Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Properties of Electric ChargesMore Properties of ChargeProperties of Charge, finalSlide 11ConductorsInsulatorsSemiconductors, etc.Slide 15Slide 16Slide 17Charging by InductionSlide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Physics 213General PhysicsLecture 1TodaySyllabusAdministrative detailsClass OverviewIntroduction to Electricity and MagnetismStatic electric chargeCoulomb’s lawElectric field2Syllabus, notes, and homework assignmentshttp://www.pa.uky.edu/~drstrach/phy213S11.phpCan also be accessed from http://www.pa.uky.edu/~drstrach/index.phpNotes will be available at this website.Feb. 2 – last day to drop without recordApr. 1 – last day to withdraw with W grade3456DemoComb and PaperScotch Tape – repulsion and attractionPivoting Plank7Properties of Electric Charges•Two types of charges exist–They are called positive and negative–Named by Benjamin Franklin•Like charges repel and unlike charges attract one another•Nature’s basic carrier of positive charge is the protonMore Properties of Charge•Nature’s basic carrier of negative charge is the electron–Gaining or losing electrons is how an object becomes charged•Electric charge is always conserved–Charge is not created, only exchanged–Objects become charged because negative charge is transferred from one object to anotherProperties of Charge, final•Charge is quantized–All charge is a multiple of a fundamental unit of charge, symbolized by e•Quarks are the exception–Electrons have a charge of –e–Protons have a charge of +e–The SI unit of charge is the Coulomb (C)•e = 1.6 x 10-19 C11Conductors•Conductors are materials in which the electric charges move freely in response to an electric force–Copper, aluminum and silver are good conductors–When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the materialInsulators•Insulators are materials in which electric charges do not move freely–Glass and rubber are examples of insulators–When insulators are charged by rubbing, only the rubbed area becomes charged•There is no tendency for the charge to move into other regions of the materialSemiconductors, etc.•The characteristics of semiconductors are between those of insulators and conductors•Silicon and germanium are examples of semiconductorsSemiconductors, etc.•The characteristics of semiconductors are between those of insulators and conductors•Silicon and germanium are examples of semiconductors16DemoGlass, Silk, Amber, Fur conduction/induction17Charging by ConductionA charged object (the rod) is placed in contact with another object (the sphere)Some electrons on the rod can move to the sphereWhen the rod is removed, the sphere is left with a chargeThe object being charged is always left with a charge having the same sign as the object doing the chargingCharging by Induction•When an object is connected to a conducting wire or pipe buried in the earth, it is said to be grounded•A neutral sphere has equal number of electrons and protons19PolarizationIn most neutral atoms or molecules, the center of positive charge coincides with the center of negative charge In the presence of a charged object, these centers may separate slightlyThis results in more positive charge on one side of the molecule than on the other side surface of the insulatorThis realignment of charge on the surface of an insulator is known as polarizationA charged comb attracts bits of paper due to polarization of the paper20DemoComb and PaperPivoting Plank2122�23Vector Nature of Electric ForcesTwo point charges are separated by a distance rThe like charges produce a repulsive force between themThe force on q1 is equal in magnitude and opposite in direction to the force on q224Vector Nature of Forces, cont.Two point charges are separated by a distance rThe unlike charges produce a attractive force between themThe force on q1 is equal in magnitude and opposite in direction to the force on q225Vector Nature of Electric ForcesMagnitude of the forceIn the direction ofThe precise Coulomb’s law1 22| || |q qF kr=1 2122q qF kr=12r12r1 212 12212ˆ ˆ ( is a unit vector)rq qF k rr=r21 12r r=-r r26Unit VectorsA unit vector is a dimensionless vector with a magnitude of exactly 1.Unit vectors are used to specify a direction Reference: Serway and Vuille, 8th ed. Chapter 3.27Unit Vectors, cont.The symbolsrepresent unit vectorsThey form a set of mutually perpendicular vectors Right-hand convention ˆˆ ˆ, ,i j and k28Unit Vectors in Vector NotationAx is the same as Ax and Ay is the same as Ay etc.The complete vector can be expressed asiˆjˆˆ ˆˆx y zA A A= + +A i j k29Adding Vectors Using Unit VectorsUsing R = A + BThenand so Rx = Ax + Bx and Ry = Ay + By( ) ( )( )( )ˆ ˆ ˆ ˆˆ ˆx y x yx x y yx yA A B BA B A BR i j i jR i jR R R= + + += + + += +2 2 1tanyx yxRR R RRq-= + =30Vector Nature of Forces, cont.Two point charges are separated by a distance rThe unlike charges produce an attractive force between themThe force on q1 is equal in magnitude and opposite in direction to the force on q2pair)reaction -(action ˆˆ21221211222112rrqqkFrrqqkF 31Four point charges, each of the same magnitude, with varying signs are arranged at the corners of a square as shown. Which of the arrows labeled A, B, C, and D gives the correct direction of the net force that acts on the charge at the upper right corner?a. Ab. Bc. Cd. De. The net force on that charge is zeroAnswer: bQuestion :3233312Answer:( )2122223223 3223 321 32 3 in the direction D21cos 45221sin 452200The answer is B.xyxyx x xy y yQF kaQF kaQQF kaQF F kaQF F kaF F FF F F-=-== == == == + <= + <oorrSuperposition Principle34221rmmGF