurs when two systems' temperatures remain constant over time.
2. Chemical equilibrium: Chemical equilibrium occurs when two systems' compositions remain constant over time.
3. Mechanical equilibrium: Mechanical equilibrium occurs when two systems' pressures do not vary over time.

System and Surroundings

System: Any distinct part of the universe surrounded by a boundary that allows for the exchange of heat or energy.

Surroundings: Everything outside of the defined system that makes up the broader environment.

Different System Types

Open System: Characterised by the unrestricted exchange of energy and matter with its environment. An everyday example is a cup of coffee boiling in an open pan, allowing heat and water vapour to escape freely.

Closed System: A closed system allows for the exchange of energy but restricts the flow of matter. A gas-filled balloon is an example of a closed system because it allows for energy transfer (thermal expansion or contraction) while maintaining a constant volume of gas.

Isolated System: There is no exchange of matter or energy with the surroundings. A thermos flask is a classic example of how insulation prevents heat loss or gain, allowing the contents to remain at a constant temperature for an extended period of time.

Different Types of Walls

Adiabatic wall: The adiabatic wall acts as an insulating barrier, preventing heat transfer between systems. This type of wall keeps the temperatures of the interacting systems constant over time.

Diathermic wall: The diathermic wall acts as a conducting barrier, allowing heat to be transferred between two systems. A diathermic wall, as opposed to an adiabatic wall, allows heat to flow through it, allowing temperature adjustments between connected systems.

Thermodynamics' Zeroth Law

According to the Zeroth Law of Thermodynamics, if two systems are in thermal equilibrium with a third system on their own, they are also in thermal equilibrium with each other. This fundamental principle provides a foundation for defining and measuring temperature by emphasising thermal equilibrium as a transitive relation.

Thermodynamic state variables

Thermodynamic state variables are macroscopic quantities that define an equilibrium state in a system. These variables contain important information about the system's properties.

Examples of Thermodynamic State Variables:

1. Extensive Variables: These variables are proportional to the system's size and are influenced by its mass or particle count. Examples include volume, mass, and internal energy. For example, when dealing with a larger mass system, the volume and internal energy are larger as well, demonstrating the system's size dependence.
2. Intensive variables: Intensive variables are unaffected by the system's size. These parameters remain constant regardless of the system's mass or number of particles. Intensive variables include pressure and temperature. These variables maintain their values regardless of system size.

Equation of State

The equation of state establishes a link between key variables such as pressure, mass, volume, and density, describing the state of matter when certain physical conditions are met. In the case of an ideal gas, this equation provides information about its behaviour under various conditions, making it a useful tool for understanding and predicting its state.

Consider an ideal gas, whose state equation is

$PV=\mu RT$

Here, P, V, and T are state variables, with μ = no. of moles.

Heat, Work and Internal Energy

Heat: Heat is the energy exchanged between a system and its surroundings when the temperature differs. This transfer occurs due to the thermal gradient, resulting in changes in the system's internal energy.

Work: Work occurs when a body or system moves in response to a force. Work, mathematically expressed as,

$\mathrm{dW}=\mathrm{PdV}$

Where p is the pressure of the gas in the cylinder.

Internal energy: The internal energy (U) of a system of molecules is the sum of their kinetic and potential energies. Internal energy, denoted by U = Ek + Ep, where Ek and Ep are the molecular kinetic and potential energies, is a macroscopic variable that is solely determined by the state of the system. It is not determined by the path taken, but rather by the energy resulting from molecular motion and configuration within the system.

• Unlike internal energy, heat and work are not state variables. They are forms of energy transfer to a system that cause an internal energy change.

Thermodynamics First Law

The First Law Of Thermodynamics is synonymous with the principle of energy conservation, which states that energy cannot be created or destroyed but can be transformed into other forms.

According to the First Law of Thermodynamics, the change in internal energy of a closed system equals the heat added to the system minus the work done by the system on its surroundings.

Examples: Consider a ball dropping from a building's roof. Initially, the ball possesses potential energy at the top, which diminishes as it falls, transforming into kinetic energy. By the time it reaches the ground, the ball only has kinetic energy.

