Thermal Properties of Matter
Easy Overview
Why does a metal spoon feel colder than a wooden spoon even though both are at room temperature? That's thermal conductivity. This chapter is about heat — how it flows, how we measure it, and how materials behave when they heat up. It's the science behind why you can fry an egg on a hot pan but not on a hot plastic plate.
Heat and Temperature — They're Not the Same
Temperature tells you how hot something is — it's the average kinetic energy of molecules. Heat is energy transferred between objects because of temperature difference. Think of temperature as a 'how excited are the molecules?' meter and heat as the actual energy moving around. Heat flows from hot to cold until thermal equilibrium (same temperature) is reached.
Thermal Expansion — Everything Grows When Hot
Most things expand when heated. Solids expand in length (linear), area (areal), and volume (cubical). Linear expansion: ΔL = α L₀ ΔT, where α is the coefficient of linear expansion. ΔA = β A₀ ΔT (β ≈ 2α), ΔV = γ V₀ ΔT (γ ≈ 3α). That's why railway tracks have gaps and bridges have expansion joints — to avoid buckling in summer.
Specific Heat and Calorimetry
Specific heat capacity (c) is the heat needed to raise 1 kg of a substance by 1°C. Water has a high specific heat (4200 J/kg·°C) — it takes a lot of energy to heat water up, and it cools down slowly. That's why coastal cities have milder climates. Calorimetry is just the heat balance equation: heat lost = heat gained. The principle of conservation of energy applied to thermal systems.
Latent Heat — Hidden Heat During Phase Change
When ice melts at 0°C, the temperature doesn't change until all ice has become water. Where does the heat go? Latent heat — it breaks the bonds between molecules instead of raising temperature. Latent heat of fusion (L_f) for melting/freezing, latent heat of vaporization (L_v) for boiling/condensation. L_v is much larger than L_f — breaking all bonds to turn liquid to gas takes way more energy than melting.
Heat Transfer — Conduction, Convection, Radiation
Three ways heat moves. Conduction: through direct contact (metal spoon in hot tea). Rate = kA(ΔT)/L, where k is thermal conductivity. Convection: through fluid movement (hot air rises, cool air sinks) — that's how room heaters work. Radiation: through electromagnetic waves (Sun's heat reaching Earth) — no medium needed. Dark surfaces absorb and radiate better than shiny ones.
Key Points
- •Temperature = average KE of molecules; Heat = energy transferred
- •Linear expansion: ΔL = α L₀ ΔT
- •β = 2α, γ = 3α for most solids
- •Specific heat c = Q / (m ΔT)
- •Latent heat: Q = mL — no temperature change during phase change
- •Conduction: kA(ΔT)/L, Convection: fluid movement, Radiation: electromagnetic waves
- •Water has high specific heat (4200 J/kg·°C) — great coolant
Practice Questions
- A 0.5 kg metal piece at 150°C is dropped into 2 kg of water at 30°C. Final temp is 35°C. Find the specific heat of the metal.
- Distinguish between conduction, convection, and radiation.
- Why do railway tracks have small gaps between sections? Explain using thermal expansion.
- Calculate the heat required to convert 1 kg of ice at −10°C to steam at 100°C. (c_ice = 2100, L_f = 3.34×10⁵, c_water = 4200, L_v = 2.26×10⁶)