Material in the Course

Syllabus for PHYS 181.002

Spring 2008

Instructor: Len Zane, BPB-202, 895-1789, len.zane@unlv.edu

Course Website: www.unlv.physics.edu/~lenz/

Texts: Physics for Scientists and Engineers with Modern Physics: A Strategic Approach by Randall D. Knight and the Solution Manual for chapters 20-42 by Pawan Kahol and Donald Foster

Chapters 25 through 35 (Electricity and Magnetism)

Tuesday

Thursday

January 22, Intro and chapter 25

January 24, chapter 25

January 29, chapter 26

January 31, chapter 26

February 5, chapter 26

February 7, chapter 27

February 12, chapter 27

February 14, chapter 28

February 19, chapter 28

February 21, review for test

February 26, test over 25 - 28

February 21, return test & chapter 29

March 4, chapter 29

March 6, chapter 29

March 11, chapter 30

March 13, chapter 30

March 18, spring break

March 20, spring break

March 25, chapter 31

March 27, chapter 31

April 1, test over 29-31 (No kidding!!)

April 3, return test & chapter 32

April 8, chapter 32

April 10, chapters 32-33

April 15, chapter 33

April 17, chapter 33

April 22, chapter 34

April 24, chapter 34

April 29, chapter 34 & 35

May 1, chapter 35

May 6, chapter 35

May 8 review for final

May 13, Final Exam, 1:00 p.m., BPB-106

Material in the Course

The material covered in the three semester engineering physics sequence is the culmination of literally thousands of years of observation, experimentation, and codification. During this semester we will cover the topics dealing with electricity and magnetism. It is important to keep in mind that the laws presented in this book, the laws that describe the behavior of physical systems involving electricity and magnetism, were not and are not obvious. But instead they represent the end point of a long and difficult process that began with casual observations, followed by more careful experimentation, until finally, some very smart person was able to find a way to describe a range of phenomena with a “simple” mathematical statement leading to a new “law” of physics. Oftentimes the “simple” statement requires learning the definitions of new quantities, for example, Electric or Magnetic Field, or Electrical or Magnetic Flux. The list below summarizes the laws we will explore this semester but not the quantities needed to actually use a particular law to solve a problem.

Coulomb’s Law – Chapter 25

Gauss’ Law – Chapter 27

Kirchoff’s Laws – Chapter 31

Biot-Savart Law – Chapter 32

Ampere’s Law – Chapter 32

Faraday’s Law – Chapter 33

Lenz’s Law – Chapter 33

Lorentz Force Law – Chapter 34

Maxwell’s Equations – Chapter 34

Maxwell’s Equations are actually a condensation and clarification of the information in the laws of Gauss, Ampere, Lenz, and Faraday, into one self-consistent set of equations. Maxwell accomplished this synthesis in the middle of the 19^th century.

The challenges of this course are multidimensional. Starting from the laws listed above, the first challenge is to understand a particular law, that is the physical content of the law and how to use the law to calculate something useful. For example Coulomb’s law allows you to calculate the force on a point charge due to an array of other point charges. (The underlying assumption is that if you know the force acting on a particle, then you can use what you learned in PHYS 180 to figure how the point charge will move under the influence of the electric force.)

If the array consists of a discrete number of point charges, the problem reduces to one of adding vectors. Therefore if you have trouble adding vectors, you will have trouble using Coulomb’s law! When dealing with a continuous array of charges, a line of charges for example, integral calculus must be used to find the force on a charge near a line of charges. Also, very early on, the concept of Electric Field is introduced as a way of visualizing how a set of charges acts on another charge. Consequently understanding Coulomb’s law requires certain mathematical skills coupled with the ability to grasp and incorporate new concepts like the Electric Field and then to correctly calculate required quantities using this information. The other laws listed above require similar skills and abilities.

