The primary type of heat transfer that occurs in solids is conduction. Unlike liquids and gases, which can transfer heat through convection (the physical movement of heated fluid), solids hold their atoms in fixed positions. Heat moves through them by energy passing from one vibrating particle to the next, traveling from hotter regions toward cooler ones without any material actually flowing.
How Conduction Works at the Atomic Level
Atoms in a solid aren’t sitting still. They constantly vibrate around fixed positions within a structured arrangement called a lattice. When one end of a solid gets heated, the atoms there vibrate more intensely. These energized atoms bump into their neighbors, transferring kinetic energy down the line. Think of it like a chain reaction: no single atom travels far, but the energy ripples through the material from the hot side to the cold side.
Physicists describe these vibrations using a concept called phonons, which are essentially packets of vibrational energy moving through the lattice. The more efficiently phonons travel through a material, the better that material conducts heat. In non-metallic solids like rock, ceramic, or glass, phonon transport is the main way heat gets around. When the lattice structure disrupts or scatters these vibrations, as it does in materials like wood or foam, thermal conductivity drops significantly.
Why Metals Conduct Heat So Well
Metals have a second, much faster mechanism on top of lattice vibrations: free electrons. In a metal, the outermost electrons of each atom aren’t bound to any single nucleus. They move freely through the lattice, almost like a gas of tiny charged particles drifting among a grid of positive ions. When one region of the metal is hotter, these free electrons pick up energy there and carry it rapidly to cooler regions, transferring it through collisions along the way.
This electron-based conduction is so efficient that it overwhelms the phonon contribution entirely. It’s also the reason metals that conduct heat well tend to conduct electricity well: the same free electrons are responsible for both. Silver leads all common metals with a thermal conductivity of about 406 W/m·K, and copper follows closely at 385 W/m·K. By comparison, ordinary glass conducts heat at roughly 0.8 W/m·K, and wood sits between 0.04 and 0.12 W/m·K. That’s a difference of roughly 3,000 to 10,000 times between a metal like silver and a piece of wood.
What Governs the Rate of Conduction
The basic rule governing conduction is Fourier’s Law, which boils down to a straightforward idea: the amount of heat flowing through a solid depends on three things. First, the material’s thermal conductivity (how easily it passes heat). Second, the cross-sectional area the heat flows through (a wider path means more heat transfer). Third, the temperature difference across the material divided by its thickness. A thin copper sheet with a large temperature difference on either side will conduct a lot of heat very quickly. A thick wall of wood with a small temperature difference will barely conduct any.
Beyond the material itself, several conditions change how fast conduction happens in practice. Temperature matters: many solids conduct heat differently at different temperatures. Moisture content plays a surprisingly large role, especially in building materials, because water conducts heat much better than air does. A damp wall loses heat faster than a dry one. Density also factors in. Denser materials generally have atoms packed more closely together, giving vibrations a shorter hop between neighbors.
Conduction in Everyday Life
You encounter conduction in solids constantly, even if you don’t think about it in those terms. A metal spoon left in a hot pot gets warm at the handle because heat conducts up through the metal from the submerged end. The heat sink inside your laptop or phone is a small block of aluminum or copper designed to conduct heat away from the processor and spread it across a larger surface area where it can dissipate. Heat exchangers in industrial systems rely on conduction through metal walls to transfer thermal energy between two fluids without mixing them.
Insulation works by exploiting the opposite end of the spectrum. Materials like fiberglass, foam, and wood have low thermal conductivity because their structures trap pockets of air and scatter phonon vibrations. A well-insulated wall doesn’t stop conduction entirely, but it slows it down enough that your heating system can keep up with the loss. The choice of solid material in any thermal application, whether the goal is to move heat quickly or block it, comes down to understanding how conduction behaves in that specific material.
Can Other Types of Heat Transfer Happen in Solids?
Conduction dominates, but it’s not always the only mechanism at work. In semitransparent solids like glass or certain polymers, infrared radiation can pass through the material’s interior rather than being absorbed immediately. This means a small amount of radiative heat transfer occurs within the solid itself, not just at its surface. For most everyday solids, this effect is negligible. In opaque materials like metals, wood, or concrete, radiation cannot penetrate the interior at all, and conduction is effectively the sole internal mechanism.
Convection, by definition, requires fluid movement, so it does not occur inside a solid. It can happen at a solid’s surface when air or liquid carries heat away, but within the rigid structure of the solid itself, conduction is what moves thermal energy from one point to another.