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At any node (junction of two or more wires) the total current entering the node equals the total current exiting the node.
In the diagram, I1 + I3 = I2 + I4 + I5
The sum of voltages in a closed loop is zero.
In the diagram, V1 + V2 + V3 + V4 = 0
related search terms: kirchoff, kirchoffs, kirchoffs laws