As if this wasn’t daunting enough, a physics course requires that you use your new found understanding of various physical phenomena to solve word problems involving charges, currents, batteries, etc., in various situations and configurations. Some of the situations will involve how an object moves under the influence of electric and/or magnetic fields, questions that will require you to use stuff learned in PHYS 180!

The good news is that all the topics in this semester’s course are related to one another so the material forms a coherent set of topics. This will be more apparent at the end of the semester than it is in the beginning, but it will be useful for you to look for similarities, connections, and analogies as the course moves from chapter to chapter.

MIT Open Course work on the Web

The following link http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-and-_{MagnetismSpring2002}/_{VideoLectures}/index.htm will take you to the webpage of the physics course 8.02 that was taught at MIT in the spring of 2002. In particular, the page lists the video lectures of Professor Walter Lewin who taught the course and has become a cyber-space physics guru. The lectures cover the material in PHYS 181 but not in the same order in the same way since the MIT course is built around a different but equivalent textbook. I strongly encourage you to watch the lectures which require RealPlayer to be loaded onto your computer, a free download. Professor Lewin has some very entertaining demonstrations which typically occur at the end of his lectures.

Lecture 1 – Has an excellent introduction to electricity and mirrors much of the material in Chapter 26 of our text, Electric Charges and Forces.

Lecture 2 – Covers much of the material in Chapter 27, The Electric Field.

Lecture 3 – This lecture meshes with the material in Chapter 28, Gauss’s Law.

As of January 11, those are the three that I have seen and they were informative and entertaining. The other lectures that cover material in our course are, based on their descriptions, as follows:

Lectures 4 and 5 – Chapters 28, 29, and 30

Lecture 6 – Fun stuff that ties together the material covered so far

Lectures 7 and 8 – Chapter 30 plus other stuff

Lectures 9 and 10 – Chapter 31

Lectures 11, 13, and 14 – Chapter 32

Lecture 12 – Professor Lewin reviews material for an exam. Probably valuable!

Lectures 15, 16, and 17 – Chapter 33

Lecture 18 and 22 – Chapter 34

Lecture 19 – Fun stuff about magnetism

Lectures 20, 24, and 25 – Chapter 35

Lecture 21 is the one that seems least connected to our material. So it can be skipped or watched.

Lecture 23 – Another review. Probably useful.

If you watched all or most of these lectures please let me know if you found them valuable. Thanks.

Words of Advice

In my experience, one of the biggest challenges in any physics course is that in order to correctly solve a problem you have to understand the underlying physical laws that apply, then you need to know how to apply those laws to the particular situation defined by that problem. If this is done correctly, you are typically left with some equations that must be solved. And if the equations are solved correctly, the answer will be right, that is agrees with some solution manual answer. A “wrong” answer can mean that you used the wrong physical law, for example conservation of kinetic energy instead of momentum, or that you applied the correct principle incorrectly, or that you set the problem up correctly but made a mistake in mathematics, or the answer in the manual is written in a different but equivalent way, or, perish the thought, the answer manual has the incorrect answer! Figuring out where you went astray is the biggest obstacle to learning physics.

Suggestions for Problem Solving

1. One of the easiest ways of checking an answer is to make sure your answer has the correct units. This presumes that you know what the correct units are and that your answer is in form that allows a simple check of units. To this end, I suggest using things like Q1 and Q2 or q and Q for charges and to not immediately put in the values of the charges. The same is true for any distances in the problem or any other quantity. This will encourage you to develop an eye for units. For example, in chapter 25 you will learn that the force between two point charges is given by kqQ/d² , where k is a constant that has the correct units to give force when multiplied by two charges in Coulombs and divided by a distance in meters squared. Consequently, any answer in chapter 25 that is supposed to be a force ought to have the constant k times two charges with a distance squared in the denominator. Any answer that does not have those pieces in the correct spots to the appropriate powers is by necessity wrong! If you are very careful to always write out the units of any numbers inserted into an equation, you could check units after replacing the symbolic quantities with their actual values in a particular problem. I have never been able to do this in a consistent fashion and find working in symbols much, much better. Then I replace the symbols with their values as the last step in the problem after making sure the units are correct.

