PHYS 1444 – Section 004 Review #1GradingElectric Charge and ConservationCoulomb’s Law – The FormulaVector ProblemsThe Electric FieldExample 21 – 14Electric FluxGauss’ LawElectric Potential EnergySlide 11Electric PotentialComparisons of Potential EnergiesProperties of the Electric PotentialE Determined from VElectrostatic Potential Energy; Three ChargesCapacitorsSlide 18Example 24 – 1Electric Energy StorageDielectricsElectric CurrentOhm’s Law: ResistanceResistivityElectric PowerAlternating CurrentWeds. Feb. 28, 2007 PHYS 1444-004, Spring 2007Dr. Andrew Brandt1PHYS 1444 – Section 004 Review #1Wednesday Feb. 28, 2007Dr. Andrew Brandt1. Test Monday Mar. 5 on Ch 21--25Weds. Feb. 28, 2007 2 PHYS 1444-004, Spring 2007Dr. Andrew BrandtGrading•Exams: 50%–Best two of three exams (2 midterms + final)–Comprehensive final –Exams will be curved if necessary–No makeup tests•Homework: 20% (no late homework)•Pop quizzes10%•Lab score: 20%Weds. Feb. 28, 2007 3 PHYS 1444-004, Spring 2007Dr. Andrew BrandtElectric Charge and Conservation•Two types of electric charge–Like charges repel while unlike charges attract•The net amount of electric charge produced in any process is ZERO!!•When a positively charged metal object is brought close to an uncharged metal object–If the objects touch each other, the free charges flow until an equilibrium state is reached (charges flow in a conductor.)–If the objects are close, the free electrons in the neutral object still move within the metal toward the charged object leaving the opposite end of the object positively charged.(induced charge)Weds. Feb. 28, 2007 4 PHYS 1444-004, Spring 2007Dr. Andrew Brandt1 2Q Q�Coulomb’s Law – The FormulaA vector quantity. Newtons•Direction of electric (Coulomb) force (Newtons) is always along the line joining the two objects.–If two charges have the same sign: forces are directed away from each other.–If two charges are of opposite sign: forces are directed toward each other. •Unit of charge is called Coulomb, C, in SI.•Elementary charge, the smallest charge, is that of an electron: F1Q1 22Q QF kr=2r�2Q�Formula191.602 10e C-= �Weds. Feb. 28, 2007 5 PHYS 1444-004, Spring 2007Dr. Andrew BrandtVector Problems •Calculate magnitude of vectors (Ex. force using Coulomb’s Law)•Split vectors into x and y components and add these separately, using diagram to help determine sign•Calculate magnitude of resultant |F|=(Fx2+Fy2)•Use = tan-1(Fy/Fx) to get angleWeds. Feb. 28, 2007 6 PHYS 1444-004, Spring 2007Dr. Andrew BrandtThe Electric Field•The electric field at any point in space is defined as the force exerted on a tiny positive test charge divided by magnitude of the test charge•The electric field inside a conductor is ZERO in a static situationFEq=rr2014Qrpe=Weds. Feb. 28, 2007 7 PHYS 1444-004, Spring 2007Dr. Andrew BrandtExample 21 – 14 •Electron accelerated by electric field. An electron (mass m = 9.1x10-31 kg) is accelerated from rest in a uniform field E (E = 2.0x104 N/C) between two parallel charged plates (d=1.5 cm), andpasses through a tiny hole in the positive plate. (a) With what speed does it leave the hole? F =qE =ma2 202v v ax= +Dipoles, torque, etc.Weds. Feb. 28, 2007 8 PHYS 1444-004, Spring 2007Dr. Andrew BrandtElectric Flux0.EE dAF = � =�rr�Weds. Feb. 28, 2007 9 PHYS 1444-004, Spring 2007Dr. Andrew BrandtGauss’ Law•The precise relation between flux and the enclosed charge is given by Gauss’ Law• 0 is the permittivity of free space in the Coulomb’s law•A few important points on Gauss’ Law–Freedom to choose!!•The integral is performed over the value of E on a closed surface of our choice in any given situation. –Test of existence of electrical charge!!•The charge Qencl is the net charge enclosed by the arbitrary closed surface of our choice. –Universality of the law! •It does NOT matter where or how much charge is distributed inside the surface. –The charge outside the surface does not contribute to Qencl. Why?•The charge outside the surface might impact field lines but not the total number of lines entering or leaving the surface0enclQE dAe� =�rr�Weds. Feb. 28, 2007 10 PHYS 1444-004, Spring 2007Dr. Andrew BrandtElectric Potential Energy•Concept of energy is very useful solving mechanical problems•Conservation of energy makes solving complex problems easier. •Defined for conservative forces (independent of path)Weds. Feb. 28, 2007 11 PHYS 1444-004, Spring 2007Dr. Andrew BrandtElectric Potential Energy•What is the definition of change in electric potential energy Ub –Ua?–The potential gained by the charge as it moves from point a to point b.–The negative work done on the charge by the electric force to move it from a to b.•Let’s consider an electric field between two parallel plates w/ equal but opposite charges–The field between the plates is uniform since the gap is small and the plates are infinitely long…•What happens when we place a small charge, +q, on a point at the positive plate and let go?–The electric force will accelerate the charge toward negative plate. –What kind of energy does the charged particle gain?•Kinetic energyWeds. Feb. 28, 2007 12 PHYS 1444-004, Spring 2007Dr. Andrew BrandtElectric Potential•The electric field (E) is defined as electric force per unit charge: F/q (vector quantity)•Electric potential (V) is defined as electrical potential energy per unit charge U/q (scalar)Weds. Feb. 28, 2007 13 PHYS 1444-004, Spring 2007Dr. Andrew BrandtComparisons of Potential Energies •Let’s compare gravitational and electric potential energies2mm•What are the potential energies of the rocks?–mgh and 2mgh•Which rock has a bigger potential energy?–The rock with a larger mass•Why?–It’s got a bigger mass.•What are the potential energies of the charges?–+QVba and +2QVba •Which object has a bigger potential energy?–The object with a larger charge.•Why?–It’s got a bigger charge.The “potential” is the same but the heavier rock or larger charge can do a greater work.Weds. Feb. 28, 2007 14 PHYS 1444-004, Spring 2007Dr. Andrew Brandt•What are the differences between the electric potential and the electric field?–Electric potential•Electric potential energy per unit charge•Inversely proportional to the distance•Simply add the potential from each of the charges to obtain the total
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