Current, Resistance, Power

This section contains documents created from scanned original files, which are inaccessible to screen reader software. A "#" symbol is used to denote such documents.

Video Clips

Video RealVideo®
6:52 minutes (0:00 - 6:52)

Definition, with example of calculating the drift velocity current for a copper wire.

Prior Knowledge: None
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
6:22 minutes (6:52 - 13:14)

Statement of Ohm's Law with definitions of conductivity, resistance, and resistivity.

Prior Knowledge: Current (beginning of video lecture 9)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
3:59 minutes (13:14 - 17:13)

Comparison of the properties of good conductors and good insulators.

Prior Knowledge: Ohm's Law (6:52 of video lecture 9)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
5:56 minutes (17:13 - 23:09)

Explanation that temperature can cause Ohm's Law to break down. Includes a demonstration of Ohm's Law holding for one resistor and not for another.

Prior Knowledge: Ohm's Law (6:52 of video lecture 9)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
4:01 minutes (23:09 - 27:10)

Ohm's Law failing to hold for a light bulb with a resistance that varies with temperature.

Prior Knowledge: Ohm's Law (6:52 of video lecture 9)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
5:28 minutes (27:10 - 32:38)

Definition and comparison of values for resistance, current, and voltage.

Prior Knowledge: Ohm's Law (6:52 of video lecture 9)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
4:15 minutes (32:38 - 36:53)

Using a candle to increase the conductivity of air near an electroscope.

Prior Knowledge: Ohm's Law (6:52 of video lecture 9)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
6:15 minutes (36:53 - 43:08)

Using salt to increase the conductivity of water and light a bulb.

Prior Knowledge: Ohm's Law (6:52 of video lecture 9)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
6:08 minutes (43:08 - 49:16)

Calculation of resistance between body and Earth through a pair of shoes, including a demonstration that this resistance is not high enough to keep charge from flowing.

Prior Knowledge: Ohm's Law (6:52 of video lecture 9)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
7:16 minutes (0:00 - 7:16)

Definition and explanation of how a battery works, with examples.

Prior Knowledge: None
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
3:34 minutes (7:16 - 10:50)

Symbol for a battery in a circuit diagram, definition of electromotive force and internal resistance.

Prior Knowledge: None
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
4:30 minutes (10:50 - 15:20)

Connecting two batteries in series to demonstrate that the electromotive force doubles.

Prior Knowledge: Power Supplies (beginning of video lecture 10)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
2:54 minutes (15:20 - 18:14)

Derivation of the equation for power P=IV, including definition of Watts as unit of power.

Prior Knowledge: None
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
5:43 minutes (18:14 - 23:57)

Finding the power consumed by a resistor, with examples including light bulbs, heaters, and the human body.

Prior Knowledge: Power (15:20 of video lecture 10)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
4:33 minutes (23:57 - 28:30)

Finding the power supplied by a battery, with a demonstration of a 9 V battery being shorted out.

Prior Knowledge: Power (15:20 of video lecture 10)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
2:48 minutes (28:30 - 31:18)

Showing the large current and power supplied by a car battery when it is shorted out.

Prior Knowledge: Power (15:20 of video lecture 10)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
11:27 minutes (32:47 - 44:14)

Finding currents and power in a circuit of batteries and resistors.

Prior Knowledge: Current, Resistance, Power (video lecture 9 and video lecture 10)
Instructor: Prof. Walter Lewin
Back to Top
Video RealVideo®
12:19 minutes (20:39 - 32:58)

Discussion of the advantages of transporting electric energy at high voltages to minimize power loss. Calculation of the maximum voltage possible without causing corona discharge, including a slide of corona discharge on a power line during a storm.

Prior Knowledge: Electric Breakdown (32:02 of video lecture 6)
Instructor: Prof. Walter Lewin
Back to Top

Lecture Notes

Document PDF - 1.2 MB
Page 4 to page 26

Current I and current density J are defined, with diagrams and explanation of current flow. Conductivity and resistivity defined. Ohm's Law delta V = IR for relationship between potential and current. Symbols and sign conventions for batteries, resistors, and capacitors in circuits. Resistors in series and in parallel.

