This time I explain more about how ‘cospans’ represent gadgets with two ends, an input end and an output end:
I describe how to glue such gadgets together by composing cospans. We compose cospans using a category-theoretic construction called a ‘pushout’, so I also explain pushouts. At the end, I explain how this gives us a category where the morphisms are electrical circuits made of resistors, and sketch what we’ll do next: study the behavior of these circuits.
These lecture notes provide extra details: