in the top circuit they want to find northon/thevenin
and as in the bottom circuit there is no independent sources
so they add a test source to know how much current go threw it
i notices that on the top circuit we have 2 intersection
and they want to find the potential on them (v1 and v2)
but on the bottom circuit we have 3 intersections and we put
two variables instead of three .
why??
in this kind of thing
in what intersections do i need to find there potential
??
why they a KCL only on two of the intersection instead of the three present?
in both cases you have two nodes only with unknown voltages, so maked as V1 & V2. voltages are maked on nodes that are unknown, in the 2nd diagram the said 3rd intersection(node) is having the source voltage.