Conduction
dQ/dt = kA·ΔT/L — heat flow in a rod.
Key Notes
Conduction: heat transfer through a material WITHOUT bulk motion of matter — vibrating molecules and electrons share energy.
Dominant mechanism in solids, especially metals (free electrons).
Fourier's law: heat current dQ/dt = −kA·(dT/dx), where k = thermal conductivity (W/m·K).
Steady-state through a slab: dQ/dt = kA·ΔT/L, where L is thickness.
Good conductors: metals (Cu: ~400 W/m·K, Ag: ~430, Al: ~240). Poor conductors (insulators): wood, plastic, air.
Thermal resistance R = L/(kA) — analogous to electrical resistance. Series and parallel rules apply.
Free electrons in metals carry heat efficiently (Wiedemann-Franz law: k/σ ∝ T).
Used in: heat sinks, thermos flasks (insulation), thermal grease (improve conduction).
Formulas
Fourier's law
Heat current proportional to temperature gradient.
Steady-state slab
Across slab of thickness L, area A.
Thermal resistance
Analogous to electrical R = ρL/A.
Slabs in series
Heat passes through each in turn.
Slabs in parallel
Heat splits between paths.
Important Points
Conduction is dominant in solids (especially metals).
Metals conduct heat well because of free electrons (same reason they conduct electricity).
Wood, glass, air have low k — used as INSULATORS.
Series-parallel rules for thermal resistance — just like electrical circuits.
Wiedemann-Franz law: k/σ ∝ T for metals — direct link between electrical and thermal conduction.
Vacuum has no conduction (or convection) — Thermos flask uses double-walled vacuum to block both.
Conduction notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.