These equations may be expanded by taking the indicated partial derivatives, but it is usually more helpful to have them in the forms just given;furthermore,it is much easier to expand them later if necessary than it is to put the broken pieces back together again. Laplace’s equation is all-embracing, for, applying as it does wherever volume charge density is zero, it states that every conceivable configuration of electrodes or conductors produces a field for which ∇2V = 0. All these fields are different, with different potential values and different spatial rates of change, yet for each of them ∇2V = 0. Because every field (if ρν = 0) satisfies Laplace’s equation, how can we expect to reverse the procedure and use Laplace’s equation to find one specific field in which we happen to have an interest? Obviously,more information is required, and we shall find that we must solve Laplace’s equation subject to certain boundary conditions. Every physical problem must contain at least one conducting boundary and usually contains two or more. The potentials on these boundaries are assigned values, perhaps V0, V1,..., orperhaps numerical values. These definite equipotential surfaces will provide the boundary conditions for the type of problem to be solved. In other types of problems,the boundary conditions take the form of specified values of E (alternatively, a surface charge density, ρS) on an enclosing surface, or a mixture of known values of V and E. Before using Laplace’s equation or Poisson’s equation in several examples, we muststatethatifouranswersatisfiesLaplace’sequationandalsosatisfiestheboundary conditions, then it is the only possible answer. This is a statement of the Uniqueness Theorem, the proof of which is presented in Appendix D.