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 secreted away 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 2018 Schedule

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

Week Dates Chapter
1 August 27, 29, 31 1
2 September 3, 5, 7 1, 2
3 September 10, 12, 14 2
4 September 17, 19, 21 2
5 September 24, 26, 28 2, 3
7 October 1, 3, 5 3
8 October 8, 10, 12 Fall Break, 3
9 October 15, 17, 19 3, 4
10 October 22, 24, 26 4
11 October 29, 31, November 2 4, 5
12 November 5, 7, 9 5
13 November 12, 14, 16 5, 6
14 November 19, 21, 23 6, Thanksgiving Break
15 November 26, 28, 30 7
16 December 3, 5, 7 9

Please keep in mind that these dates are subject to change. In the past the course has met on TTh, but this semester it is moving to MWF. I may move some things around as a result, or I might decide to spend more or less time on a particular topic. I will always notify you about any changes I make to this schedule.

Assignments

Homework is assigned each week (except for exam weeks) and collected the following week. Only some of the problems from each assignment will be graded. I won't tell you which ones, so you need to complete all the problems. Current and past assignments are listed below. Solutions are available for some (not all) problems, but I am no longer making them available for download — please stop by my office if you'd like to see the solutions for a particular assignment.

Assignment 11
The Vector Potential, Magnetization
Due on Friday, November 30.

This assignment covers calculations of the vector potential (including the multipole expansion), and the properties of materials with a permanent magnetization.

Assignment 10
Magnetostatics, Biot-Savart, and Ampère's Law
Due on Monday, November 12

This is a chance to practice calculating the magnetic field using the Biot-Savart law and Ampère's Law. (Material similar to problems 1 and 2 may appear on our second exam, on November 16.)

Assignment 9
Electric Fields in Matter
Due on Monday, November 5

This assignment covers dipoles, polarization, and the response of linear dielectric materials to electric fields..

Assignment 8
The Multipole Expansion
Due on Monday, October 29

This is the last assignment for Chapter 3, covering the sections on the Multipole Expansion of the potential.

Assignment 7
Separation of Variables
Due on Monday, October 22

Separation of variables is a very important technique for solving the Laplace and Poisson equations. It is often dramatically easier than evaluating the integrals for \(V\) and \(\vec{E}\).

Special Assignment
Post-Exam Problems
Solutions
Complete by October 19

This optional assignment addresses problem areas on the first exam. There are two questions, and if you complete them I will consider adding a small amount of extra-credit to your grade on the test. Read the instructions carefully, and email me if you have any questions.

Assignment 6
Method of Images
Due on October 15

This is the first homework for Chapter 3, with problems that address the “method of images”.

Assignment 5
Electrostatic Potential Energy
Due on Monday, October 1

Homework 5 covers work and potential energy in electrostatics, as well as some common features of \(1/r^{2}\) forces.

Assignment 4
Electrostatic Potential
Due on Monday, September 24
(Update: Corrects a typo in problem 6)

This is the second homework for Chapter 2, covering the electrostatic potential. Notice the question at the end of problem 6, after you calculate the potential.

Assignment 3
Electrostatics
Due on September 13

This is the first homework for Chapter 2. The rules about using Mathematica and similar tools are stated at the top of the assignment. (They are not allowed, just like on the last assignment.)

Assignment 2
More Vector Analysis
Due on September 5

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 August 27

This assignment is due at the beginning of the first class. It is a review to get you up-to-speed on some aspects of vector analysis that 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.

A 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

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 (dates TBA) count 15% each. A cumulative final on Friday, December 16 (from 1:0-3:00 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. There should also be a copy of the lectures in Isaac & Al's.

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

Notes

Linear Dielectrics
A few comments about linear dielectrics, with an example of how to use Gauss's Law to find \(\vec{D}\) and then relate it to \(\vec{E}\) using \(\vec{D} = \epsilon_{0} \vec{E} + \vec{P}\).

Forces Between Multipoles
The Coulomb force between two point charges is simple: it is either attractive or repulsive, proportional to the product of the charges and inversely proportional to the square of the distance between them. But forces between a point charge and a dipole or between two dipoles, both of which are derived in this note, are much more complex.

The Multipole Expansion
A quick review of the Multipole Expansion, with a few example calculations.

Separation of Variables for a Spherical Shell with Surface Charge
These notes provide a detailed discussion of an example we worked out in class: the potential inside and outside a spherical shell with the azimuthally symmetric surface charge density \(\sigma(\theta) = \sigma_{0} \cos\theta\). Please take a look, especially if you have questions about Assignment 7.

A Tricky Integral
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 variables, trig substitutions, and the method of partial fractions.

Calculating the Electrostatic Potential
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 contributions \(dq(\vec{r}\,')/|\vec{r}-\vec{r}\,'|\) from a distribution of charge.

The Electrostatic Potential
These notes explain why the electric field has a scalar potential, and how to find it based on the distribution of charge. Please familiarize yourself with the results in these notes before Dr. Tangarife's lecture on Friday, September 14.

Using Gauss's Law
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.

Another Integral from Homework 2
I've gotten a few questions about an integral that shows up in problems 1 and 4 of Homework 2. It involves a trig substitution that might not be obvious -- read these notes for an explanation.

An Electric Field Example

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, and working out the behavior of the electric field far away from and very close to the disk.

Coulomb Integrals

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.

The Helmholtz Theory of Vectors

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 Few Useful Integrals

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

The Dirac Delta

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

Examples of Line, Surface, and Volume Integrals

A very quick and very rough 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.

Orthogonal Coordinate Systems
Vector Calculus

A review of orthogonal coordinate systems and vector calculus for students who did not take Phys 301 (Math Methods) in the Spring 2018 semester.

Line Integrals

This is a very 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 various 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.