electrodynamics

"I have also a paper afloat, with an electromagnetic theory of light, which, till I am convinced of the contrary, I hold to be great guns."

Welcome to Physics 351! In this class we will study charges, currents, electric and magnetic fields, and their interactions. Much of the physics is expressed in a single, remarkable set of equations

\begin{gather} \vec{\nabla} \cdot \vec{E} = \frac{1}{\epsilon_{0}} \rho \vphantom{\frac{\partial\vec{B}}{\partial t}} \\ \vec{\nabla} \times \vec{E} + \frac{\partial\,\vec{B}}{\partial \,t} = 0 \\ \vec{\nabla} \cdot \vec{B} = 0 \vphantom{\frac{\partial\vec{B}}{\partial t}}\\ \vec{\nabla} \times \vec{B} - \mu_{0}\,\epsilon_{0}\,\frac{\partial\,\vec{E}}{\partial\,t} = \mu_{0}\,\vec{J} \end{gather}

This formulation of electromagnetism is due primarily to the Scottish physicist James Clerk Maxwell. His equations, in one form or another, describe phenomenon ranging from the propagation of light to the deflection of a compass needle by a magnetic field.

James Clerk Maxwell (1831-1879)

The impact of Maxwell's equations extends well beyond electromagnetism: the Theory of Special Relativity is hidden inside them, and they are the prototype for a unified description of the basic forces of Nature.

Syllabus

Basic information about our schedule, homework assignments, grades, and more can be found below. Click here to download a pdf version of the full syllabus. The syllabus has more detailed information, and you should be familiar with the policies and rules it describes.

Fall 2024 Schedule

We will cover most of the first seven chapters of the textbook, along with parts of chapters 8 and 9. The table below is an estimate of how we'll spend our time.

Week Dates Chapter
1 August 26, 28, 30 1
2 September 2, 4, 6 Labor Day; 1, 2
3 September 9, 11, 13 2
4 September 16, 18, 20 2
5 September 23, 25, 27 3
6 September 30; October 2, 4 3, Exam 1
7 October 7, 9, 11 Fall Break; 3
8 October 14, 16, 18 4
9 October 21, 23, 25 4, 5
10 October 28, 30; November 1 5
11 November 4, 6, 8 5, 6
12 November 11, 13, 15 6, Exam 2
13 November 18, 20, 22 7
14 November 25, 27, 29 8; Thanksgiving
15 December 2, 4, 6 9
16 December 12 Final Exam (1-3 pm)

Please keep in mind that these dates are subject to change. This is the absolute minimum that we have to cover. But if you are engaged and active in class we can go faster and cover additional (and interesting!) material besides what is listed here.

Assignments

Homework is assigned each week (except for exam weeks) and collected the following week. With a few exceptions it will usually be due on Monday at the beginning of class. That way you can ask questions during our Friday discussion section.

Only some of the problems from each assignment will be graded. I won't tell you which ones, so you need to complete them all. We will talk more about how this works in class. Current and past assignments are listed below. You can see solutions for some (not all) problems, but they are not available for download. Please stop by my office if you'd like to see the solutions for a particular assignment.

Assignment 5
Electrostatic Potential Energy
Due on Monday, September 30

Homework 5 covers work and potential energy in electrostatics. There is also a bonus problem that investigates some common features of $$1/r^{2}$$ forces. That problem won't be graded , but it's an interesting exercise!

Assignment 4
Electrostatic Potential
Due on Monday, September 23

This is the second homework for Chapter 2, covering the electrostatic potential, Gauss's Law, and the behavior of the electric field at surfaces where there is a charge density.

Assignment 3
Electrostatics
Due on Monday, September 16

This is the first homework for Chapter 2, covering Coulomb integrals and Gauss's Law. The rules about using Mathematica and similar tools are stated at the top of the assignment. Click here for some additional tips on problems 5 and 6. Some of the integrals you need are discussed in "A Few Useful Integrals," in the Notes section.

Assignment 2
Vector Calculus
Due on Wednesday, September 4

This assignment covers the rest of our Math Methods review. Read the instructions at the top of the page – Mathematica and similar tools are not allowed!

Assignment 1
Review of Vector Analysis
Due on Wednesday, August 28

This assignment is due at the beginning of the second class. It is a review to get you up-to-speed on some aspects of vector analysis we will frequently use in class.

Working with your classmates on these assignments is encouraged! But you should only hand in work that you've completed on your own. If your solution looks just like someone else's work then you need to go back and redo it from scratch. If you can't explain each step of your solution then you haven't completed the problem on your own. Remember: the only way to be ready for the exams is to do the homework yourself.