2. My next suggestion is more of a strategy for doing the exercises and problems at the end of each chapter. In my experience, students have a tendency to do the problems by “fishing” for the correct formula in the chapter. That is, after a problem is read, the chapter is scanned for what looks like an appropriate equation. The definition of an appropriate equation is one that gives the “correct” answer. This is a seriously flawed strategy. Instead I suggest the following. Xerox the problems at the back of the book. Then put some physical distance between the problems and the textbook. Now sit down with the problems and attempt to do them without having the textbook at hand. If you can’t, then it is time to re-read the text trying to focus in on the material you did not know that kept you from doing that particular problem. When you can do the exercises and problems at the end of the chapter without using the textbook as a crutch, you will have made great progress in learning the material. This is also great preparation for the examinations since the textbook is not available during tests!

3. The last thing I encourage you to do is to attempt to evaluate the “correctness” of your answer before checking it against the solution manual. Are the units correct? Is the sign of the answer correct? If d is the distance between two charges, d has to be a positive real number. Is the answer the correct mathematical entity? For example, is the answer a vector instead of a scalar. If a vector is called for and your answer is a scalar, your answer cannot be correct! If the answer is a numerical value, is it a “reasonable” value? If the answer is in symbolic form, does it have the right dependence on the parameters that constitute the answer? I will give examples of these checks during the semester. Asking these questions helps you develop important problem solving skills.

Learning how to assess the “correctness” of an answer is extremely valuable because it encourages you to develop a better understanding of the simpler, limiting situations, which are then used to develop a better grasp of more complicated situations.

Grading

Physics is learned by working problems. The number of problems that have to be worked by you to understand the material in a particular chapter is impossible for me to know. Each chapter in the text lists has a summary of the most important concepts for that chapter. Doing problems that help you learn those highlighted concepts is obviously a useful to study each chapter. It is your responsibility to manage your time efficiently and to learn the material in each chapter. I will also do some representative problems from each chapter to help you see how mathematics and physics work in unison to solve problems. Homework will not be collected or graded but below is a list of ten problems from each chapter that sample the material covered. The list is not exhaustive but is just a guide. If you are having trouble with a particular type of problem, do more problems covering that topic!

Chapter 25 B problems 7, 14, 27, 30, 36, 46, 53, 58, 60, 66

Chapter 26 B problems 9, 19, 22, 27, 28, 34, 38, 44, 48, 52

Chapter 27 B problems 2, 5, 13, 23, 26, 28, 34, 44, 48, 50

Chapter 28 B problems 3, 9, 16, 21, 28, 32, 37, 44, 46, 52

Chapter 29 B problems 3, 5, 15, 21, 23, 30, 38, 40, 48, 70

Chapter 30 B problems 9, 19, 34, 37, 43, 45, 54, 66, 70, 74

Chapter 31 B problems 11, 25, 33, 34, 38, 41, 43, 62, 66, 70

Chapter 32 B problems 4, 7, 19, 23, 27, 40, 48, 50, 61, 62

Chapter 33 B problems 3, 7, 13, 23, 27, 34, 44, 48, 68, 71

Chapter 34 B problems 5, 8, 13, 15, 28, 37, 44, 45, 47, 54

Chapter 35 B problems 6, 14, 18, 26, 33, 37, 38, 47, 51, 60

There will be two one hour exams. Each exam will count for 30% of your grade. The dates and chapters covered on those exams is listed on the syllabus. The final will be comprehensive but will emphasize the material covered after the second exam. The final will count for 40% of your grade. Note that calculators will not be allowed for any of the examinations. The numbers on the examinations will either be simple enough to be calculated by hand or the answers will be symbolic, that is letters without numbers!

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