1. Lecture 22: FRI 17 OCT DC circuits II Ch27.4-9 Physics 2113
Jonathan Dowling
Lecture 22: FRI 17 OCT
DC circuits II
Ch27.4-9

2. How to Solve Multi-Loop Circuits

3. Monster Maze
If all resistors have a resistance of 4Ω, and all batteries are ideal and have an emf of 4V, what is the current through R?
Step 1: Find loop that only goes through R and through batteries but no other resistors.
Step 2: Assume i is clockwise on the loop and walk the loop using the loop rules.
Step 3: Solve for i.
Step 4: If i is negative, reverse the direction of the current in the diagram.

4. What If We Chose Wrong Direction for i ???
If all resistors have a resistance of 4Ω, and all batteries are ideal and have an emf of 4V, what is the current through R?
Step 1: Find loop that only goes through R and through batteries but no other resistors.
Step 2: Assume i is counter-clockwise on the loop and walk the loop using the loop rules.
Step 3: Solve for i.
Step 4: If i is negative, reverse the direction of the current in the diagram.

5. One Battery? Simplify! Resistors Capacitors Key formula: V=iR Q=CV
In series: same current dQ/dt same charge Q
Req=∑Rj /Ceq= ∑1/Cj
In parallel: same voltage same voltage
1/Req= ∑1/Rj Ceq=∑Cj
P = iV = i2R = V2/R
U = QV/2 = Q2/2C = CV2

6. Many Batteries? Loop & Junction!
Three Batteries: Straight to Loop & Junction
One Battery: Simplify First

7. Conservation of ENERGY!
Apply Loop Rule
Around every loop add +E if you cross a battery from minus to plus, –E if plus to minus, and –iR for each resistor. Then sum to Zero: +E1 –E2  – iR1 – iR2  = 0. R1 R2 E1 E2 + –
Conservation of ENERGY!

8. Conservation of CHARGE!
Apply Junction Rule
At every junction sum the ingoing currents and outgoing currents and set them equal.
i1  =  i2 + i3 i2 i1 i3
Conservation of CHARGE!

9. Equations to Unknowns +E1 –E2 – i1R1 – i2 R2 = 0 and Solve for i2 , i3
Continue applying loop and junction until you have as many equations as unknowns!
Given: E1 , E2  , i1 , R1 , R2  
+E1 –E2  –  i1R1 –  i2 R2  = 0
and
i1  =  i2 + i3
Solve for i2 , i3

10. Example Find the equivalent resistance between points (a) F and H and
(b) F and G.
(Hint: For each pair of points, imagine that a battery is connected across the pair.)
Compile R’s in Series
Compile equivalent R’s in Parallel F H
Slide Rule
Series
Parallel

11. Apply loop rule three times and junction rule twice.
Example
Assume the batteries are ideal, and have emf E1=8V, E2=5V, E3=4V, and R1=140Ω, R2=75Ω and R3=2Ω.
What is the current in each branch?
What is the power delivered by each battery?
Which point is at a higher potential, a or b?
Apply loop rule three times and junction rule twice.

12. Apply loop rule three times and junction rule twice.
Example
What’s the current through resistor R1?
What’s the current through resistor R2?
What’s the current through each battery?
Apply loop rule three times and junction rule twice.

13. Too many batteries: loop and junction!
There are three loops and two junctions.
Assume all currents are clockwise.
Both junctions give same result:
Walk three loops clockwise from a to a:
Finally reverse i3.
Three equations and three unknowns i1, i2, i3. Tough in general but trick here is to realize red and green loop equations depend on only one of the i’s. Solve for i1 and i3 and use junction to get i2.

14. E1=8V, E2=5V, E3=4V, and R1=140Ω, R2=75Ω and R3=2Ω.
What is the current in each branch?
What is the power delivered by each battery?
Which point is at a higher potential, a or b?
What’s the current through resistor R1
What’s the current through resistor R2?
What’s the current through each battery?

15. Non-Ideal Batteries
You have two ideal identical batteries, and a resistor. Do you connect the batteries in series or in parallel to get maximum current through R?
Does the answer change if you have non-ideal (but still identical) batteries?
Apply loop and junction rules
until you have current in R.

16. More Light Bulbs
If all batteries are ideal, and all batteries and light bulbs are identical, in which arrangements will the light bulbs as bright as the one in circuit X?
Does the answer change if batteries are not ideal?
Calculate i and V across each bulb.
P = iV = “brightness” or
Calculate each i with R’s the same:
P = i2R

17. RC Circuits: Charging a Capacitor
In these circuits, current will change for a while, and then stay constant.
We want to solve for current as a function of time i(t)=dq/dt.
The charge on the capacitor will also be a function of time: q(t).
The voltage across the resistor and the capacitor also change with time.
To charge the capacitor, close the switch on a.
VC=Q/C
VR=iR
A differential equation for q(t)! The solution is:
Time constant: τ = RC
Time i drops to 1/e. CE
i(t)
E/R

18. RC Circuits: Discharging a Capacitor
Assume the switch has been closed on a for a long time: the capacitor will be charged with Q=CE.
+++
---
Then, close the switch on b: charges find their way across the circuit, establishing a current. C + -
Solution:
i(t)
E/R

20. Time constant: τ = RC
Time i drops to 1/e.
i(t)
E/R

21. Example
The three circuits below are connected to the same ideal battery with emf E. All resistors have resistance R, and all capacitors have capacitance C.
Which capacitor takes the longest in getting charged?
Which capacitor ends up with the largest charge?
What’s the final current delivered by each battery?
What happens when we disconnect the battery?
Simplify R’s into
into Req. Then apply charging formula with ReqC = τ

22. Example
In the figure, E = 1 kV, C = 10 µF, R1 = R2 = R3 = 1 MΩ. With C completely uncharged, switch S is suddenly closed (at t = 0).
What’s the current through each resistor at t=0?
What’s the current through each resistor after a long time?
How long is a long time?
Simplify R1, R2, and R3
into Req. Then apply discharging formula with ReqC = τ