Physics 2102 Gabriela Gonz lez Charles Augustin de Coulomb 1736 1806 Gabriela Gonz lez Office hours Nicholson 271 C Tue 5 30 6 30pm Th 5 6pm or by appt Phone 578 0468 Email gonzalez lsu edu Research Detection of Gravitational Waves ligo org einsteinmessengers org 1 2102 Class website www phys lsu edu classes spring2011 phys2102 Our Section website www phys lsu edu faculty gonzalez Teaching Phys2102 Schedule grading policy syllabus all posted here Check both often Lectures will be posted in our section s website Textbook Fundamentals of Physics Halliday Resnick and Walker 9th edition We will cover chapters 21 38 in this class You have access to the online textbook in WileyPlus com Exams Two midterms 6 7pm Thursdays Feb 24 and Mar 31 Final Exam cumulative Wed May 11 3 5pm Web based system WileyPlus com To register go to http edugen wiley com edugen class cls211589 Notice that this is only for section 5 Email me gonzalez lsu edu ASAP if you have any trouble There will be one assignment per week due Wed 2am Tue late night The first assignment is due Wed Jan 26 on Ch 21 2 Score Feb 24 exam 100 pts Mar 31 exam 100 pts Final Exam 150 points Homework 25 points Course grade is guaranteed to be at least as follows 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 All together Maxwell s equations Electromagnetic waves Optical images Matter waves 3 Two types of charges positive negative Like charges repel Opposite charges attract Charles Augustin de Coulomb 1736 1806 or or 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 4 For charges in a VACUUM k Often we write k as 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 5 q1 q2 q3 20 C q1 Three equal charges form an equilateral triangle of side 1 5 m as shown Compute the force on q1 What is the force on the other charges d d q3 d q2 y 1 Solution Set up a coordinate system compute vector sum of F12 and F13 x d d 2 q 3 d Charge q placed at center What is the force on central particle 6 Total amount of charge in an isolated system is fixed conserved Example 2 identical metal spheres have charges 1C and 2C You connect these together with a metal wire what is the final charge distribution 1C 2C Charge is always found in INTEGER multiples of the charge on an electron proton why Electron charge e 1 6 x 10 19 Coulombs Proton charge p 1 6 x 10 19 Coulombs Unit of charge Coulomb C in MKS units One cannot ISOLATE FRACTIONAL CHARGE e g 0 8 x 10 19 C 1 9 x 10 19 C etc but what about quarks 7 negative electron cloud nucleus of positive protons uncharged neutrons Z atomic number of protons of electrons in a neutral atom A mass number of protons Z of neutrons N electron charge e 1 6 x 10 19 Coulombs proton charge electron mass 9 10938188 10 31 kilograms proton mass 1 67262158 10 27 kilograms neutron mass In a conductor electrons move around freely forming a sea of electrons This is why metals conduct electricity Charges can be induced moved around in conductors Blue background mobile electrons Red circles static positive charge nuclei 8 In an insulator each electron cloud is tightly bound to the protons in a nucleus Wood glass rubber Note that the electrons are not free to move throughout the lattice but the electron cloud can distort locally An object can be given some excess charge giving electrons to it we give it negative charge or taking electrons away we give it positive charge How do we do charge an object Usually moving charges from one surface to another by adhesion helped by friction or by contact with other charged objects If a conductor the whole electron sea redistributes itself If an insulator the electrons stay where they are put 9 Electric charges come with two signs positive and negative Like charges repel opposite charges attract with a magnitude calculated from Coulomb s law F kq1q2 r2 Atoms have a positive nucleus and a negative cloud Electron clouds can combine and flow freely in conductors are stuck to the nucleus in insulators We can charge objects by transferring charge or by induction Electrical charge is conserved and quantized 10
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