<math>q = -k\nabla T(\mathbf{r})</math>
The distribution where q is the heat transfer rate or heat flux density with units of W/m<sup>2</sup>, T is the temperature expressed in °C or °K, ∇ is the gradient operator and k is the thermal conductivity with the units of W/(m.K). It can be modeled shown that the distribution of temperature is governed by the heat diffusion equation:
<math>\Delta T(\mathbf{r}) = \nabla^2 T(\mathbf{r}) = -\frac{\rho(\mathbf{r})}{\epsilon}</math>