Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Field (physics) wikipedia , lookup
Time in physics wikipedia , lookup
Aharonov–Bohm effect wikipedia , lookup
Elementary particle wikipedia , lookup
Magnetic monopole wikipedia , lookup
Fundamental interaction wikipedia , lookup
Electrical resistivity and conductivity wikipedia , lookup
Maxwell's equations wikipedia , lookup
History of electromagnetic theory wikipedia , lookup
Electromagnetism wikipedia , lookup
Lorentz force wikipedia , lookup
Physics 7D Classical Physics Prof. Michael Cooper [email protected] FRH 2123 Administration Instructor: Prof. Michael Cooper - Office Hours: Th 9:30-10:30am in FRH 2111 (or by appointment) - Office Location: 2123 Frederick Reines Hall (FRH) Course Website: https://eee.uci.edu/17w/47340 => Syllabus, Lectures, etc. TAs: Alexis Romero, Cody Combs, Michael Fitzgerald - Office Hours: by appointment only - Tutor center: see Schedule in course website contacts Contact: [email protected], [email protected], [email protected], [email protected] - When you email us w/ questions include: - 1) your name, 2) student ID, 3) discussion session #, 4) TA name Registration Qs: All add/drop questions should be addressed to Physics Student Affairs Office (FRH 4109): [email protected] Grading ■ ■ ■ ■ ■ ■ ■ Final Exam: 40% Midterm: 25% Homework: 15% (lowest score dropped) Quizzes: 15% (lowest score dropped) Clickers: 5% Midterm Exam: Thursday, Feb 23 Final Exam: Tuesday, Mar 21 Textbook & Mastering Physics Textbook: University Physics with Modern Physics, 13th Edition, Young & Freedman (Pearson / Addison Wesley publishers). Modified Mastering Physics for online homework. Info on how to register & enroll available on course website. See a how-to video here: https://www.youtube.com/watch?v=ZojLfmTArb4 Must pay for access unless you’ve already paid when you took a prior P7 course at UCI. Self-enroll: cooper46886 Self enroll with this code: cooper46886 Title: Classical Physics: Electromagnetism [Physics 7D, W17, Cooper] Clickers Clickers will be used during lecture. They help test your level of understanding and encourage you to engage in class. • 3/4 of your score is participation; 1/4 performance (i.e. getting the right answer). • You must register your iclicker at http://iclicker.com • Even if you have registered your iclicker in the past, DO IT AGAIN. • Be sure to register with your 8-digit UCI student ID exactly as it appears in WEBREG. • An iclicker can be used by only one student during a given quarter. If you have borrowed someone else’s, make sure you register that iclicker in your name/ID #. Typical Discussion Section 1. ~25 minutes: questions/discussion regarding homework problems (or previous week’s quiz) 2. ~25 minutes: short quiz Discussion sections will NOT meet tomorrow; first discussion sections will be on Jan 18th. Chapter 21 Electric Charge and Electric Field Goals for Chapter 21 ̣To study electric charge and charge conservation ̣To see if/how objects become charged (conductors and insulators) ̣To calculate the electric force between objects using Coulomb’s Law ̣Learn the distinction between electric force and electric field ̣To calculate the electric field due to many charges (E-field superposition) ̣To visualize and interpret electric fields History Electromagnetic phenomena known by the ancients - static electricity (~600 BCE, Thales) - magnets (~100 BCE China, Greeks, Central America(?)) Observation: Static electricity exhibits both attraction and repulsion. Many early scientists thought that there were two types of forces or “fluids”. B. Franklin: No, there are two charges! (he was right) BTW: Franklin coined several electrical terms that we still use today: battery, charge, conductor, plus, minus, positively, negatively, condenser (= capacitor). Convention that electrons are (-) goes back to Franklin History 1600 - William Gilbert - Saw effects of static electricity in many materials (e.g. rubbed glass). Identified differences among substances we now know as conductors / insulators. 1660-1750 - Early electric generators (based on friction) c. 1750 - B. Franklin: lightning=electricity, same as rubbed glass. A single kind of charge “fluid” (not two; now we know that electrons are “flowing”). 1785 - Charles Coulomb => electric force follows inverse square law 1819 - Hans Oersted: compass needles deflected when near a wire carrying an electric current (i.e. current / changing electric field produces a magnetic field). 