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Transcript
Chapter 21 Electric Charge and Electric Field http://m.wmmr.com/2012/02/video-water-droplets- / orbiting-a-needle-in-space Our approach • Consider some initial questions and interest • Outline ideas of the electrical interaction • Consider the form charge takes • Work on visualizing charge • Do some math review (through the week) • Look at the charging processes • Grasp Coulomb’s Law (on electric force) Our approach (cont’d.) • Grasp the Electric Field • Work on visualizing the Electric Field • Understanding the Electric Field in and around conductors and insulators • Understand the dynamics of charged objects in an Electric Field • Look at electric diploes (+ other moments) • Master Electric Field and Electric force calculations Your ideas, experience • What are some phenomena involving electric charge and the electrical interaction? • Make a list on individual whiteboards, share with neighbors, class Why study E&M? • Chemistry, Biology, Geology • Technology • Bridge to conceptual understanding (conceptual technology) • Limits? (Sure: Classical theory, one interaction, within current scope of understanding) There’s unanswered questions! Electrical outline • • • • 21: Electric Charge and Electric Field 22: Gauss’s Law (use of symmetry) 23: Electric Potential 24: Capacitance, Dielectrics, Electrical Energy Storage • 25: Electric Currents and Resistance • 26: DC Circuits Magnetic outline • 27: Magnetism • 28: Sources of Magnetic Field • 29: Electromagnetic Induction and Faraday’s Law • 30: Inductance, Electromagnetic Oscillations, and AC Circuits • 31: Maxwell’s Equations and Electromagnetic Waves Do we have good answers to … • • • • Why does charge exist? What is charge? Why is charge conserved? How is the electrical interaction related to other kinds of interactions? (demo: strength of the electrical interaction) • In what forms does charge exist? (later) • What interactions involve charge? and what are their characteristics? (next) The electrical interaction (charges at rest) • As force at a distance • As force from a field - the Electric Field • As a more general understanding of interaction as the exchange of different “particles” understood as quantum mechanical states of corresponding fields • In quantum electro-dynamics (as an exchange of photons) • See diagram next for lowest order terms. An EM higher order term In what form is charge found? • (microscopically) The Standard Model – http://www.pha.jhu.edu/~dfehling/#chart • (macroscopically) Discrete q1, q2, . . . and continuous charge distributions ρ, σ, λ • Represented in terms of monopole, dipole, quadrupole, etc. Visualizing charge • Explanation activity (groups, white boards, equipment, notes, write up to hand in) • Charged balloon – http://phet.colorado.edu/en/simulation/balloons • “John Travoltage” – http://phet.colorado.edu/en/simulation/travoltage • Electroscope demonstration • Tape on table-top demonstration BUT WAIT!!! physics ≠ revealed knowledge experiment disproves (not proves) • Consider alternate one charge theory charge + + + o o o force repel attract neutral • Design an experiment to disprove this. • Write up for next class; see handout Math Review • Introduction and comments • Review (from MIT, posted) • Mostly at end of week, but you may look ahead Charging (Use whiteboards) • Describe (using diagrams) three ways of charging objects: charging by friction, charging by contact, charging by induction. – http://physci.kennesaw.edu/physlets/electricity /tm-cond-insu-17.htm – http://web.mit.edu/8.02t/www/802TEAL3D/vis ualizations/electrostatics/chargebyinduction/c hargebyinduction.htm Coulomb’s Law (use whiteboards) • What simple situation does the scalar form of Coulomb’s Law describe? (sketch the physical situation for two positive charges and include vector arrows) • Among what quantities does Coulomb’s Law provide a relation? (make a list) • Make the sketch (above) for the vector form of Coulomb’s Law? Coulomb’s Law (cont’d) (use white boards) • Sketch a more complicated situation involving discrete charges in which Coulomb’s Law could be applied. • Sketch a more complicated situation involving a continuous charge distribution in which Coulomb’s Law could be applied. • Ranking tasks (handout) The Electric Field • What, precisely, is a vector field? • How is the electric field defined operationally at a point in space? • What is the underlying physical reason the electric field can be defined this way? • How is the force on a specific charge figured out from the electric field? • Why does the electric field satisfy the superposition principle? The Electric Field representation • How is the electric field represented mathematically? Give an example. • How is the electric field represented by vector arrows? Question • How is the electric field represented by lines (of oriented grass seed)? http://web.mit.edu/8.02t/www/802TEAL3D/ visualizations/electrostatics/DestroyField/d estroyField_640.mpg … Electric Field representation • The electric field represented by the motion of test charges http://www.falstad.com/vector3de/index.html • How is the electric field represented by field lines? http://physics.bu.edu/~duffy/semester2/c0 2_visualize.html The Electric Field (cont’d) • What is an example of a complicated situation involving discrete charges in which the electric field could be defined? • Set up the sum. (Use whiteboard.) • What is an example of a complicated situation involving a continuous charge distribution in which the electric field could be defined? • Set up the integral. (Use whiteboard.) Electric field lines • What do the electric lines look like for a variety of simple charge distributions? – http://physics.bu.edu/~duffy/semester2/c02_vi sualize.html – More • What, in general, is the relation between electric field lines and charges (in a static situation)? E-fields & conductors • What is implied about E inside a conductor in a static situation? • What is implied about E at the surface of a conductor in a static situation? • What is implied about E inside a closed conducting box with no charge inside? Movement of charges in an E-field • What is the starting point for determining the motion of a charge in an E-field? Electric Dipoles • How is the dipole moment vector defined? • What is the torque on an electric dipole in an E-field? • What is the potential energy of an electric dipole in an E-field? (How is the zero of potential energy defined?) • What is the E-field produced by an electric dipole? – http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/electrostatics/PithBallsCreate/PithCr eate_640.mpg E-field calculations • along the axis away from a line of charge • along a mid-line perpendicular from a line of charge • off a mid-line perpendicular from a line of charge • along the middle perpendicular from a square of charge the end • Review these slides to refresh your memory of what we did, what you learned during our classes, and what is left to understand of chapter 21 subject matter. • What would the electric field at a point look like as a positive charge goes by in a straight line near the point? – http://web.mit.edu/8.02t/www/802TEAL3D/vis ualizations/electrostatics/MovingChargePosEl ec/MovChrgPosElec_640.mpg • … a negative charge goes by in a straight line near the point? – http://web.mit.edu/8.02t/www/802TEAL3D/vis ualizations/electrostatics/MovingChargeNegEl ec/MovChrgNegElec_640.mpg back