Passive linear networks are used in a wide variety of engineering applications, but the best studied are electrical circuits made of resistors, inductors and capacitors. We describe a category where a morphism is a circuit of this sort with marked input and output terminals. In this category, composition describes the process of attaching the outputs of one circuit to the inputs of another. We construct a functor, dubbed the `black box functor', that takes a circuit, forgets its internal structure, and remembers only its external behavior. Two circuits have the same external behavior if and only if they impose same relation between currents and potentials at their terminals. The space of these currents and potentials naturally has the structure of a symplectic vector space, and the relation imposed by a circuit is a Lagrangian linear relation. Thus, the black box functor goes from our category of circuits to a category with Lagrangian linear relations as morphisms. We prove that this functor is symmetric monoidal and indeed a hypergraph functor. We assume the reader is familiar with category theory, but not with circuit theory or symplectic linear algebra.
Keywords: passive linear network, electric circuit, principle of minimum power, black box, decorated cospan, compact closed category, hypergraph category, Lagrangian relation
2010 MSC: 18C10, 18D10, 53D12, 94C05
Theory and Applications of Categories, Vol. 33, 2018, No. 38, pp 1158-1222.