Response to a Constant Flux Switched On at

It has been shown (Section 4.2.9) that after Laplace transformation, Pick s second law takes the form 

The solution of an equation of this type is facilitated if the second term is zero. This objective can be attained by introducing a new variable c defined as 

The variable c, can be recognized as the departure c - c of the concentration from its initial value c°. In other words, c, represents heperturbation from the initial concentration (Fig. 4.19). 

After Laplace transformation of Eq. (4.47), the differential equation becomes 

This equation is identical in form to the linearized P-b equation [cf Eq (3 21)] and therefore must have the same general solution, i.e., 

---