1. Kirchhoff’s Laws Physics 102: Lecture 06
Note opposite sign convention compared to your handouts!
Lecture timing is fine. No need to rush 1

2. Last Time Last Lecture Today Resistors in series:
Current thru is same; Voltage drop across is IRi
Last Lecture
Resistors in parallel:
Voltage drop across is same; Current thru is V/Ri
Solved Circuits
What about this one?
Today 5

3. Kirchhoff’s Rules Kirchhoff’s Junction Rule (KJR):
Current going in equals current coming out.
Kirchhoff’s Loop Rule (KLR):
Sum of voltage drops around a loop is zero. 12

7. Using Kirchhoff’s Rules
(1) Label all currents
Choose any direction R1 ε1 R2 R3 ε2 ε3 R5 A B I1 I3 I2 I4 I5
(2) Label +/- for all elements
Current goes +  - (for resistors) + - + -
Choose loop and direction R4
Have students label I5, since it isn’t shown in their drawing
Write down voltage drops
Be careful about signs 17

8. Loop Rule Practice Example Find I: B e1= 50V A e2= 10V R1=5 W I
Can ask if R1 and R5 are in series, parallel. Also make sure they understand potential at B is higher than at A. 22

9. Loop Rule Practice Example Find I: B e1= 50V A e2= 10V
R1=5 W I
Find I: + -
e1= 50V
Label currents
Label elements +/-
Choose loop
Write KLR A
R2=15 W
e2= 10V
+e1 - IR1 - e2 - IR2 = 0
I I = 0
I = +2 Amps
Opposite sign convention compared to your handouts!
Can ask if R1 and R5 are in series, parallel. Also make sure they understand potential at B is higher than at A. 22

10. ACT: KLR Resistors R1 and R2 are
R1=10 W
Resistors R1 and R2 are
1) in parallel ) in series 3) neither
E2 = 5 V I2
R2=10 W
Definition of parallel:
Two elements are in parallel if (and only if) you can make a loop that contains only those two elements. IB - +
E1 = 10 V
Note that nothing is in series or in parallel!
Upper loop contains R1 and R2 but also ε2. 25

11. Preflight 6.1 Calculate the current through resistor 1. 21% 69% 11%
R=10 W + -
21% % %
E2 = 5 V I2
R=10 W
1) I1 = 0.5 A 2) I1 = 1.0 A 3) I1 = 1.5 A
E1 - I1R = 0
 I1 = E1 /R = 1A IB - +
E1 = 10 V
Note that nothing is in series or in parallel! 27

12. Preflight 6.1 ACT: Voltage Law
Calculate the current through resistor 1. I1
R=10 W + -
21% % %
E2 = 5 V I2
R=10 W
1) I1 = 0.5 A 2) I1 = 1.0 A 3) I1 = 1.5 A
E1 - I1R = 0  I1 = E1 /R = 1A IB - +
E1 = 10 V
ACT: Voltage Law
How would I1 change if the switch was opened?
1) Increase 2) No change ) Decrease 32

13. Preflight 6.2 Calculate the current through resistor 2. 66% 23% 11%
R=10 W
66% % %
1) I2 = 0.5 A 2) I2 = 1.0 A 3) I2 = 1.5 A
E2 = 5 V I2
R=10 W + -
E1 - E2 - I2R = 0
 I2 = 0.5A IB - +
E1 = 10 V 35

14. Preflight 6.2 How do I know the direction of I2?
It doesn’t matter. Choose whatever direction
you like. Then solve the equations to find I2.
If the result is positive, then your initial guess
was correct. If result is negative, then actual
direction is opposite to your initial guess. I1
R=10 W
E2 = 5 V I2
R=10 W - +
Work through preflight with opposite
sign for I2? IB - +
E1 = 10 V
+E1 - E2 + I2R = 0 Note the sign change from last slide
 I2 = -0.5A Answer has same magnitude as before but opposite sign. That means current goes to the right, as we found before. 35

15. Kirchhoff’s Junction Rule
Current Entering = Current Leaving
I1 = I2 + I3 I1 I2 I3
R=10 W
E1 = 10 V IB I1
E = 5 V I2 + -
Preflight 6.3
Calculate the current through battery.
11% % %
1) IB = 0.5 A 2) IB = 1.0 A 3) IB = 1.5 A
IB = I1 + I2 = 1.5 A 38

16. Kirchhoff’s Laws (1) Label all currents (2) Label +/- for all elements
Choose any direction R4 R1 ε1 R2 R3 ε2 ε3 I1 I3 I2 I4 R5 A B
(2) Label +/- for all elements
Current goes +  - (for resistors)
Choose loop and direction
Your choice!
Have them go back to this slide and fill in (5)
Write down voltage drops
Follow any loops
Write down junction equation
Iin = Iout 39 36

17. Example
You try it!
In the circuit below you are given e1, e2, R1, R2 and R3. Find I1, I2 and I3. R1 R2 R3 I1 I3 I2 e1 e2
1) This will be given on the exam. Comment on 1 equation 2 unknowns. 2 eqs 3 unknowns finally 3 and 3 45

18. You try it!      Example e1 e2
In the circuit below you are given e1, e2, R1, R2 and R3. Find I1, I2 and I3. 
Label all currents (Choose any direction) 
2. Label +/- for all elements (Current goes +  - for resistor) 
3. Choose loop and direction (Your choice!) 
Write down voltage drops (Potential increases or decreases?)
Loop 1: +e1 - I1R1 + I2R2 = 0
Loop 2
+e1- I1R1 - I3R3 - e2 = 0
1) This will be given on the exam. Comment on 1 equation 2 unknowns. 2 eqs 3 unknowns finally 3 and 3
Loop 2: R1 + - I1 I3 I2 +  -
5. Write down junction equation + e1
Loop 1 R2 R3 - -
Node: I1 + I2 = I3 + - + e2
3 Equations, 3 unknowns the rest is math! 45

19. Let’s put in real numbers
Example
Let’s put in real numbers
In the circuit below you are given e1, e2, R1, R2 and R3. Find I1, I2 and I3.
Loop 2 2 5 10 I1 I3 I2 + - 20
1. Loop 1: I1+10I2 = 0
2. Loop 2: I1-10I3-2=0
3. Junction: I3=I1+I2
Loop 1
solution: substitute Eq.3 for I3 in Eq. 2:
20 - 5I1 - 10(I1+I2) - 2 = 0
rearrange: 15I1+10I2 = 18
rearrange Eq. 1: I1-10I2 = 20
Now we have 2 eq., 2 unknowns. Continue on next slide
1) This will be given on the exam. Comment on 1 equation 2 unknowns. 2 eqs 3 unknowns finally 3 and 3 45

20. 15I1+10I2 = 18
5I1 - 10I2 = 20
Now we have 2 eq., 2 unknowns.
Add the equations together:
20I1=38 I1=1.90 A
Plug into bottom equation:
5(1.90)-10I2 = 20 I2=-1.05 A
note that this means direction of I2 is opposite to that shown on the previous slide
Use junction equation (eq. 3 from previous page)
I3=I1+I2 =
I3 = 0.85 A
We are done!

21. See you next time…