In addition to current conservation, energy must also be conserved in an electrical circuit. Since energy is directly related to electrical voltage, the sum of all partial voltages around a closed loop must always equal zero. That is, for $n$ resistors in a loop:
\[
\boxed{\sum_{k=1}^n U_k = 0}
\]
The signs are determined analogously to the junction rule, based on the direction of current flow. The traversal direction of the loop can be chosen arbitrarily, but the signs must be adjusted if the direction is reversed.
For example, consider a loop with five different resistors. Since the current flows in the same direction everywhere in this loop, the sum of the partial voltages is
\[
U_1 + U_2 + U_3 + U_4 + U_5 = 0
\]
When applying the loop rule, one must always be careful with sign conventions. If a voltage source is present in the loop, then either the source voltage or the individual resistor voltages must be given a negative sign.
Example problem: Two resistors are connected in a loop with a 12 V voltage source. One resistor has a measured voltage of -5 V. What is the voltage drop across the other resistor? Solution: -7 V