1. Kirchhoff’s Rules
Some circuits cannot be broken down into series and parallel connections.
For these circuits we use Kirchhoff’s rules.
Junction rule: The sum of currents entering a junction equals the sum of the currents leaving it.
At junction - a

2. Loop rule: The sum of the changes in potential around
a closed loop is zero.

4. Problem Solving: Kirchhoff’s Rules
Label each current.
Identify unknowns.
Apply junction and loop rules; you will need as many independent equations as there are unknowns.
Solve the equations, being careful with signs.

5. For the circuit shown in diagram, find the potential difference between points a and b. Each resistor has R = 75 Ω and each battery is 1.5 V. R a b
Kirchhoff’s loop rule:

6. What is the resistance between points X and Y?
A Cube of 10W Resistors X Y 10
What is the
resistance between
points X and Y?
2004 AP Physics Workshop, Ted Vittitoe

7. 6 A Y Symmetry 2 6 A ammeter Variable power supply 1 2 1 2 2 1 X 6 10
2004 AP Physics Workshop, Ted Vittitoe

8. 6 A V One path through the cube Y 2 A 1 Kirchhoff’s Loop Rule X 2 10
2004 AP Physics Workshop, Ted Vittitoe

9. Y 10
I = 6 A
V = 50 V 10 10 10 10
Ohm’s Law 10 10 10 10 10 10 10 X
2004 AP Physics Workshop, Ted Vittitoe