Lecture 17 Charges at microscopic level understand insulators conductors Quantify force Coulomb s law Concept of electric field Electric field due to point charge Charge at microscopic level I 2 types of charges behave like positive and negative numbers e g metal sphere is neutral after receiving equal amounts of 2 which is positive is convention Franklin glass rod positive electron attracted to it electron negative Atomic level fundamental unit of charge e for proton e for electron inherent property no other sources of charge q Np e Ne e Np Ne e charge quantization acquire positive charge by losing electron ionization negative ion extra electron Charge at microscopic level II charging by rubbing molecular ions from breaking of bonds charge conservation transferred by electrons ions qwool qplastic charge diagrams show net charge conserve charge in next diagram Insulators and Conductors insulators charges immobile Conductors e g metals valence electrons weakly bound respond to electric forces salt water ions Charging conductors in electrostatic equilibrium excess charge located on surface if in interior forces exerted causing move Discharging human body salt water is large conductor 2 conductors in contact share charge grounding object connected to earth conductor thru conductor to prevent build up of charge Charge polarization charged objects either sign force on neutral separation of charges in neutral Electric Dipole Polarization force attractive both signs of charged rods charged rod picks up paper insulator atoms polarized electrons still bound net force electric dipole two charges with separation Charging by Induction Coulomb s law equal in magnitude opposite in direction along line joining attractive for opposite repulsive for like vectors point charges size separation between static charges speed of light Using Coulomb s law Units of charge derived from current 9 2 2 19 K 9 10 N m C e 1 6 10 C Rewrite in terms of 0 F 1 4 K 8 85 1 q1 q2 4 0 r 2 10 12 C2 N m2 Superposition multiple charges 1 2 3 F net on j F 1 on j F 2 on j Strategy pictorial representation show charges forces vectors graphical vector addition x and y components Example Two 1 0 g spheres are charged equally and placed 2 0 cm apart When released they begin to accelerate at 150 meter per second squared What is the magnitude of the charge on each sphere Concept of a Field gravity electric forces long range action at a distance mechanism force changes instantly alteration of space around a mass charge gravitational electric field Faraday and Maxwell other masses charges respond to field f x y z cf particle exits at 1 point Electric Field Model more complex 2 types of charges forces materials field is agent exerting force F q E vector at every point same direction as F for q 0 independent of q since F on q q source charge create electric field E probe charge experiences F exerted by E Electric Field of Point Charge Use Coulomb s law F on q 1 q E q 4 0 r2 away from q field diagram sample vectors at tail of vector does not stretch Unit Vector Notation mathematical notation for away from q i j k magnitude 1 no units purely directional information r unit vector pointing from origin to point straight outward from point like E applies to q 0 r points towards charge
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