node circuit analysis

Hi.
how do I know when looking at a circuit that I can't replace one of the node voltage variables with a given voltage source? (for example V1=U0) All the other examples I've done worked perfectly like this. (for example picture 2)Am I correct to conclude that this supernode situation (pocture 1) happens when the voltage source is the only element between 2 nodes so we use one of the nodes as a reference point? Would it be the same with a current source? thank youu in advance 🌟🌟

IMG_20240202_135932_edit_43259044990794.jpg349.6KB

IMG_20240202_134813_edit_42588450750792.jpg488.8KB

Reply 1

Joinedup

It's quite usual to have the supernode connected directly to ground... But it doesn't need to be directly connected to ground.

Assuming we're looking at the first picture.
You've drawn a direction for your assumed current which is good, I also like to draw an arrow next to resistors to show the direction of the PD across the resistor implied by that current so my brain doesn't get flipped around when I start to write my equations.