1831 - Michael Faraday and Joseph Henry: when a wire is moved near a magnet, an electric current flows in the wire (i.e. changing magnetic field creates an electric field). History 1873 - James Clerk Maxwell formulated laws of electromagnetism. This course will culminate with Maxwell’s equations. Modern view of the atom Atoms are neutral (0 charge) Equal numbers of equal magnitude, opposite-sign charged particles Electrons (-) Protons (+) Neutron (neutral) Standard unit of charge is the Coulomb e = 1.6 ×10-19 Coulombs (C) Need 6.24×1018 e for 1 C Typical charge will be in µC where µC=10-6 C 13 Static electricity + What makes plastic wrap cling? Static electricity. Some materials (like your hair or wool) give electrons off more easily than others. ← When you rub a balloon on your sweater, you’re transferring electrons from sweater to balloon. Other materials (like silk) hold on to their electrons more tightly. When you rub a glass rod on silk, the silk steals electrons from the glass rod. The glass becomes +ve. Static electricity 1st record of static electricity dates to ancient Greeks (~600 BCE) - noticed that polishing amber with fur allowed it to attract bits of straw. Amber (The ancients didn’t have plastic combs, balloons, or glass!) - The word electric derives from the Greek word for amber (elektron). Conservation of electric charge. Electric charge is conserved. Charge is not created in the process of rubbing two objects together – the electrification is due to a transfer of charge from one object to another Electric charge ̣ Materials that elucidate the basics of electrostatics: glass rods, plastic tubes, silk, and fur - how people figured out that there were 2 types of charges and that they attracted/repelled. Quiz: Electric Charge I Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges? A.one is positive, the other is negative B.both are positive C.both are negative D.both are positive or both are negative Quiz: Electric Charge I Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges? A.one is positive, the other is negative B.both are positive C.both are negative D.both are positive or both are negative Quiz: Electric Charge II From the picture, what can you conclude about the charges? A. have opposite charges B. have the same charge C. all have the same charge D. one ball must be neutral (no charge) Quiz: Electric Charge II From the picture, what can you conclude about the charges? = = = = Quiz: Electric Charge II From the picture, what can you conclude about the charges? A. have opposite charges B. have the same charge C. all have the same charge D. one ball must be neutral (no charge) Insulators and Conductors Insulators: electrons are somewhat tightly bound to atomic nuclei ̣Electron cannot move relatively freely through material ✓ ✓ When a good insulator is charged (by doing work: e.g. rubbing) in a small region, the charge is unable to move to another region Most non-metals are insulators (e.g. glass, rubber, wood) Conductors: electrons easily flow (like a liquid) from one nucleus to another ̣Electrons can move relatively freely through the material ✓ ✓ When a good conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material Most metals are conductors (e.g. copper, aluminum, silver) Charging by induction The metal ball is now “polarized” Quiz: Conductors I Two neutral conductors are connected by a wire and a charged rod is brought near, but does not touch. The wire is taken away, and then the charged rod is removed. What are the charges on the conductors? A B C D E 0 0 0 ? ? ? ? ? ? 0 + - - + + + - - Quiz: Conductors I Two neutral conductors are connected by a wire and a charged rod is brought near, but does not touch. The wire is taken away, and then the charged rod is removed. What are the charges on the conductors? A B C D E 0 0 + - - + + + - - 0 0 -+ - - + -? +? Coulomb’s law q1q2 F12 = ke 2 rˆ12 r 1736-1806 € Charles Coulomb measured the magnitudes & directions of electric forces between two small charged spheres Coulomb’s law ■ ■ q1q2 F12 = ke 2 rˆ12 r The electrical force between two stationary point charges is given by Coulomb’s Law € Force is ● inversely proportional to the square of the separation between the charges ● proportional to the product of the charges, q1 and q2, on the two particles ● directed along the line joining them ● attractive if charges have opposite signs ● repulsive if they are the same sign Coulomb’s law: Units & Constants q1q2 F12 = ke 2 rˆ12 r The SI unit of charge is the coulomb (C) ke is called the Coulomb constant €. 109 ke = 8.9876 x N m2/C2 = 1/(4πεo) (~ 9x109 N.m2/C2) εo is the permittivity of free space εo = 8.8542 x 10-12 C2 / N.m2 ● q1q2 1 q1q2 |F2 on 1| =|F1 on 2| = k 2 = r12 4πε 0 r122 usually use: ke = 9 x 109 N.m2/C2 ● ● e = 1.6 x 10-19 C Typical charges ~ µC range 1 µC needs 6.24 x 1012 electrons or protons Electric forces are vectors Force of 1 on 2 q1q2 F12 = ke 2 rˆ12 r r̂12 is a unit vector directed from q1 to q2 Like charges produce a repulsive force between them. Obeys Newton’s third law ! ! F21 = −F12 Electric forces are much stronger than gravity Compare gravitational and electric forces between an electron and a proton for a given separation r: mpme Fg = G 2 r 2 Fe = k qeqp r2 ke2 = 2 r 2 (9 ×10 9 )(1.6 ×10 −19 ) Fe ke = = Fg Gmpme (6.67 ×10 −11 )(1.67 ×10 −27 )(9.11×10 −31 ) Fe ≈ 2 ×10 39 Fg The electric force is almost 40 orders of magnitude stronger than the gravitational force. So why don’t we usually notice electric forces ? See you Next Tuesday No discussion sections tomorrow and no lecture Thursday.