Mathematically,

$\mathrm{\Delta }\mathbf{Q}=\mathrm{\Delta }\mathbf{U}+\mathrm{\Delta }\mathbf{W}$

For Cyclic Process
$\sum \mathrm{\Delta }Q=\sum \mathrm{\Delta }W$

Where: ΔQ is the heat provided to the system by the environment.

ΔW denotes the amount of work done by the system in relation to the environment.

And ΔU is the change in the system's internal energy.

Relation between C_p and C_v

Mayer's Formula describes the relationship between the specific heat at constant pressure (C_p) and the specific heat at constant volume (C_v) for an ideal gas. According to Mayer's relation, an ideal gas:

$C_p-C_V=R$

Specific Heat Capacity

Specific heat capacity is the amount of heat required to raise the temperature of a substance per unit mass.
$S=\frac{\mathrm{\Delta }Q}{m\mathrm{\Delta }T}$

Where,

m is the body's mass, ΔQ is the quantity of heat that a substance absorbs or rejects and ΔT stands for temperature change.

Molar Specific Heat Capacity

The heat capacity per mole is the amount of heat (in moles) absorbed or rejected (instead of mass m in kg) by a substance to change its temperature by one unit.

$\mathbf{C}=\mathbf{S}/\mu =\mathrm{\Delta }\mathbf{Q}/\mu \mathrm{\Delta }\mathbf{T}$

Where: μ= the mass of a material in moles, C is the substance's molar specific heat capacity, ΔQ is the quantity of heat that a substance absorbs or rejects and ΔT stands for temperature change.

At constant pressure, molar specific heat capacity (Cp): The equivalent molar specific heat capacity at constant pressure is called molar specific heat capacity at constant pressure if the gas is retained at constant pressure during the heat transfer (Cp).

At constant volume, the molar specific heat capacity (Cv): The equivalent molar specific heat capacity at constant volume is called molar specific heat capacity at constant volume if the volume of the gas is maintained during the heat transfer (Cv).

Isothermal Processes

An Isothermal Process is defined as the condition in which the temperature of a system undergoing a thermodynamic process remains constant throughout. In other words, during an isothermal process, the system's other thermodynamic variables change (such as pressure, volume, or internal energy), but the temperature remains constant.

The equation for an ideal gas undergoing an isothermal process describes the relationship between pressure (P), volume (V), and temperature (T):

PV=constant

This means that the product of pressure and volume for an ideal gas remains constant as long as the process takes place at the same temperature. A hyperbolic curve is commonly used to represent the behaviour of an isothermal process on a pressure-volume (PV) diagram.

Graphically,

Points in the Graph of the Isothermal Process:

• Isotherms are curves on the pressure-volume (P-V) graph that form during an isothermal process. These isotherms have a hyperbolic shape.
• The slope of an isothermal curve on the P-V graph:

$\tan \theta =\frac{dP}{dV}=\frac{-P}{V}$

• The area between the isothermal curve and the volume axis on the P-V graph represents the work done during the isothermal process.

  The magnitude of work done in the isothermal process:

$\begin{array}{rl}& W=nRT{\log }_e\left(\frac{V_f}{V_i}\right)=2.303nRT{\log }_{10}\left(\frac{V_f}{V_i}\right)\\ & W=nRT{\log }_e\left(\frac{P_i}{P_f}\right)=2.303nRT{\log }_{10}\left(\frac{P_i}{P_f}\right)\end{array}$

Adiabatic Processes

An Adiabatic process is a thermodynamic process in which no heat is exchanged between the system and its surroundings. In other words, the system is thermally isolated, which prevents heat from entering or leaving the system.

Equations of Adiabatic process:

$\begin{array}{rl}& P{V}^{\gamma }=\text{ constant; where }\gamma =\frac{C_P}{C_V}\\ & T{V}^{\gamma -1}=\text{ constant }\Rightarrow T_1{V}_1^{\gamma -1}=T_2{V}_2^{\gamma -1}\text{ or }T\propto V^{1-\gamma }\\ & \frac{T^{\gamma }}{P^{\gamma -1}}=\text{ constant }\\ & \Rightarrow T_1^{\gamma }{P}_1^{1-\gamma }=T_2^{\gamma }{P}_2^{1-\gamma }\text{ or }T\propto P^{\frac{\gamma -1}{\gamma }}\text{ or }P\propto T^{\frac{\gamma }{\gamma -1}}\end{array}$

Isochoric Processes

Isochoric Process refers to a thermodynamic process in which the system's volume remains constant while other variables, such as pressure and temperature, vary. Because the volume is constant in an isochoric process, no work is done to or by the gas.

