Kirchoff's Rules (20-10)
• For nasty circuits in which no two resistors are in
series or parallel, which cannot be simplified
• Junction: where at least three circuit paths meet
(e.g. B, E above)
• Loop: a closed path around the circuit
(e.g. BCDEB, BAFEB, BAFEDCB)
• Branch: path connecting two junctions (BE,BCDE)
Junction/Node Rule (Current)
• At any junction, the sum of currents entering the
junction = the sum of currents leaving the junction
 I enter =  I leaving
• Within any branch, choose the current directions
arbitrarily – if you get a negative value for a
current, you chose the wrong direction.
Loop Rule (Potential)
I
+
+5V
-5V
5V
Loop Rule:
∑ ∆V = 0
loop
The sum of the changes in potential around any
closed path of the circuit must be zero.
∆V :
I
I
+
_
+
_
Find the currents I1, I2, and I3
The following four equations supposedly result from
applying Kirchoff's junction rule to the four junctions
in the circuit shown below. However, one of the
equations is written incorrectly.
Which one is wrong?
A) I4 + I5 = I1
B) I1 + I2 = I3
C) I6 = I3 + I5
D) I2 = I4 + I6
Which one of the following equations results from a
correct application of Kirchoff's loop rule to the top
loop?
A) I1R1 – I2R2 –2.0V + I4R4 = 0
B) I1R1 + I2R2 –2.0V – I4R4 = 0
C) –I1R1 + I2R2 –2.0V + I4R4 = 0
D) –I1R1 – I2R2 +2.0V – I4R4 = 0