The heat transfer at an interface is considered a transient heat flow. To analyze this problem, the Biot number is important to understand how the system behaves. The Biot number is determined by: Bi = h L k The heat transfer coefficient h , is introduced in this formula, and is measured in J m 2 s K . If the system has a Biot number of less than 0.1, the material behaves according to Newtonian cooling, i.e. with negligible temperature gradient within the body. If the Biot number is greater than 0.1, the system behaves as a series solution. The temperature profile in terms of time can be derived from the equation q = − − h Δ Δ T , which becomes T − − T f T i − − T f = exp ⁡ ⁡ ( − − h A t ρ ρ C p V ) .