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Physics 272: Electricity and Magnetism Instructor: Mark Palenik Office hours: by appointment Monday June 11th 2012 Course Info Syllabus and schedule posted on course website Homework through webassign, due Thursday and Sunday Bring iclickers to class Help center available for homework assistance (room 12) I will be teaching weeks 1-4 (Mark Palenik) Office hours available by appointment Jonathan Nistor will teach weeks 5-8 Dr. Hirsch Says. . . Dr. Hirsch Textbook The textbook is Matter & Interactions, vol III: Electric & Magnetic Interactions by R. Chabay & B. Sherwood (John Wiley & Sons 2007). We will cover almost all of the topics in this volume. See the table of contents at the front of the book (which also includes the contents of Volume I). Make sure it is the Third Edition. The new book comes with a free coupon for WebAssign, the on-line homework service. Follow the instructions and get yourself registered. General Information Room PHYS 144: Undergraduate office Room PHYS 12: Help center Room PHYS 290: Physics Library We will use WebAssign for homework and lab assignments. You will be able to access your scores in CHIP. See “Important Links” on course web page for details! For questions concerning WebAssign contact: V.K. Saxena: Office: PHYS 176, Phone: 49575 Activities and Responsibilities • In-class activities and responsibilities – You are responsible for attending all classes, and attendance will count toward your grade. – Bring a scientific calculator to class. – If you miss class, it is your responsibility to find out what you missed. • Homework – Homework and lab assignments will be posted on the web. Assignments are due on Thursdays and Sundays. • Outside class – Study assigned textbook sections. – An assignment to study sections of the textbook means: • Read the assigned textbook sections thoughtfully. • Do the "stop and think" activities. • Write brief solutions to the in-line "exercises" and keep them in a notebook. Quizzes, Exams, Grades • Clicker Questions in Lecture: – Short multiple choice questions will be posed in lecture. The purpose is to assess your understanding. It will also be used to check attendance. – You have to purchase an iClicker ( http://www.iclicker.com ) from the bookstore. – You must register your clicker ID in CHIP!!!! • Exams: – There will be a two hour midterm and a 2-hour final exam. Exams will be during lab. Exams are closed-book, but relevant formulas and constants will be provided. • Grades: – This was how the course was graded last semester—possibly still. – The final grade will be determined on the following basis (Course Total = 700 points): • 200 points - final exam • 200 points midterm • 75 points - WebAssign homework • 100 points – Labs – lab quizzes need to be added to CHIP • 25 points - Clicker Questions & Attendance • 100 points – Recitation Problems Lectures don’t work?? • Say hello to the people next to you • Form small groups with your surrounding people. You’ll be discussing with them. • Don’t worry about sitting next to the same people from lecture to lecture, but you can if you want Today • • • • Charges Review vectors and force Force between two charged objects Electric field of a point charge Electric Charge • • • • Two kinds of charges, positive (+) and negative (-) Protons have positive charge (1.602x10-19 C) Electrons have negative charge (-1.602x10-19 C) Two positive charges repel – Proton/proton • Two negative charges repel – Electron/electron • Opposite charges attract – Proton/electron Protons and Electrons • Protons and electrons have equal and opposite charges • Protons are much more massive (~2000x) than electrons e = 1.602x10-19 C Charge (C) Mass (kg) Proton 1.602x10-19 1.7x10-27 Electron -1.602x10-19 9x10-31 • Electrons are fundamental particles, protons are composed of 3 particles (quarks) U P D U E Review: Force • From Newton’s laws (II): 𝐹= 𝑑𝑃 𝑑𝑡 𝑑 𝑚𝑣 𝑑𝑡 m is constant = 𝑑𝑣 =𝑚 = 𝑚𝑎 𝑑𝑡 • Also recall (III): For every action, there is an equal and opposite reaction* *There