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Electricity: It’s All Around Us Courtesy Dr. Joseph Alward, University of the Pacific Physics Dept. Electricity • In innumerable gadgets & high technology •It holds atoms together •It’s in lightning & other sparks Electricity Comes From Charged Particles in Atoms Nucleus contains positive protons Negative electrons buzz around nucleus Nucleus also contains neutral neutrons Net charge of atoms is zero - neutral Courtesy Science Museum, London Courtesy www.energy.ca.gov/education/story/ story-html/chapter07.html Electricity Basics Like charges repel Unlike charges attract Courtesy psych.hanover.edu/Krantz/ neural/charge2anim.html Attraction and Repulsion Evidence for Two Kinds of Charge Acrylic rod charged by rubbing with wool attracts pith ball After touching, rod repels ball Rubber rod rubbed with silk attracts same ball Conservation of Charge In atoms positive and negative charges balance Removing electron produces positive ion Ions are charged atoms Imbalance can also come from adding an electron to make negative ion Electrons are neither created nor destroyed, merely transferred No one has ever witnessed electric charges, + or -, destroyed or created Positive or Negative? You rub cloth and add electrons. Are you positive or negative? Negative - - - - - - - - - You rub something else and lose electrons. Are you positive or negative? Positive + + + + + + + Coulomb’s Law F = k q1 q 2 /d2 Like gravity law) F = Gm1m2/d2 q1 is amount of charge of one particle q2 is amount of charge of other particle d is distance between particles Unit of charge is the coulomb, abbreviated C k = 9 000 000 000 N-m2 /C2 = 9 x 109 N-m2 /C2 ___________ Repelling Students Two students one meter apart each carry a charge of one Coulomb. What is the force between them? F = k q1 q 2 /d2 = 9 x 109 N-m2 /C2 x 1C2 /1 m2 = = 9 x 109 Newtons What would happen to our school if this were actually the case? How would they accelerate? a = F/m = 9 x 109 N/(6 x101 kg) = 1.5 x 108 m/s2 v = at = 1.5 x 108 m/s2 (1 sec) = 1.5 x 108 m/s Speed of light = 3 x 108 m/s Conductors and Insulators Conductors are materials in which electrons are free to move around, especially metals. •Good conductors of electricity In insulators such as rubber, paper, glass and styrofoam electrons are tightly bound to atoms. • Poor conductors of electricity Electrons Are Free To Move in Metals Semiconductors Good insulators when pure Become much better conductors when tiny amounts of impurities are added Ex. Germanium and silicon, used in transistors and microchips Superconductors Metals that become infinitely conducting at low temperatures Electric current can flow forever without energy input Methods of Charging •Friction – rubbing transfers charge •Contact – touching leads to charge transfer •Induction – bringing charged object near causes redistribution of charge •Grounding – charges repel or attract to or from an effectively infinite reservoir of charge such as the ground Charging by Friction Charging by Contact Some electrons transfer from rod to ball Charging by Induction(conductors) Courtesy of the Physics Classroom Charging by Induction(insulators) Such separation of charge is called charge polarization Polarizing Atoms Explain This Explain This A charged comb attracts little bits of paper Courtesy Dr. Joseph Alward, University of the Pacific Physics Dept. Explain This Applications of Electrostatic Charge Negatively charged paint adheres to positively charged metal Application Fine mist of negatively charged gold particles adhere to positively charged protein on fingerprint. (From Eugene Hecht's Physics, 2nd Edition Brooks/Cole Publishing) Electrostatic Air Cleaner Xerox machine You find out how it works Charge Coupled Device (CCD) A semiconductor device used to record light and make images. Capacitor A device used to store charge Basically consists of two metal plates oppositely charged, and not touching + - In between there may be an insulating material called a dielectric to boost the amount of charge it can store A Capacitor Stores Electric Energy A battery produces electric energy bit by bit A capacitor is NOT a type of battery A battery can be used to charge a capacitor The energy stored is the work done to charge it Capacitor Design One style is like a rolled up sandwich of foil and wax paper Capacitance Units Defined by Q = CV Symbol C Unit: coulombs per volt = farad 1 pf = 1 picofarad = 10-12 farad 1nf = 1 nanofarad = 10-9 f 1mf = 1 microfarad = 10-6 f C is Constant for a Given Capacitor Does not depend on Q or V Proportional to area Inversely proportional to distance between plates C = e0 A/d If dielectric like oil or paper between plates use e = Ke0; K is called dielectric constant e is called permittivity. e0 is permittivity of free space Capacitors Photos courtesy Illinois Capacitor, Inc Charging and Discharging Capacitors can charge slowly and discharge quickly, as in an electronic flash This is why your digital camera flash needs time in between shots Applications In automotive ignitions In strobe lights In electronic flash In power supplies In nearly all electronics Capacitor Safety A high voltage capacitor can kill Do not open a computer monitor or old style TV set Electronic technicians know to discharge large capacitors with a screwdriver or other insulated tool How Big Are Capacitors? Previously capacitors were usually no bigger than millifarads. A one Farad capacitor would be as big as a refrigerator. But today’s supercapacitors can pack several Farads into a few cubic centimeters. The biggest can be thousands of Farads Supercapacitors* use double layer electrolyte technology, usually with activated carbon The carbon has huge surface area *Also called ultracapacitors Supercapacitor Advantages Charge much more quickly than batteries Deliver energy much more quickly than batteries Store much more energy than regular capacitors Supercapacitor applications Power hybrid electric motor vehicles for short bursts Backup power for computers Operate emergency doors and slides in commercial aircraft Lower energy density but greater power density than batteries More info