Junction rule: At any junction, the sum of the currents entering the junction is equal to the sum of currents leaving the junction.
Alternatively, ξi=0, where ξi is the total current.
Justification: Conservation of charge
Loop rule: The algebraic sum of charges in the potential around a closed loop involving resistors and cells in the loop is zero.
Alternatively, ∆V=0, where ∆V is the change in potential.
Justification: Conservation of energy