Understanding Thermal Conductivity

Thermal conductivity measures how readily heat passes through a material. High conductivity means heat flows easily. Low conductivity means the material insulates. This property shapes choices from cooking pans to building insulation.

What Thermal Conductivity Measures

Thermal conductivity, written as the symbol k, is a property of the material itself. It does not depend on the size or shape of an object, only on what that object is made of. The idea comes from Fourier's law of heat conduction. In one dimension, the rate of heat flow through a slab is Q = k · A · ΔT / L, where A is the area facing the heat, ΔT is the temperature difference across it, and L is the thickness. Rearranged, that reads k = Q · L / (A · ΔT).

Units and How It Is Measured

The SI unit is the watt per meter-kelvin, W/(m·K). Because a change of one kelvin equals a change of one degree Celsius, W/(m·°C) means the same value. Other unit systems remain in use, so conversions are common.

Laboratories read the value directly. Steady-state methods such as the guarded hot plate hold a fixed temperature difference across a sample and measure the heat passing through. Transient methods such as laser flash analysis time a short heat pulse and work back to the conductivity.

Material Conductivity Values

A Worked Example

Take an exterior wall insulated with fiberglass at k = 0.04 W/(m·K). The area is 10 m², the layer is 0.1 m thick, and the inside sits 20 K warmer than the outside. Heat loss works out to Q = 0.04 · 10 · 20 / 0.1 = 80 watts. Build the same wall from copper at k = 400 and it would pass 800,000 watts. That factor of ten thousand is why we insulate with the low numbers and cool electronics with the high ones. To convert a rating, multiply by the table above: wood near 0.15 W/(m·K) is about 0.087 BTU/(hr·ft·°F).

Where the Numbers Get Used

• Electronics: copper and aluminum heat sinks draw heat off processors, helped by paste that fills tiny air gaps.
• Insulation: fiberglass, mineral wool, and foam slow heat loss, and an R-value is simply thickness divided by conductivity.
• Cookware: aluminum and copper bases spread heat evenly, while plastic or wooden handles stay cool.
• Windows: a sealed gas layer between panes trims the heat that escapes.

Conclusion

Thermal conductivity varies by 10,000× across common materials. Metals conduct heat quickly because of their free electrons. Insulators work by trapping still air to slow heat transfer. Knowing these values guides material choice for heat sinks, insulation, cookware, and countless other applications.

Related Articles

• Insulation R-Values: Understanding Thermal Resistance
• Thermal Conductivity of Materials: A Practical Guide