Instructions
Total Questions: 20 | Marks: 4 each | No Negative Marking
Thermodynamics – IIT JEE Notes (Set 21)
Introduction to Thermodynamics
Definition
Thermodynamics is the branch of physics that deals with heat, temperature, work, and energy transformations in physical systems.
Scope
It explains how heat energy converts into mechanical work and vice versa.
Thermodynamic System
Definition
A thermodynamic system is a specified portion of matter under study.
Types of Systems
Open system, closed system, and isolated system.
Thermodynamic Variables
State Variables
Pressure, volume, temperature, and internal energy.
Equation of State
Relation between thermodynamic variables of a system.
Zeroth Law of Thermodynamics
Statement
If two systems are separately in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
Importance
This law defines the concept of temperature.
First Law of Thermodynamics
Statement
Heat supplied to a system equals increase in internal energy plus work done by the system.
Formula
ΔQ = ΔU + W
Key Insight
It is based on conservation of energy.
Internal Energy
Definition
Total kinetic and potential energy of molecules inside a system.
Ideal Gas
Internal energy of an ideal gas depends only on temperature.
Heat and Work
Heat
Energy transferred due to temperature difference.
Work
Energy transferred when a system changes volume against external pressure.
Work Formula
W = ∫PdV
Specific Heat Capacity
Definition
Amount of heat required to raise temperature of unit mass by one degree.
Specific Heat at Constant Volume
Cv
Specific Heat at Constant Pressure
Cp
Mayer’s Relation
Formula
Cp – Cv = R
Importance
Valid for ideal gases.
Ratio of Specific Heats
Formula
γ = Cp/Cv
Importance
Used in adiabatic processes.
Ideal Gas Equation
Formula
PV = nRT
Variables
P = pressure, V = volume, n = number of moles, R = gas constant, T = temperature.
Isothermal Process
Definition
Process occurring at constant temperature.
Condition
PV = constant
Key Insight
Internal energy change is zero for ideal gas.
Adiabatic Process
Definition
Process in which no heat exchange occurs between system and surroundings.
Condition
PVᵞ = constant
Key Insight
Temperature changes during adiabatic expansion or compression.
Isochoric Process
Definition
Process occurring at constant volume.
Work Done
Work done is zero because volume does not change.
Isobaric Process
Definition
Process occurring at constant pressure.
Work Done
W = PΔV
PV Diagram
Importance
Area under PV curve represents work done by the gas.
Cyclic Process
In cyclic process, system returns to initial state.
Second Law of Thermodynamics
Kelvin-Planck Statement
No engine can convert all heat into work completely.
Clausius Statement
Heat cannot flow spontaneously from colder body to hotter body.
Entropy
Definition
Entropy is a measure of randomness or disorder of a system.
Key Insight
Entropy increases in irreversible processes.
Heat Engine
Definition
A device that converts heat energy into mechanical work.
Efficiency
η = W/Q₁
Carnot Engine
Importance
Ideal heat engine with maximum possible efficiency.
Efficiency Formula
η = 1 – T₂/T₁
Key Insight
Efficiency depends only on source and sink temperatures.
Refrigerator
Working Principle
Works as reverse heat engine.
Coefficient of Performance
COP = Q₂/W
Kinetic Theory of Gases
Basic Assumptions
Gas molecules are in random motion and collisions are perfectly elastic.
Pressure of Gas
Pressure arises due to collisions of molecules with container walls.
Root Mean Square Speed
Formula
vrms = √(3RT/M)
Key Insight
Higher temperature increases molecular speed.
Degrees of Freedom
Definition
Independent ways in which molecules can store energy.
Examples
Monatomic gases have 3 degrees of freedom.
Equipartition of Energy
Statement
Energy is equally distributed among all degrees of freedom.
Average Energy
Each degree contributes (1/2)kT energy.
Conceptual Insights
Key Understanding
Thermodynamics connects heat transfer with mechanical work and energy conservation.
Common Mistakes
Students often confuse adiabatic and isothermal processes and forget sign conventions in thermodynamics.
Important Exam Concepts
Conceptual Traps
Internal energy of ideal gas depends only on temperature, not pressure or volume.
JEE Strategy
Practice PV diagrams, thermodynamic processes, and numerical problems on heat engines thoroughly. Focus on derivations and conceptual clarity.