Box models for 2D geometry

For some quick estimates of the expected behavior of the current we can use a bold approximation which is referred to as the box-model solution. We start with the assumption that the current has a simple shape, a well defined “box.” In the present 2D case the box is a rectangle of length xN (t) and height hN (t). The main objective is to determine the behavior of these two variables, which means that we need two equations.