Electricity and Magnetism plus Optics and Modern Physics Spring 2018 Electricity and Magnetism plus Optics and Modern Physics Instructor: Robin Côté

Course Info Course has several components: Lecture: (me talking, demos and Active learning). Homework Sets: problems from the book. Clicker questions (AA): short concept questions in class Team Problems: small questions solved on blackboard Tests: two midterms and a final. Questions on tests will look like those we do in the rest of the class; in homework and during lectures. No surprises Office hours: to answer additional questions Labs: (group exploration of physical phenomena). 2

How to do well in the course ? FINAL GRADE WILL BE MADE OF: 2 Midterms 30% Final Exam 20% Homeworks 15% Labs 25% Clicker questions 7% Team problems 3% Remember: if you miss 1 HW (out of ~10 given during the semester), you miss 2% of the final score ! if you miss more than two LABs => fail

Announcements Most of the info about the class will be posted on: www.phys.uconn.edu/~rcote lecture notes (.ppt and .pdf formats) Exam samples and solutions Syllabus Follow the link to 1202 Homework on Masteringphysics Labs start during the week of January 23.

Announcements Register for MasteringPhysics Course ID: cote90751 Homeworks will be posted on Mastering Physics Register for MasteringPhysics Course ID: cote90751 Clicker frequency: AA HW will be due usually Fri. afternoons (5:00 pm) No Late HW accepted HELP: Become familiar with the Physics Resource Center for help with problem sets. Also, the Q-Center. Note that the ACT icon is actually a hyperlink to ACT 1. Select it to view the ACT (once the lecture has been given). ACTs can also be viewed from the syllabus and the ACTs link on the homepage.

Format of Lectures Roughly 1/2 to 2/3 of the time in class devoted to presentation of material by instructor InterACTive periods during lectures where students work together on problems an “ACT” Note that the ACT icon is actually a hyperlink to ACT 1. Select it to view the ACT (once the lecture has been given). ACTs can also be viewed from the syllabus and the ACTs link on the homepage. Clicker questions to check understanding of concepts A few team problems presented (~5 minutes each) Occasional demos to illustrate key concepts

The World According to Physics 1201 • Things Specified by geometry and mass • Forces Gravity: Others: Tension, Normal, Friction • Space and Time Euclidean with Galilean Invariance “ordinary” 3D space; “slow” velocities

The World According to Physics 1202 • Things -- Bodies and Fields (E,B) Specified by geometry and mass and charge Forces Gravity: Electromagnetic: Space and Time Euclidean with Lorentz Invariance “ordinary space” but can be really really fast...

Chapter 19: Electric charge The effects of electric charge were first observed as static electricity: After being rubbed on a piece of fur, an amber rod acquires a charge and can attract small objects.

Where Does Our Study Start? Benjamin Franklin The Phenomena (known for a long time) Silk on glass Þ glass ® positive Fur on rubber Þ rubber ® negative The Concept Unlike charges attract Like charges repel

More facts about charges 2 types of charges Electric charge is always conserved Charge is not created, only exchanged Origin of charges Electrons and protons (later) Charges quantized ( e ) electron = - e proton = + e e = 1.6 x 10-19 C SI unit: C = Coulomb

How it works ... Polarization & attraction Some materials can become polarized their atoms “rotate” in response to an external charge. This is how a charged object can attract a neutral one.

19.2: Insulators vs. Conductors Insulators – wood, rubber, styrofoam, most ceramics, etc. Conductors – copper, gold, exotic ceramics, etc. Sometimes just called metals Insulators – charges cannot move. Will usually be evenly spread throughout object Conductors – charges free to move. on isolated conductors all charges move to surface. Semiconductors – in between e.g. : silicon & germanium

Charging by conduction A charged object (the rod) is placed in contact with another object (the sphere) Some electrons on the rod can move to the sphere When the rod is removed, the sphere is left with a charge The object being charged is always left with a charge having the same sign as the object doing the charging

