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Conduction is the transfer of heat through direct molecular collisions without bulk movement of matter. The rate of heat flow is given by Fourier's law: P = kA(ΔT/L), where k is the thermal conductivity of the material, A is cross-sectional area, ΔT is the temperature difference, and L is thickness. Metals are excellent conductors because free electrons transport energy efficiently; insulators like wood and fiberglass have low k values.
Compare thermal conductivities of materials and interpret why a metal doorknob feels colder than a wooden door at the same room temperature. Solve steady-state conduction problems in layered materials (R-value problems) by analogy with electrical resistance in series.
You know from thermal equilibrium that heat flows from hot regions to cold regions until temperatures equalize. Conduction is the microscopic mechanism by which this happens inside a solid or stationary fluid: energetic molecules collide with their neighbors and pass kinetic energy along, without any bulk flow of matter. The quantitative description is Fourier's Law: the rate of heat flow P (in watts) through a material is proportional to the temperature difference, the cross-sectional area, and the inverse of the thickness — P = kA(ΔT/L), where k is the thermal conductivity of the material.
The factor k is a material property that varies enormously: copper has k ≈ 400 W/m·K, while air has k ≈ 0.025 W/m·K — a factor of 16,000. This explains why a metal doorknob at 20°C feels cold while a wooden door at the same temperature feels neutral. Your hand is at 37°C; both objects are at 20°C and will extract heat from your hand. But copper extracts it 1,000× faster than wood, so your hand cools rapidly — which your nervous system interprets as "cold." The temperature is the same; the sensation is different because k is different.
Fourier's Law has a direct analogy with Ohm's Law for electricity: P (heat flow rate) corresponds to current I, temperature difference ΔT corresponds to voltage ΔV, and the thermal resistance R_th = L/(kA) corresponds to electrical resistance. For composite materials — like a wall made of plaster, insulation, and brick — thermal resistances in series simply add: R_total = R₁ + R₂ + R₃. This is the basis of R-values in building insulation: a higher R-value means higher thermal resistance, so less heat escapes in winter. Doubling the thickness doubles R; using a material with half the k also doubles R.
In steady state, the heat current is the same through every layer of a composite wall — just as current is the same through resistors in series. The temperature drops across each layer proportionally to its thermal resistance: most of the temperature drop occurs across the most resistive layer. This is exactly why adding a thin air gap (very low k but also very thin) contributes some insulation, while thick fiberglass batting (low k, large L) contributes much more. Understanding this layered resistance framework lets you design thermal systems — from building envelopes to heat sinks in electronics — using the same intuition you would apply to a resistor network.