Statement of Fourier's law of heat conduction:
"The rate of heat conduction through a plane wall is proportional to the temperature difference across the layer and heat transfer area, but is inversely proportional to the thickness of the layer"
Fourier's law in differential form is given by,
k = thermal conductivity in W/m^2.K
A = area normal to the heat flow in m^2
dT/dx = temperature gradient