Thermal Conductivity

by Dr. Patrick Hopkins

How is Thermal Conductivity Defined?

Thermal conductivity is defined as the quantity that relates the heat flux through the material, Q, to the temperature gradient that is established across the material, dT/dx:

Q=-k dT/dx

This equation, known as “The Fourier Law”, predicts the temperature difference across some thickness of material in steady state conditions (when the temperature is not varying in time). So, if you want to make sure your hand does not get too hot when you are holding your cup of coffee, you can either:

  • Make the walls of the coffee cup thicker to make sure your hand is further away from the hot coffee (maybe put a handle on the mug, spacing your hand away from the heat source).
  • Make the walls of the coffee cup out of a lower thermal conductivity material (maybe use a coffee sleeve which is made from porous materials with air pockets, making for very low thermal conductivity walls!).

In the International System of Units, thermal conductivity is expressed using W/m/K (Watts per meter-Kelvin).