Charging by Induction Grounded: an object connected to a conducting wire or pipe to the A negatively charged rubber rod is brought near an uncharged sphere The charges in the sphere are redistributed electrons in the sphere are repelled from electrons in the rod A grounded conducting wire is connected allows electrons to move from the sphere to the ground The wire to ground is removed, the sphere is left with an excess of induced positive charge Charging by induction requires no contact with the object inducing the charge

19-3 The Force of Electric Charges Assume that the electrical force between two charged objects acts along the line joining the centers of the charges (a Central Force). It increases if the magnitude of one of the charges increases. It increases if the distance between the charges is decreased, i.e. the charges get closer

19-3 The Force of Electric Charges Charles Coulomb (1736-1806) The electric force between two charged particles: is inversely proportional to the square of the distance between particles; increases if the magnitude of the charges increases; is attractive if the charges are of opposite sign and repulsive if the charges have the same sign.

What We Call Coulomb's Law q2 r F12 q1 F21 q1q2 1 F12= r 4pe0 r2 SI Units: r in meters q in Coulombs F in Newtons Þ 1 4pe0 = k = 8.987 109 N m2/C2 We call this group of constants “k” as in: F = k q1q2/r2 This force has same spatial dependence as gravitational force, BUT there is NO mention of mass here!! The strength of the FORCE between two objects is determined by the charge of the two objects.

Chapter 19, ACT 1 Q2 A charged ball Q1 is fixed to a horizontal surface as shown. When another charged ball Q2 is brought near, it achieves an equilibrium position at a distance d12 directly above Q1. d23 Q2 Q3 d12 g Q1 When Q1 is replaced by a different charged ball Q3 , Q2 achieves an equilibrium position at distance d23 (< d12) directly above Q3. 1: A) The charge of Q3 has the same sign as the charge of Q1 B) The charge of Q3 has the opposite sign as the charge of Q1 C) Cannot determine the relative signs of the charges of Q3 & Q1

Chapter 19, ACT 2 A charged ball Q1 is fixed to a horizontal surface as shown. When another charged ball Q2 is brought near, it achieves an equilibrium position at a distance d12 directly above Q1. Q2 Q2 d12 d23 g Q1 Q3 When Q1 is replaced by a different charged ball Q3 , Q2 achieves an equilibrium position at distance d23(< d12) directly above Q3. 2: A) The magnitude of charge Q3 < the magnitude of charge Q1 B) The magnitude of charge Q3 > the magnitude of charge Q1 C) Cannot determine relative magnitudes of charges of Q3 & Q1

What happens when you consider more than two charges? If q1 were the only other charge, we would know the force on q due to q1 . F ® 1 2 -q +q1 +q2 If q2 were the only other charge, we would know the force on q due to q2 . What is the force on q when both q1 and q2 are present?? The answer: just as in mechanics, we have the Law of Superposition: The TOTAL FORCE on the object is just the VECTOR SUM of the individual forces. F ® = 1 + 2

Chapter 19, ACT 3 Q2 Q1 3R +Q +2Q Two balls, one with charge Q1 = +Q and the other with charge Q2 = +2Q, are held fixed at a separation d = 3R as shown. Q2 Q1 R +Q +2Q Q3 2R Another ball with (non-zero) charge Q3 is introduced in between Q1 and Q2 at a distance = R from Q1 . Which of the following statements is true? (a) The force on Q3 can be zero if Q3 is positive. (b) The force on Q3 can be zero if Q3 is negative. (c) The force on Q3 can never be zero, no matter what the charge Q3 is.

Force Comparison Electrical vs Gravitational q2 r q1 Þ For a proton, q = 1.6 X 10-19 C m = 1.67 X 10-27 kg Þ * Note: smallest charge seen in nature ! G=6.7 10-11 N m2/kg2

Recap of today’s lecture Course details Charges Two types (+ and -) Like repel, unlike attract Type of materials Conductors / insulators Coulomb force