is a case you will learn about in magnetism where this is not quite true, although we still have conservation of momentum which usually follows from the 3rd law. Review: Vectors • Vectors have magnitude and direction – e.g. 4 meters east, <4, 0, 0>, or 4𝑥 ? Direction Magnitude Point in the 𝑟 direction 𝑟=<x,y,z> 1 𝑟 𝑟 = 2 2 2< x , y , z > = 𝑥 +𝑦 +𝑧 𝑟 More with vectors • Charge q1 is located at <x1,y1,z1> • Charge q2 is located at <x2,y2,z2> 1. Write an expression for the position of q2 relative to q1 2. Write an expression for the position of q1 relative to q2 q1 q2 1. <x2 – x1, y2 – y1, z2 - z1> = 𝑟2 − 𝑟1 = 𝑟21 2. <x1 – x2, y1 – y2, z1 - z2> = 𝑟1 − 𝑟2 = 𝑟12 Conceptual question • Given two charges, q1 and q2, is this a possible expression for force: • The force on q2 due to q1: 𝐹= 1 𝑞1 𝑟21 2 4𝜋𝜖0 𝑟21 The force on q1 due to q2 1 𝑞2 𝐹= 𝑟12 2 4𝜋𝜖0 𝑟12 Conceptual question 2 • Given two charges, q1 and q2, is this NEW expression a possible expression for force: • The force on q2 due to q1: −1 𝑞1 𝐹= 𝑟21 2 4𝜋𝜖0 𝑟21 The force on q1 due to q2 −1 𝑞2 𝐹= 𝑟12 2 4𝜋𝜖0 𝑟12 Force on charges • Two charges, q1 and q2 • Force on q1 due to q2 𝐹= 1 𝑞1𝑞2 𝑟 4𝜋𝜖0 𝑟12 2 12 • Force on q2 due to q1 𝐹= Equal magnitude, opposite sign Satisfies Newton’s 3rd law 1 𝑞1𝑞2 𝑟21 2 4𝜋𝜖0 𝑟21 We will see there is a relationship between this force and something called the electric field “Clicker” question • Two positively charged particles start from rest at point A) • Which of the following is true about the particles at B) and C) A) B) C) a) The particles are moving faster at B) than at C) b) The particles are moving faster at C) than at B) c) The particles are moving the same speed at B) and C) Recall: 𝐹 = 1 𝑞1𝑞2 𝑟12 2 4𝜋𝜖0 𝑟12 Electric fields • We could rewrite the previous equation as 𝐹 = 𝑞𝐸 • Force acting on a particle = charge of the particle * the electric field it is sitting in • Electric fields point away from negative charges and toward positive charges • Point in direction that positive charge would be pushed. Electric Fields (Coulomb’s law) • The electric field produced by a particle with charge q is 1 𝑞 𝑟 2 4𝜋𝜖0 𝑟 • Where r is the distance from the particle, and 𝑟 is a unit vector pointing away from the particle. • Pointing stuff? • If a particle at <-1, -1, -1> has a charge of 2C, what is the electric field at <1, 1, 1>? • What is the force on a particle at <1, 1, 1> with a charge of 3C? • What is the force on the first particle, with a charge of 2C? Structure of atoms • Atoms are made up of protons, neutrons, and electrons • The nucleus is a tiny object at the center of the atom • An electron cloud surrounds the nucleus • Question: If the nucleus is made up of protons and neutrons, which are Larger than electrons, why is the electron cloud so big and the nucleus So small? nucleus Electron cloud Charge of atoms • Atoms are usually neutral (equal number of protons and electrons) • Charged atoms are called ions • Electrons can be pulled from the nucleus, but it requires work – Must remove an integer number of electrons • The protons at the center attract the electrons, but the electrons repel each other. • The attractive force must be slightly larger than the repulsive force for electrons to remain bound Extra: Electric fields in other dimensions • Later on you will learn about flux, which explains why E goes as 1/r2. • In the mean time, let’s see if we can figure out how E would behave in 1 or 2 dimensions. • Maxwell’s equations say • In 1d, this becomes 𝜕𝐸𝑥 𝜕𝑥 𝜕𝐸𝑥 𝜕𝑥 = 𝑞 𝜖0 + 𝜕𝐸𝑦 𝜕𝑦 + 𝜕𝐸𝑧 𝜕𝑧 = 𝑞 𝜖0 Extra: 1d Electric field • Consider a 1 dimensional world, where everything is on a line (x axis). Point charge Q at x=0 X’ • What is the charge at x=x’? X’ can take on any value except zero. 𝜕𝐸𝑥 𝑞 • Using Maxwell’s equation = what is the derivative of 𝜕𝑥 𝜖0 the electric field at x’? • What does this tell us about the electric field? Extra: Electric field in n dimensions • • • • In 1d: Electric field goes as 1/r0 In 2d: Electric field goes as 1/r In 3d: Electric field goes as 1/r2 In nd: Electric field goes as 1/rn-1