Physics XI - Chapter 11: Thermal Properties of Matter
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- Heat: Form of energy transferred due to temperature difference; measured in Joules.
- Temperature: Measure of average kinetic energy of molecules; determines heat flow direction.
- Thermal Expansion: Increase in dimensions with temperature; solids expand least, gases most.
- Coefficient of Linear Expansion (α): Fractional change in length per °C; α = ΔL/(LΔT).
- Coefficient of Area Expansion (β): β ≈ 2α for isotropic solids.
- Coefficient of Volume Expansion (γ): γ ≈ 3α for solids and liquids; γ = 1/273 °C⁻¹ for ideal gases.
- Anomalous Expansion of Water: Contracts from 0°C to 4°C; maximum density at 4°C.
- Specific Heat Capacity (c): Heat required to raise temperature of unit mass by 1°C; c = Q/(mΔT).
- Calorimetry Principle: Heat lost by hot body = Heat gained by cold body (conservation of energy).
- Water Equivalent: Mass of water that would absorb same heat as given body for same ΔT.
- Latent Heat: Heat absorbed/released during phase change at constant temperature.
- Latent Heat of Fusion: Solid ↔ Liquid transition; for ice = 80 cal/g = 334 kJ/kg.
- Latent Heat of Vaporization: Liquid ↔ Gas transition; for water = 540 cal/g = 2260 kJ/kg.
- Conduction: Heat transfer through molecular collisions without bulk motion; best in metals.
- Thermal Conductivity (k): Measures material's ability to conduct heat; SI unit: W/m·K.
- Convection: Heat transfer through bulk fluid motion; natural (density differences) or forced.
- Radiation: Heat transfer through electromagnetic waves; requires no medium; IR region.
- Black Body: Perfect absorber and emitter of radiation; theoretical concept.
- Stefan-Boltzmann Law: E = σAT⁴; total radiation ∝ T⁴ (σ = 5.67×10⁻⁸ W/m²K⁴).
- Wien's Displacement Law: λ_max T = constant (0.0029 m·K); hotter objects emit shorter wavelengths.
- Newton's Law of Cooling: Rate of cooling ∝ temperature difference with surroundings; dT/dt = -k(T-T₀).
- Greenhouse Effect: Trapping of IR radiation by atmospheric gases; maintains Earth's temperature.
- Triple Point of Water: 0.01°C, 611.657 Pa; all three phases coexist.
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Chapter Summary
This chapter unveils the fascinating thermal behavior of matter - how substances respond to heat and temperature changes. We begin by distinguishing between heat (energy in transit) and temperature (measure of molecular kinetic energy), establishing the fundamental concepts that govern thermal interactions in our daily lives.
The phenomenon of thermal expansion explains why railway tracks have gaps, why bridges have expansion joints, and why mercury rises in thermometers. We explore linear, area, and volume expansions through their respective coefficients (α, β, γ), with the crucial relationship γ ≈ 3α for isotropic solids. The anomalous expansion of water between 0°C to 4°C is particularly remarkable - this unique property allows aquatic life to survive in frozen lakes and makes water essential for Earth's climate regulation.
Calorimetry provides the tools to measure heat transfer, based on the principle of energy conservation. The concepts of specific heat capacity and latent heat help us understand why some materials heat up quickly while others don't, and why phase changes occur at constant temperatures. Water's exceptionally high specific heat capacity makes it an ideal coolant and explains coastal climate moderation.
The three modes of heat transfer - conduction, convection, and radiation - each operate through different mechanisms. Conduction dominates in solids through molecular vibrations, convection in fluids through bulk motion, and radiation through electromagnetic waves that can travel through vacuum. The laws governing radiation - Stefan-Boltzmann and Wien's displacement - allow us to understand stellar temperatures and design efficient thermal systems.
From designing thermos flasks to understanding global warming, from cooking food to launching spacecraft, the principles in this chapter form the foundation of thermal physics with profound implications across science, engineering, and environmental studies.