Loop Rule

Kirchhoff's second rule, the loop rule, is based upon conservation of energy and says that "the sum of the changes in potential around any closed path of a circuit must be zero," (Giancoli 529).

Kirchhoff’s Loop Rule
Σ∆V= 0

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Kirchhoff's loop rule goes hand in hand with Ohm's law which says V = IR.  Ohm's law, a description of metal conductors, says that the current through a conductor in a circuit is proportional to the voltage difference within that same circuit. 
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To use Kirchhoff's loop rule you must first label the currents and the direction they run in.  Next, you must identify the unknowns which results in three equations which come from using Kirchhoff's junction rule.  Then, you apply the loop rule to two different loops within the circuit and use Ohm's Law.  Finally, you solve for the three equations.