 The differential form of Fourier's law of thermal conduction shows that the local heat flux density, formula_2, is equal to the product of thermal conductivity, formula_3, and the negative local temperature gradient, formula_4.The heat flux density is the amount of energy that flows through a unit area per unit time.where (including the SI units)
The thermal conductivity, formula_3, is often treated as a constant, though this is not always true.