The change in temperature (T) as a function of depth (z) measured towards the center of the planet is called the thermal gradient (dT/dz). In regions where conduction dominates, heat flow is related to the thermal gradient by Fourier' s law:
The thermal conductivity (k) is related to the density (p), specific heat at constant pressure (cp), and thermal diffusivity (j) of the material:
The rate at which layers gain (or lose) heat is given by
Combining this with Eq. (3.27) gives the thermal diffusion equation, also called the heat conduction equation:
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