Warning

Never, ever hand in an assignment that has been copied from a solutions manual. You won't learn anything that way, and it will earn you a grade of zero for that assignment. If it happens more than once it will be reported to the Department Chair and the Dean. Consider yourself warned. Click here to see the College of Arts and Sciences Statement on Academic Integrity.

Grades in the course are primarily determined by homework assignments and exams. The weekly homework grades contribute 35% of your final grade in the class, and two exams (on October 4 and November 15) count 15% each. A cumulative final on Thursday, December 12 (from 1 - 3 pm) is worth 30%. The remaining 5% depends on attendance and participation. Asking questions, taking advantage of office hours, and attending both lectures and discussion sections will earn you the full 5%. Check the pdf syllabus for more details.

References

The main text for the class is Introduction to Electrodynamics (4th Ed) by Griffiths. The tone of the book is casual and most students find it very accessible. When I was an undergraduate I used the the books by Wangsness and Purcell. Those texts might be useful if something in Griffiths isn't clear. A more advanced treatment is given in Jackson's Classical Electrodynamics, which is the text for practically every graduate E&M course.

1. Introduction to Electrodynamics
David J. Griffiths
2. Electromagnetic Fields
Roald K. Wangsness
3. Electricity and Magnetism
Edward M. Purcell
4. Classical Electrodynamics
J.D. Jackson

Griffiths' book has a very complete (for our purposes) discussion of vector calculus as it is used to describe electricity and magnetism. If you'd like to see additional discussions of this material, I recommend the math methods book by Boas, and also the book by Riley, Hobson, and Bence. For a more advanced treatment refer to Arfken and Weber.

1. Mathematical Methods in the Physical Sciences
Mary L. Boas
2. Mathematical Methods for Physics and Engineering
K.F. Riley, M.P. Hobson, and S.J. Bence
3. Mathematical Methods for Physicists
George Arfken and Hans Weber

The Feynman Lectures on Physics, which include a few nice discussions about some of the things we'll talk about in class, are available online. The Physics Club should also have a copy downstairs.

From time to time I will supplement material from the book with my own notes, which will be posted below.

Lecture Notes

The full set of lecture notes is available on Sakai, organized in the "Lecture Notes" folder under the "Resources" tab. Click here to access the notes.

Notes

One of the problems on Assignment 4 leads to an integral of the form \begin{gather} \int dx\,\sqrt{x^2 + \alpha^2} ~. \end{gather} Evaluating this integral requires the application of several different integration techniques, including changes of variable, trig substitution, and the method of partial fractions.

These notes review two calculations of the electrostatic potential: one obtained by integrating $$d\vec{l} \cdot \vec{E}$$ for a known electric field, and the other by adding up the infinitesimal contributions \begin{gather} dV(\vec{r}, \vec{r}\,') = \frac{dq(\,\vec{r}\,')}{|\,\vec{r}-\vec{r}\,'|} \end{gather} to the potential from each bit of a distribution of charge.

When a charge distribution is very symmetric, Gauss's Law can help us determine the electric field without having to set up and evaluate Coulomb integrals. These notes briefly review Gauss's Law, Gaussian surfaces, and how to find the electric field for a spherically symmetric distribution of charge.

Here are a few examples of integrals containing three-dimensional Dirac deltas, that you can work through if you'd like a little more review.

There is an integral requiring a trig substitution that shows up a few places on Homework 3. If you are stuck you should read these notes for an explanation.

In class we worked out the electric field at a point above or below the center of a disk. These notes go through that calculation in detail, showing all the steps of setting up and evaluating the integral.

Some extra discussion of charge distributions, the transition from a collection of point charges to an infinite number of infinitesimal charges, and the Coulomb integrals for the electric field produced by line, surface, and volume charge densities.

These notes give a brief overview of the Helmholtz theory of vectors, and some important facts about vectors with vanishing divergence or curl. A more complete discussion is given in Appendix B of the text. Some of these ideas will be developed more fully in later chapters.

A quick review of a few integrals that show up again and again on the homework.

The Dirac delta can be a little tricky, so here are some notes that expand on our quick review in class.

A quick review of line, surface, and volume integrals with several examples. The part on volume integrals isn't finished, but the stuff on line and surface integrals is there.

A review of orthogonal coordinate systems and vector calculus from my Phys 301 (Math Methods) course.

This is a basic review of line integrals – what they are, how to evaluate them, etc. It may be useful if you're a little rusty on this topic. The file is big (about 22 MB) because of the embedded plots. Let me know if you find any typos or mistakes and I will post a corrected version.

E&M Stress Relief

Sometimes the E&M wears you out, and you need a picture of an adorable little kid doing physics to get you back on track. Not a problem.