NEET Physics 2027 — Thermodynamics and Kinetic Theory: Complete Notes and Practice MCQs
Thermodynamics and Kinetic Theory is a crucial chapter for NEET Physics, contributing 3-5 questions annually. These topics test conceptual understanding of heat, work, energy transformation and molecular behaviour of gases. This complete guide covers all key laws, formulae and 10 practice MCQs for NEET 2027.
Laws of Thermodynamics
The four laws govern thermodynamic processes: Zeroth Law — if A is in thermal equilibrium with B, and B with C, then A is in equilibrium with C (basis of temperature measurement). First Law — energy cannot be created or destroyed: Q = W + delta-U (heat supplied = work done + change in internal energy). Second Law — entropy of an isolated system never decreases; heat flows spontaneously from hot to cold. Third Law — entropy of a perfect crystal at absolute zero is zero.
Thermodynamic Processes
| Process | Condition | Work Done |
|---|---|---|
| Isothermal | T = constant | W = nRT ln(V2/V1) |
| Adiabatic | Q = 0 | W = -delta-U = nCv(T1-T2) |
| Isochoric | V = constant | W = 0 |
| Isobaric | P = constant | W = P*delta-V = nR*delta-T |
Carnot Engine Efficiency
Carnot engine is the most efficient heat engine operating between temperatures T1 (source) and T2 (sink). Efficiency = 1 – T2/T1 = (T1-T2)/T1. Key points: (1) No real engine can exceed Carnot efficiency. (2) Efficiency is never 100% as long as T2 is not 0 K. (3) Higher the temperature difference, higher the efficiency. Coefficient of Performance (COP) for refrigerator = T2/(T1-T2).
Kinetic Theory of Gases — Key Formulae
- Ideal Gas Law: PV = nRT (n = moles, R = 8.314 J/mol/K)
- RMS velocity: vrms = sqrt(3RT/M) — proportional to sqrt(T)
- Mean velocity: vmean = sqrt(8RT/piM)
- Most probable velocity: vmp = sqrt(2RT/M)
- Speed ordering: vmp < vmean < vrms (ratio 1 : 1.128 : 1.225)
- Average KE per molecule: (3/2)kT, where k = Boltzmann constant = 1.38 x 10^-23 J/K
Specific Heat and Degrees of Freedom
| Gas Type | Degrees of Freedom | Cv | Cp | gamma |
|---|---|---|---|---|
| Monoatomic (He, Ar) | 3 | (3/2)R | (5/2)R | 5/3 = 1.67 |
| Diatomic (H2, N2) | 5 | (5/2)R | (7/2)R | 7/5 = 1.4 |
| Polyatomic (CO2) | 6 | 3R | 4R | 4/3 = 1.33 |
Mean Free Path and Brownian Motion
Mean free path is the average distance between two successive molecular collisions. It increases with decreasing pressure and increases with temperature. Brownian motion is the random motion of small particles suspended in a fluid due to bombardment by molecules — first observed by Robert Brown (1827), theoretically explained by Einstein (1905).
Frequently Asked Questions — NEET Thermodynamics 2027
How many questions come from Thermodynamics in NEET?
Thermodynamics and Kinetic Theory together contribute 3-5 questions in NEET Physics each year. Carnot engine efficiency, First Law application, and kinetic theory speed formulae are the most frequently tested topics.
What is the Carnot engine efficiency formula?
Carnot Engine efficiency = 1 – (T2/T1) = (T1 – T2)/T1, where T1 is the hot source temperature and T2 is the cold sink temperature, both in Kelvin. A Carnot engine at T1=500K and T2=300K has efficiency = 1 – 300/500 = 40%.
What is the relation between Cp and Cv?
Mayer’s relation: Cp – Cv = R for one mole of ideal gas. The difference arises because at constant pressure, the gas does additional work (P*delta-V) during expansion, requiring more heat than at constant volume.
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