# Thermal Conductivity

Thermal conductivity (λ) is the intrinsic property of a material which relates its ability to conduct heat. Heat transfer by conduction involves transfer of energy within a material without any motion of the material as a whole. Conduction takes place when a temperature gradient exists in a solid (or stationary fluid) medium. Conductive heat flow occurs in the direction of decreasing temperature because higher temperature equates to higher molecular energy or more molecular movement. Energy is transferred from the more energetic to the less energetic molecules when neighboring molecules collide.

Thermal conductivity is defined as the quantity of heat (Q) transmitted through a unit thickness (L) in a direction normal to a surface of unit area (A) due to a unit temperature gradient (ΔT) under steady state conditions and when the heat transfer is dependent only on the temperature gradient. In equation form this becomes the following:

Thermal Conductivity = heat x distance / (area x temperature gradient)

$\lambda = \frac{Q \times L}{A \times \Delta T}$

Approximate values of thermal conductivity for some common materials are presented in the table below.

 Material Thermal ConductivityW/m, oK Thermal Conductivity(cal/sec)/(cm2, oC/cm) Air at 0 C 0.024 0.000057 Aluminum 205.0 0.50 Brass 109.0 - Concrete 0.8 0.002 Copper 385.0 0.99 Glass, ordinary 0.8 0.0025 Gold 310 - Ice 1.6 0.005 Iron - 0.163 Lead 34.7 0.083 Polyethylene HD 0.5 - Polystyrene expanded 0.03 - Silver 406.0 1.01 Styrofoam 0.01 - Steel 50.2 - Water at 20 C - 0.0014 Wood 0.12-0.04 0.0001