Thermal Properties of Matter

Heat is not a substance a body contains — it is energy in transit, and it flows for exactly one reason: a difference in temperature. The chapter starts there, with the zeroth law of thermodynamics that makes temperature measurable at all, then builds the absolute scale from the ideal-gas thermometer and the triple point of water at $273.16$ K. Matter responds to heat first by growing: thermal expansion, with $\Delta L = \alpha L\,\Delta T$ and the ratio $\alpha : \beta : \gamma = 1 : 2 : 3$ that ties linear, areal and volume expansion together — the physics behind expansion joints, shrink fits, bimetallic thermostats, and the anomalous behaviour of water that lets a lake freeze from the top down and its fish survive. Then it absorbs: specific heat capacity $\Delta Q = ms\,\Delta T$, water's exceptionally large value and the climate that follows from it, and calorimetry, where heat lost equals heat gained. At a change of state the temperature stops rising altogether and the energy goes into latent heat instead, which is why steam scalds far worse than boiling water. The second half is transport. Conduction carries heat through a solid, $H = KA\frac{\Delta T}{L}$, and becomes almost easy once written as a thermal resistance $R = \frac{L}{KA}$ that adds in series and in parallel exactly like an electrical one. Convection carries it by moving the fluid itself, from a pan of water to the sea breeze and the monsoon. Radiation needs no medium at all: the blackbody spectrum, Wien's law $\lambda_m T = b$ that reads a star's temperature from its colour, the Stefan-Boltzmann law $H = \sigma A e T^4$ whose fourth power is unforgiving, and Kirchhoff's law, that a good absorber is a good emitter. The chapter closes on Newton's law of cooling — an approximation with a stated range, derived rather than asserted — and the greenhouse effect that keeps the planet about 33°C warmer than it would otherwise be. Topics the rationalised syllabus trimmed — areal expansion and the $1:2:3$ ratio, bimetallic strips, thermal resistance in series and parallel, Kirchhoff's law and Prevost's theory, and the greenhouse effect — are restored in full, because Boards, JEE Main and NEET ask about them every year. Dedicated JEE and NEET Corners follow, each with a full-length exam-pattern practice drill.