ΔW =0

Isobaric Processes

The term "Isobaric Process" is derived from the combination of "iso," meaning the same, and "baric," which refers to pressure. In an isobaric process, pressure remains constant throughout the operation, whereas volume and temperature may fluctuate.

$W=\mu R(V_2-V_1)$
$\mathrm{\Delta }Q=\mathrm{\Delta }U+\mu \mathbf{R}(T_2-T_1)$

Cyclic Processes

The process by which the system returns to its original condition.

ΔU = 0

This means that the entire amount of heat absorbed is equal to the amount of work done by the system.

Heat Engines

A Heat Engine is a device that allows heat to be supplied and absorbed by placing a body in alternating contact with hot and cold bodies.

A heat engine, to put it simply, is a mechanism that turns thermal energy into mechanical energy.

Refrigerators

Refrigerators operate on the same principle as a heat engine, but in reverse. The essential components are a cold reservoir (freezer) at a lower temperature and a heat reservoir (environment) at a higher temperature.

Thermodynamics' Second Law

The Second Law Of Thermodynamics states that any spontaneous process will inevitably result in an increase in the total entropy (S) of the universe. Simply put, this law states that the entropy of an isolated system does not decrease over time.

Under certain conditions, such as thermodynamic equilibrium or a reversible process, the total entropy of a system and its surroundings remains constant. The Law of Increased Entropy is an alternative term for the second law, emphasising the tendency of systems to progress towards greater disorder over time.

Reversible and Irreversible Process

Reversible Process

A thermodynamic process is considered reversible if it can be reversed, restoring the system and its surroundings to their original states while causing no net change in the universe. Reversible processes allow the system to transition from an initial state to a final state and then seamlessly reverse back to the initial state. Examples include isothermal expansion and compression, as well as electrolysis.

Irreversible Process

Irreversible processes are those that cannot be reversed in any practical way. Irreversible processes typically happen quickly. Examples include plastic deformation, combustion, diffusion, and water flow downhill. Irreversible processes cause an increase in entropy and lack reversibility.

Carnot Engine

The Carnot Engine was named after the scientist Carnot.

It's a reversible heat engine that can operate at two different temperatures.

It achieves a level of efficiency that no other engine can match.

Processes in a Carnot engine's cycle

Any heat engine's basic function is to transfer heat Q₁ from a hot reservoir at temperature T₁ to a cool reservoir at temperature T₂.

It is isothermal expansion because the system absorbs heat. At temperature T₁, the engine absorbs heat Q₁.

Inside the engine, an adiabatic process occurs, causing the temperature of the engine to rise from T₁ to T₂, but with no heat flow.

Isothermal contraction occurs as the system releases heat. At temperature T₂, the engine produces heat Q₂.

An adiabatic process occurs once more, changing the system's temperature from T₂ to T₁.

Isothermal expansion, followed by the adiabatic process, and then isothermal contraction, followed by the adiabatic process, make up one cycle of the Carnot engine.

This will continue to happen.

The efficiency of Carnot engine is given by:-

$\eta =\frac{W}{Q_1-Q_2}=1-\frac{T_2}{T_1}$

Importance of NCERT Class 11 Physics Chapter 12 Notes

• Conceptual Clarity – The notes provide a structured explanation of thermodynamics concepts, helping students understand the principles of heat, work, and energy in an organized manner.

• Concise and Exam-Oriented – The NCERT Class 11 Physics Chapter 11 notes summarize key points, making revision easier before exams.

• Coverage of Fundamental Topics – These notes cover essential aspects such as thermodynamic laws, equilibrium, heat engines, and refrigerators, ensuring students grasp the core principles effectively.

• Simplifies Complex Concepts – Thermodynamics can be challenging, but these notes break down complex ideas into simple explanations, making learning more accessible.

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