Thermodynamics is the branch of physics that deals with heat, work, temperature, and energy transfer. It explains how energy flows and transforms in physical systems.
👉 Core Idea: Heat and work are two ways of transferring energy.
1. Thermal Equilibrium
Definition
A system is said to be in thermal equilibrium if there is no net heat flow between its parts or with surroundings.
Zeroth Law of Thermodynamics
Statement
If two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.
👉 This law defines the concept of temperature.
2. System and Surroundings
System
The part of the universe under study.
Surroundings
Everything outside the system.
Types of Systems
- Open System → exchanges matter and energy
- Closed System → exchanges only energy
- Isolated System → exchanges neither matter nor energy
3. Thermodynamic Processes
Isothermal Process (T = constant)
PV = constant
Adiabatic Process (Q = 0)
PV^γ = constant
Isobaric Process (P = constant)
V ∝ T
Isochoric Process (V = constant)
P ∝ T
4. Internal Energy
Definition
Total energy contained in a system due to molecular motion and interactions.
Concept Clarity
👉 Internal energy depends only on temperature (for ideal gas).
5. Heat and Work
Heat (Q)
Energy transferred due to temperature difference.
Work (W)
Energy transferred when force causes displacement.
Sign Convention
- Heat absorbed → positive
- Work done by system → positive
6. First Law of Thermodynamics (Very Important)
Statement
Heat supplied to a system is equal to the sum of change in internal energy and work done.
Equation
Q = ΔU + W
Concept Clarity
👉 Energy is conserved (law of conservation of energy).
7. Applications of First Law
Isothermal Process
ΔU = 0 → Q = W
Adiabatic Process
Q = 0 → ΔU = -W
Isochoric Process
W = 0 → Q = ΔU
Isobaric Process
Q = ΔU + PΔV
8. Second Law of Thermodynamics
Statement
Heat cannot flow from a colder body to a hotter body without external work.
Kelvin-Planck Statement
It is impossible to convert all heat into work.
Clausius Statement
Heat cannot flow from cold to hot spontaneously.
9. Heat Engine
Definition
A device that converts heat into work.
Efficiency
η = W/Q₁
10. Refrigerator
Definition
A device that removes heat from a cold body.
Coefficient of Performance (COP)
COP = Q₂/W
11. Carnot Engine
Definition
An ideal heat engine with maximum efficiency.
Efficiency
η = 1 – (T₂/T₁)
Where:
- T₁ = temperature of source
- T₂ = temperature of sink
12. Entropy (Basic Idea)
Definition
Measure of disorder of a system.
Concept Clarity
👉 Entropy always increases in natural processes.
Important Numericals
Numerical 1
Find work done if Q = 100 J, ΔU = 40 J
W = Q – ΔU = 60 J
Numerical 2
Find efficiency if W = 200 J, Q = 500 J
η = 200/500 = 0.4
Numerical 3
Find COP if Q₂ = 300 J, W = 100 J
COP = 3
Numerical 4
Find efficiency of Carnot engine if T₁ = 500 K, T₂ = 300 K
η = 1 – (300/500) = 0.4
Important Formula Sheet
- Q = ΔU + W
- PV^γ = constant
- η = W/Q₁
- COP = Q₂/W
- η_carnot = 1 – T₂/T₁
Concept Clarity (Important)
👉 WHY heat cannot convert fully into work?
Because some energy is always lost.
👉 WHY entropy increases?
Because natural processes tend toward disorder.
👉 WHY temperature in Kelvin?
Because thermodynamic laws require absolute scale.
Common Mistakes
- Using °C instead of Kelvin
- Sign convention errors
- Confusing heat and temperature
Conclusion
Thermodynamics explains energy transfer and transformation. Understanding laws and processes is essential for mastering physics and real-world systems.
👉 Focus on laws + concepts + numericals.