When applying Kirchhoff's Current Law in nodal analysis, a voltage source situated between two unknown nodes prevents a direct current calculation for that branch. Engineers solve this by treating the two nodes and the voltage source as a single supernode. They sum the currents leaving this combined region and use the known voltage difference between the two internal nodes as a constraint equation to solve the circuit.
You can't just write a standard KCL equation for node A because of the 5-volt source, so draw a supernode around A and B.
Related terms
Nodal analysis
A systematic method for determining the voltage at every node in an electrical circuit.
Node
A continuous connection point in a circuit where two or more components meet.
Thévenin's theorem
An analytical method used to simplify complex linear circuits into a single voltage source and a single series resistor.