According to Fourier’s Law, the time it takes for the heat to transfer through a material is proportional to the temperature gradient. The equation below shows that the density flux density is equal to the product of the thermal conductivity and the negative temperature gradient.
where q is the heat flux density expressed in J/(m2*s), k is thermal conductivity constant expressed in J/(m*s*K), and ?T is the temperature gradient expressed in K/m. The negative sign in the equation indicates that the heat flows down the temperature gradient. In other words, the heat flows from a higher temperature to a lower temperature. This equation can be derived into its one-dimensional form. The below equation is Fourier’s Law in the x-direction: