The transfer of heat energy is defined as heat flux, Q. By definition, this is the flow of heat energy through a defined area over a defined time. So, the units for Q are Joules (energy) divided by area (square meters) and time (seconds). Joules/(m^2∙sec). Since power is defined as energy divided by time and 1 Watt is equal to 1 Joule/second, Q can also be expressed as Watts/m^2 .
Fourier’s Law defines the ability of a specific material to transfer heat. It defines the heat flux, Q, through a material in terms of the cross-sectional area through which the heat energy transfer occurs, A, and the temperature gradient over which the transfer occurs, ∆T/∆x:
Q=-κA ∆T/∆x (1)
k is the proportionality constant for this relationship, known as the thermal conductivity. It is a characteristic property of a material. The units of the individual components of Equation (1) can be written out (with Q defined in terms of power):
Watts =κ∙m^2∙K/m (2)
When the units of length are cancelled out, Equation (2) becomes:
Solving for the thermal conductivity, k:
For more information on thermal conductivity and its importance in a variety of industries, check out our applications page.