The First law or the junction rule: for a given junction or node in a circuit, the sum of the currents entering equals the sum of the currents leaving.
The junction rule tells
us, in the above figure I1 = I2 + I3.
The 2nd law or the loop rule: around any closed loop in a circuit, the sum of the potential differences across all elements is zero.
The loop rule tells us,
in the above figure VA+VB+VC+VD=0
