Temperature, Heat & Thermal Properties Explained | Chapter 17 of University Physics

Chapter 17 introduces the fundamental concepts of temperature and heat, showing how materials respond to thermal energy. We’ll cover how thermal equilibrium defines temperature, the scales we use to measure it, how materials expand when heated, the relationship between heat and temperature changes, phase transitions, and the three modes of heat transfer.

Watch the full video summary here for detailed examples and demonstrations.

Thermal Equilibrium & Temperature Scales

When two systems in contact share no net heat flow, they reach thermal equilibrium; this principle underlies the Zeroth Law of Thermodynamics. Temperature scales quantify this property:

• Celsius: 0 °C at water’s freeze, 100 °C at boil
• Kelvin: K = °C + 273.15, with the triple point of water at 273.16 K
• Fahrenheit: 32 °F freeze, 212 °F boil

Thermometers exploit physical changes—liquid expansion or resistance shifts—to measure temperature reliably.

Thermal Expansion

Materials expand when heated:

• Linear expansion: ΔL = α L₀ ΔT
• Volume expansion: ΔV = β V₀ ΔT, with β ≈ 3α for isotropic solids

Preventing expansion induces thermal stress, a critical factor in engineering designs.

Heat & Specific Heat

Heat (Q) is energy transferred due to temperature difference:

• Q = m c ΔT (mass m, specific heat c)
• Q = n C ΔT (mole n, molar heat capacity C)
• Units: joule (J), calorie (cal), where 1 cal = 4.186 J
• Dulong–Petit’s law approximates C ≈ 25 J/mol·K for many solids.

Calorimetry & Phase Changes

Phase changes occur at constant temperature but require latent heat:

• Fusion (melting/freezing): Q = m L_f
• Vaporization (boiling/condensation): Q = m L_v
• Sublimation: direct solid ↔ gas transition

In an isolated calorimeter, ΣQ = 0 ensures energy conservation across mixing and phase transitions.

Heat Transfer Mechanisms

Conduction

Energy flows through molecular contact. Heat current:

H = (k A ΔT) / L, where k is thermal conductivity, A area, L thickness.

Convection

Heat carried by fluid motion—either natural (density-driven) or forced (fans/pumps).

Radiation

Electromagnetic emission from all bodies. Stefan–Boltzmann law:

P = ε σ A T⁴, with σ = 5.6704×10⁻⁸ W/m²·K⁴ and emissivity ε (0–1).

Conclusion

Understanding temperature, heat capacity, phase transitions, and the modes of heat transfer is essential for thermodynamics and practical applications—from climate control to materials science. These concepts lay the groundwork for the First Law of Thermodynamics and beyond.

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