Class 11 Physics · NCERT notes for NEET

Thermodynamics: NCERT Notes, Formulas and Tricks for NEET

How heat, work and internal energy are linked: the zeroth, first and second laws, the four standard processes on a p-V graph, and reversible and irreversible processes. NEET asks from this chapter almost every year, mostly p-V graphs and first-law sign problems.

• 1. Zeroth and first law of thermodynamics
• 2. Thermodynamic processes 🔒
• 3. Second law of thermodynamics 🔒

Topic 1 is open for everyone. The full chapter, quick revision and practice open with a plan, or start free with 5 full chapters.

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Topic 1

Zeroth and first law of thermodynamics

Rub your palms and they get warm with no flame at all: work alone can raise internal energy, exactly as heat does.

• Thermodynamics studies heat, temperature and the conversion of heat into other forms of energy using macroscopic variables like $p$, $V$, $T$ (not individual molecules).
• Thermal equilibrium: the macroscopic variables of the system do not change with time. An adiabatic wall blocks heat flow; a diathermic (conducting) wall allows it.
• Zeroth law: if A and B are each in thermal equilibrium with C, then A and B are in thermal equilibrium with each other. It gives us the idea of temperature: bodies in equilibrium have the same $T$.
• Internal energy $U$ = sum of kinetic and potential energies of the random (disordered) motion of molecules, measured in the centre-of-mass frame. The KE of the whole container moving is NOT part of $U$.
• $U$ is a state variable: $\Delta U$ depends only on the initial and final states. For an ideal gas, $U$ depends only on $T$.
• Heat is energy transfer because of a temperature difference; work is energy transfer by any other means (moving piston, stirring). Neither is a state variable: both depend on the path.
• First law (energy conservation): $\Delta Q=\Delta U+\Delta W$, with $\Delta Q>0$ for heat given to the system and $\Delta W>0$ for work done by the system.
• Work done by a gas $W=\int pdV$ = area under the $p$-$V$ curve.
• Specific heat $s=\frac{1}{m}\frac{\Delta Q}{\Delta T}$ (J kg${}^{-1}$ K${}^{-1}$); molar specific heat $C=\frac{1}{\mu }\frac{\Delta Q}{\Delta T}$. For water $s=4186$ J kg${}^{-1}$ K${}^{-1}$; $1$ cal $=4.186$ J.
• For an ideal gas $C_p-C_v=R$ and $\gamma =C_p/C_v>1$. Extensive variables ($U$, $V$, mass) scale with size; intensive ones ($p$, $T$, density) do not.

Formulas

• $\Delta Q=\Delta U+\Delta W$
  First law; $\Delta W$ is work done BY the system
• $W={\int }_{V_1}^{V_2}pdV$
  Work by gas = area under $p$-$V$ curve
• $\Delta U=\mu C_v\Delta T$
  Ideal gas, valid for ANY process
• $C_p-C_v=R$
  Mayer's relation (molar, ideal gas)
• $\gamma =\frac{C_p}{C_v}=1+\frac{2}{f}$
  $f$ = degrees of freedom: $5/3$ monatomic, $7/5$ rigid diatomic
• $\Delta Q=ms\Delta T=\mu C\Delta T$
  Heat for a temperature change

Tips, tricks and hacks

Common mistakes

NEET traps

"Heat and work are state variables" is FALSE; only $U$ (with $p$, $V$, $T$) is. NEET often pairs this with a true statement about $\Delta U$ being path independent.

The zeroth law defines temperature (thermal equilibrium). It is the first law that is energy conservation; options swap them.

Heat supplied is extensive ($\Delta Q$ doubles if the system doubles); temperature and pressure are intensive.

Practise Thermodynamics questions

Topic-wise practice from Level 1 to Level 3, NCERT-based questions and NEET PYQs, with a full solution after every answer.