Physics 213 General PhysicsSlide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13AC Generators, contAC GeneratorsSelf-inductanceSlide 17Slide 18Slide 19Slide 20Inductor in a CircuitRL CircuitPhysics 213General PhysicsLecture 122Last Meeting: Faraday’s Law of Induction, Lenz’ LawToday: Finish Faraday’s and Lenz’ Laws, Self Inductance, RL Circuits, Energy Stored3456( )Blx xBl Blvt t tjeD D D=- =- =- =-D D D789Demo Rail and Magnetic Flux10Demo Levitating Floppy Disc in Large Magnet Levitating Metal Plate Magnets Falling through Copper and Plastic Tubes Eddy Current Pendulum Thomson Coil11121314AC Generators, contBasic operation of the generatorAs the loop rotates, the magnetic flux through it changes with timeThis induces an emf and a current in the external circuitThe ends of the loop are connected to slip rings that rotate with the loopConnections to the external circuit are made by stationary brushes in contact with the slip ringsAC GeneratorsThe emf generated by the rotating loop can be found byε =2 B ℓ v=2 B ℓ sin θIf the loop rotates with a constant angular speed, ω, and N turnsε = N B A ω sin ω tε = εmax when loop is parallel to the fieldε = 0 when when the loop is perpendicular to the fieldSelf-inductanceSelf-inductance occurs when the changing flux through a circuit arises from the circuit itselfAs the current increases, the magnetic flux through a loop due to this current also increasesThe increasing flux induces an emf that opposes the change in magnetic fluxAs the magnitude of the current increases, the rate of increase lessens and the induced emf decreasesThis opposing emf results in a gradual increase of the current181920=<<2120 0BN NB I BA I rl lm m pF= � = =BNFInductor in a CircuitInductance can be interpreted as a measure of opposition to the rate of change in the currentRemember resistance R is a measure of opposition to the currentAs a circuit is completed, the current begins to increase, but the inductor produces an emf that opposes the increasing currentTherefore, the current doesn’t change from 0 to its maximum instantaneouslyRL CircuitWhen the current reaches its maximum, the rate of change and the back emf are zeroThe time constant, , for an RL circuit is the time required for the current in the circuit to reach 63.2% of its final value /1tI eR
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