1. Physics 122C Electricity and Magnetism
Lecture 14
DC Circuits and Power
April 27, 2007
Martin Savage

2. Lecture 14 Announcements
Lecture HW Assignment #5 has been posted on the Tycho system and is due Wednesday at 10 PM.
Remember that Midterm 2 is next friday
11/21/2018
Physics 122B - Lecture 14

3. Circuit Elements & Diagrams
These are some of the symbols we will use to represent objects in circuit diagrams.
Variable elements are indicated by adding an arrow.
Other symbols: inductance, transformer, diode, transistor, ammeter, etc. A
11/21/2018
Physics 122B - Lecture 14

4. A Circuit Diagram Actual Circuit Circuit Diagram 11/21/2018
Physics 122B - Lecture 14

5. Kirchhoff’s Laws
11/21/2018
Physics 122B - Lecture 14

6. Basic Circuit
11/21/2018
Physics 122B - Lecture 14

7. Plumber’s Analogy* High pressure V1 I P1 I V2 Constriction Pump
Low pressure
The “plumber’s analogy” of this circuit is a pump (=battery) pumping water in a closed loop of pipe that includes a constriction (=resistor). The pressure (=V1) is the upper part of the loop is higher than in the lower part (=V2). There is a pressure drop (=V1-V2) across the constriction and a pressure rise across the pump. The water flow I is the same at all points around the loop.
Pump = Battery
Constriction = Resistor
Pressure = Potential Water Flow =Current
11/21/2018
Physics 122B - Lecture 14

8. Example: A Single-Resistor Circuit
11/21/2018
Physics 122B - Lecture 14

9. Example: A More Complex Circuit (1)
Analyze the circuit shown, to:
Find the current in and potential difference across each resistor.
Graph the potentials around the circuit
11/21/2018
Physics 122B - Lecture 14

10. Example: A More Complex Circuit (2)
11/21/2018
Physics 122B - Lecture 14

11. Question What is the DV across the indicated circuit element in
the direction of the current, I?
-4 V; (b) -2 V; (c) 0 V; (d) +2 V; (e) +4 V;
11/21/2018
Physics 122B - Lecture 14

12. Applying Kirchhoff’s Loop Law to Many Loops*
i1 – i3
Define a minimum set of current loops. Label all elements.
Write a loop equation for each loop. (Battery or 0 = SDV).
Solve equations for currents
Calculate other variables of interest.
R1=
R2= i3 i1
Vbat=
R4=
R3=
R5=
Loop equations: i2
i1 – i2
11/21/2018
Physics 122B - Lecture 14

13. Solving for the loop currents
I V
I2 = 0 I
I V
I2 = 0 I
Gaussian Elimination
I V
I2 = 0 I
I1 = 2 A , I2 = 1 A , I3 = 2/3 A
11/21/2018
Physics 122B - Lecture 14

14. Applying Kirchhoff’s Junction Law to Many Junctions*
Define a minimum set of junction potentials. You can select one ground point. Label all elements.
Write a junction equation for each unknown junction .
Solve these equations for the unknown junction potentials.
Calculate the other variables of interest.
R1= J1
R5=
Vbat V1
R2=
R3=
R4=
Junction equations: J2
V=0 V2
11/21/2018
Physics 122B - Lecture 14

15. Energy and Power (1)
Example: A 90 W load resistance is connected across a 120 V battery.
How much power is delivered by the battery?
11/21/2018
Physics 122B - Lecture 14

16. Energy and Power (2)
The energy delivered by the battery is dissipated by the resistor.
11/21/2018
Physics 122B - Lecture 14

17. Example: The Power of Light
How much current is “drawn” by a 100 W light bulb connected to a constant 120 V outlet?
11/21/2018
Physics 122B - Lecture 14

18. Example: The Power of Sound
Most loudspeakers are designed to have a resistance of 8 W. If such a loudspeaker is connected to a stereo amplifier with a rating of 100 W, what is the maximum possible current in the loudspeaker?
11/21/2018
Physics 122B - Lecture 14

19. Example: A Dim Bulb
How much power is used by a 60 W light bulb rated at 120 V if it is operated using a dimmer switch at 100 V? (Assume the resistance is constant.)
(Actually, the resistance at this lower operating potential will be less than the R-value calculated above because the temperature will be lower, so the actual power used will be somewhat higher.)
11/21/2018
Physics 122B - Lecture 14

20. End of Lecture 14
Before the next lecture, read Knight, sections 31.5 through 31.8.
Lecture HW Assignment #5 has been posted on the Tycho system and is due next Wednesday at 10 PM.
Remember that next Friday we will have Exam 2.
11/21/2018
Physics 122B - Lecture 14