Physics 2102 Jonathan Dowling Benjamin Franklin 1705 1790 Physics 2102 Lecture 01 MON 12 JAN Electric Charge I Version 1 12 09 Charles Augustin de Coulomb 1736 1806 Who am I Why am I Here Prof Jonathan P Dowling 1994 98 Research Physicist US Army Aviation Missile Command 1998 2004 Principal Scientist NASA Jet Propulsion Laboratory 2004 Present Director Hearne Institute for Theoretical Physics LSU Office hours Nicholson Hall 453 MWF 10 30 11 30AM or by appointment Phone 225 578 0887 Email jdowling lsu edu My Own Research Quantum Optics Quantum Computing Photonic Crystals Hearne Institute for Theoretical Physics Quantum Sciences Technologies Group Course Details Main Class Website for All Sections http www phys lsu edu classes spring2009 phys2102 Syllabus Schedule Grading Policy Exam Solutions Lectures will be posted in this section s website http phys lsu edu jdowling PHYS21022SP09 Text Fundamentals of Physics Halliday Resnick and Walker 8th edition We will cover chapters 21 36 Exams Midterms 6 7PM THU 05 FEB THU 05 MAR THU 02 APR Final Exam 5 30 7 30PM FRI 08 MAY Lab Meets This Week Show up or be dropped Tutoring Free Tutors in 102 Nicholson Hall Course Details Homework Web based system Web Assign To register Go to http www webassign net student html On the left frame student login Username pawsusername lsu Institution lsu Password lsuidnumber Choose credit card registration or FREE with Purchase of PHYS2102 Book from Bookstore One Assignment Per Week Due 2AM Wednesdays First HW Is Posted This Week Due Next WED 2AM Course Details Grading A 90 B 80 89 C 60 79 D 50 59 Borderline Cases Decided by Class Attendance F 50 What Are We Going to Learn A Roadmap Electric Charge Electric Force on Other Electric Charges Electric Field and Electric Potential Moving Electric Charges Current Electronic Circuit Components Batteries Resistors Capacitors Electric Currents Magnetic Field Magnetic Force on Moving Charges Time Varying Magnetic Field Electric Field More Circuit Components Inductors AC Circuits Maxwell s Equations Electromagnetic Waves Light Waves Geometrical Optics Light Rays Physical Optics Light Waves Interference Diffraction Let s Get Started Electric Charges Two Types of Charges Positive Negative Like Charges Repel Opposite Charges Attract Atomic Structure Structure Negative Electron Cloud Nucleus of Positive Protons Uncharged Neutrons The Unit of Electric Charge is the Coulomb which is C Proton Charge e 1 60 10 19 C Rules of Attraction and Repulsion Discovered by Benjamin Franklin Benjamin Franklin 1705 1790 Force Between Pairs of Point Charges Coulomb s Law q1 Charles Augustin De Coulomb 1736 1806 F12 q2 F21 F12 q1 q2 F21 F12 q1 q2 F21 Coulomb s Law the Force Between Point Charges Lies Along the Line Connecting the Charges Is Proportional to the Magnitude of Each Charge Is Inversely Proportional to the Distance Squared Note That Newton s Third Law Says F12 F21 Coulomb s Law q1 F12 F21 r12 k q1 q2 F12 2 r12 F N Newton The k is the electric constant of proportionality 2 N m k 8 99 10 9 C2 Usually we write k 1 Units q2 12 with 8 85 10 0 4 0 r m meter C2 N m2 q C Coulomb Two Inverse Square Laws Newton s Law of Gravitational Force Coulomb s Law of Electrical Force Area of Sphere 4 r2 Number of Lines of Force is Constant Hence Force Per Unit Area is Proportional to 1 r2 Superposition Question How Do We Figure Out the Force on a Point Charge Due to Many Other Point Charges Answer Consider One Pair at a Time Calculate the Force a Vector In Each Case Using Coulomb s Law and Finally Add All the Vectors Superposition Useful To Look Out for SYMMETRY to Simplify Calculations Feel the Force Example Three Equal Charges Form an Equilateral Triangle of Side 1 5 m as Shown Compute the Force on q1 What are the Forces on the Other Charges Solution Set up a Coordinate System Compute Vector Sum of F12 and F13 q1 q2 q3 20 mC q1 d d q3 d q2 60 F13 1 o F12 y d d 2 x 3 d Another Example With Symmetry k 2q q F r2 r q Charge q Placed at Center F What is the Force on Central Particle All Forces Cancel Except From 2q
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