Thermodynamics Master Cheat Sheet - NEET 2027

1. NEET Exam Weightage & PYQ Analysis

Thermodynamics is a high-yield topic for NEET 2027 Physics and Chemistry papers. On average, NTA includes 2 to 3 direct questions from this unit annually, contributing 8 to 12 marks. The NTA pattern focuses heavily on polytropic process calculations, First Law energy conservation, thermodynamic relations \(C_p - C_v = R\), and Carnot engine efficiency.

2. Fundamental Definitions & Systems

System Types

• Closed System: Mass remains constant within the boundary, but energy transfer (heat and work) is allowed.
• Open System: Both mass and energy cross system boundaries (e.g., control volume systems).
• Isolated System: Neither mass nor energy can cross the system boundaries.

State Postulate

The state of a simple compressible system is completely specified by two independent, intensive properties.

3. The Four Laws of Thermodynamics

Zeroth Law of Thermodynamics

If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. If \(T_A = T_C\) and \(T_B = T_C\), then \(T_A = T_B\).

First Law of Thermodynamics (Law of Conservation of Energy)

For a non-flow closed system, the change in internal energy is:

For a complete thermodynamic cycle, the net heat transfer equals the net work done:

For an open system (control volume), the steady-flow energy equation is:

Second Law of Thermodynamics

• Kelvin-Planck Statement: It is impossible for any device operating in a cycle to receive heat from a single reservoir and produce an equivalent amount of work.
• Clausius Statement: It is impossible to construct a device operating in a cycle that transfers heat from a lower-temperature body to a higher-temperature body without external work.

Entropy formulation of Second Law:

where \(\sigma \ge 0\) represents entropy generation.

Third Law of Thermodynamics

As absolute temperature approaches absolute zero (\(T \to 0\) K), the entropy of a pure crystalline substance approaches zero (\(S \to 0\)).

4. Thermodynamic Properties & TdS Equations

Enthalpy Formula

Enthalpy (\(H\)) is defined as:

TdS (Entropy) Relations

Specific Heats

For ideal gases, Mayer's relation holds true: \(C_p - C_v = R\), and the adiabatic index is \(\gamma = \frac{C_p}{C_v}\).

5. Ideal Gas Equations & Polytropic Processes

The equation of state for ideal gases is defined as:

Internal energy and enthalpy changes depend solely on temperature for ideal gases:

Polytropic Processes (\(PV^n = C\))

• \(n=0\): Isobaric Process (\(P = \text{constant}\))
• \(n=1\): Isothermal Process (\(T = \text{constant}\))
• \(n=\gamma\): Adiabatic Process (\(Q = 0\))
• \(n=\infty\): Isochoric Process (\(V = \text{constant}\))

Work done in a polytropic process (\(n eq 1\)):

6. Heat Engines, Refrigerators & Cycles

Thermal Efficiency of Heat Engines

Coefficient of Performance (COP)

For Refrigerators:

For Heat Pumps:

Carnot Efficiency

Maximum theoretical efficiency between two thermal reservoirs at temperatures \(T_H\) and \(T_L\):

7. People Also Ask (Frequently Asked Questions)