Prior Knowledge: Batteries and Elementary Circuits (pages 14-24 of presentation 8)
Instructors: Dr. Peter Dourmashkin, Prof. J. David Litster, Prof. David Pritchard, Prof. Bernd Surrow
Back to Top
Document PDF - 1.2 MB
Page 35 to page 41

Definition, with examples of power supplied by a battery, power dissipated by a resistor, and power absorbed by a capacitor. Balance of energy in a circuit (power supplied by a battery = power dissipated by resistors + power absorbed by capacitors).

Prior Knowledge: DC Circuits and Kirchhoff's Rules (pages 4-34 of presentation 10)
Instructors: Dr. Peter Dourmashkin, Prof. J. David Litster, Prof. David Pritchard, Prof. Bernd Surrow
Back to Top
Document PDF
Page 1 to page 1

Definition of current I and current density J, with diagrams and equations. Statement of microscopic Ohm's Law.

Prior Knowledge: Electric Field (R2)
Instructors: Dr. Peter Dourmashkin, Prof. J. David Litster, Prof. David Pritchard, Prof. Bernd Surrow
Back to Top
Document PDF
Page 2 to page 2

Definition of electromotive force. Statement of Kirchhoff's rules. Step-by-step method for solving DC circuit problems. Equations for resistance, power, and resistors in series and in parallel.

Prior Knowledge: Electric Field (R2) and Current (R10)
Instructors: Dr. Peter Dourmashkin, Prof. J. David Litster, Prof. David Pritchard, Prof. Bernd Surrow
Back to Top
Document PDF
Page 3 to page 5

Definition from E-field; car batteries; direction of current flow.

Prior Knowledge: Current
Instructor: Prof. Gabriella Sciolla
Back to Top
Document PDF
Page 10 to page 12

Using Kirchhoff's rules to solve complex circuits; internal resistance and power dissipation.

Prior Knowledge: Kirchhoff's rules
Instructor: Prof. Gabriella Sciolla
Back to Top

Online Textbook Chapters

Document PDF
Page 2 to page 4

Current as dQ/dt; current density and velocity; conductivity.

Prior Knowledge: Electric fields
Instructors: Dr. Peter Dourmashkin, Prof. J. David Litster, Prof. David Pritchard, Prof. Bernd Surrow
Back to Top
Document PDF
Page 4 to page 8

Microscopic and macroscopic Ohm's Law; resistance and resistivity; power dissipation.

Prior Knowledge: Current
Instructors: Dr. Peter Dourmashkin, Prof. J. David Litster, Prof. David Pritchard, Prof. Bernd Surrow
Back to Top
Document PDF
Page 9 to page 14

Strategy for resistance and resistivity; resistance for cable, cone, hollow cylinder; charge at a junction.

Prior Knowledge: Ohm's Law
Instructors: Dr. Peter Dourmashkin, Prof. J. David Litster, Prof. David Pritchard, Prof. Bernd Surrow
Back to Top

Practice Problems

Document PDF
Problem 1 to problem 2

Question with answer and explanation.

Prior Knowledge: None
Instructors: Dr. Peter Dourmashkin, Prof. J. David Litster, Prof. David Pritchard, Prof. Bernd Surrow
Back to Top
Document PDF
Problem 3 to problem 10

Four questions about current and power in simple circuits of batteries and lightbulbs, with answers and explanations.

Prior Knowledge: None
Instructors: Dr. Peter Dourmashkin, Prof. J. David Litster, Prof. David Pritchard, Prof. Bernd Surrow
Back to Top
Document PDF
Problem 1

5-part power problem; finding resistance, power, voltage conditions, and Lorentz force in power lines.

Prior Knowledge: None
Instructor: Prof. Walter Lewin
Back to Top

Exam Questions

Document PDF
Problem 1

5-part problem; determining which bulbs in a circuit are lit when various switches are closed and which are brightest.

Prior Knowledge: None
Instructor: Prof. Walter Lewin
Back to Top
Document PDF
Problem 1

Determining relative brightness of light bulbs in a resistor circuit.

Prior Knowledge: None
Instructors: Dr. Peter Dourmashkin, Prof. Gunther Roland
Back to Top