We will follow the steps for using Kirchoff’s loop rule:• Arbitrarily choose a direction of current. Draw arrows on your circuit to indicate this direction.
• Follow the loop in the direction you chose. When you cross a resistor, the voltage is −IR , where R is
the resistance, and I is the current flowing through the resistor. This is just an application of Ohm’s
law. (If you have to follow a loop against the current, though, the voltage across a resistor is written
+IR .)
• When you cross a battery, if you trace from the − to the + add the voltage of the battery, subtract the
battery’s voltage if you trace from + to −.
• Set the sum of your voltages equal to 0. Solve. If the current you calculate is negative, then the
direction you chose was wrong—the current actually flows in the direction opposite to your arrows.
In the case of Figures 19.6a and 19.6b , we’ll start by collapsing the two parallel resistors into a single
equivalent resistor of 170 Ω. You don’t have to do this, but it makes the mathematics much simpler.
Figure 19.6a Example circuit for using Kirchoff’s loop rule.Figure 19.6b Circuit ready for analysis via Kirchoff’s loop rule.