Slide 1Lightning ReviewReview 1: right hand ruleReview 2: EMF polarity and current direction20.4 Lenz’s law revisitedLenz’s law: energy conservationSlide 7Example: direction of the currentApplications of Magnetic Induction20.5 GeneratorsAC generatorAC generator (cont)DC generatorMotorsExample: coil in magnetic field20.6 Eddy currents (application)1101/14/1901/14/19General Physics (PHY 2140)Lecture 17Lecture 17 Electricity and MagnetismInduced voltages and inductionLenz’s lawGenerators and motorsChapter 20http://www.physics.wayne.edu/~apetrov/PHY2140/2201/14/1901/14/19Lightning ReviewLightning ReviewLast lecture: 1.1.Induced voltages and inductionInduced voltages and induction Induced EMFInduced EMFFaraday’s lawFaraday’s lawMotional EMFMotional EMFReview Problem: A long, straight wire carries a steady current I. A rectangular conducting loop lies in the same plane as the wire, with two sides parallel to the wire and two sides perpendicular. Suppose the loop is pushed toward the wire as shown. Given the direction of I, the induced current in the loop is1. clockwise.2. counterclockwise.3. need more information0B nIm=cosBA qF =NtDF=-DEBlv=E3301/14/1901/14/19Review 1: right hand ruleReview 1: right hand rule4401/14/1901/14/19Review 2: EMF polarity and current directionReview 2: EMF polarity and current direction-+Current flows from + to – terminal of the battery (source of EMF)Current flows from + to – terminal of the battery (source of EMF)Electrons move in the opposite direction.5501/14/1901/14/1920.4 Lenz’s law revisited20.4 Lenz’s law revisitedLenz's law says that the induced current will produce Lenz's law says that the induced current will produce magnetic flux magnetic flux opposing this changeopposing this change. To oppose an . To oppose an increase into the page, it generates magnetic field which increase into the page, it generates magnetic field which points out of the page, at least in the interior of the loop. points out of the page, at least in the interior of the loop. Such a magnetic field is produced by a Such a magnetic field is produced by a counterclockwisecounterclockwise current (use the right hand rule to verify). current (use the right hand rule to verify). Application of Lenz's law will tell Application of Lenz's law will tell us the direction of induced us the direction of induced currents, the direction of currents, the direction of applied or applied or produced forces, produced forces, and the polarity and the polarity of induced of induced emf's.emf's.6601/14/1901/14/19Lenz’s law: energy conservationLenz’s law: energy conservationWe arrive at the same conclusion from We arrive at the same conclusion from energy conservation point of viewenergy conservation point of viewThe preceding analysis found that the The preceding analysis found that the current is moving ccw. Suppose that this current is moving ccw. Suppose that this is not so.is not so.If the current If the current I I is cw, the direction of the is cw, the direction of the magnetic force, magnetic force, BlIBlI, on the sliding bar , on the sliding bar would be would be rightright..This would accelerate the bar to the right, This would accelerate the bar to the right, increasing the area of the loop even more.increasing the area of the loop even more.This would produce even greater force and This would produce even greater force and so on.so on.In effect, this would generate energy out of In effect, this would generate energy out of nothing violating the law of conservation of nothing violating the law of conservation of energy. energy. Our original Our original assertion that the assertion that the current is cw is current is cw is not right, so the not right, so the current is ccw!current is ccw!7701/14/1901/14/19NSNSSNSvvchange changeThe induced flux seeks to counteract the change.8801/14/1901/14/19Example: direction of the currentExample: direction of the currentFind the direction of the current induced in the resistor at the instant the switch is closed.9901/14/1901/14/19Applications of Magnetic InductionApplications of Magnetic InductionTape / Hard Drive / ZIP ReadoutTape / Hard Drive / ZIP ReadoutTiny coil responds to change in flux as the magnetic domains (encoding Tiny coil responds to change in flux as the magnetic domains (encoding 0’s or 1’s) go by.0’s or 1’s) go by.Question: How can your VCR display an image while paused?Question: How can your VCR display an image while paused?Credit Card ReaderCredit Card ReaderMust swipe cardMust swipe card  generates changing fluxgenerates changing flux–Faster swipe Faster swipe  bigger signal bigger signal101001/14/1901/14/1920.5 Generators20.5 GeneratorsGenerators and motors are two of the most important Generators and motors are two of the most important applications of induced emf (magnetic inductance). applications of induced emf (magnetic inductance). A generator is something that converts mechanical A generator is something that converts mechanical energy to electrical energy. energy to electrical energy. Alternating Current (AC) generatorAlternating Current (AC) generatorDirect Current (DC) generatorDirect Current (DC) generatorA motor does the opposite, it converts electrical energy A motor does the opposite, it converts electrical energy to mechanical energy. to mechanical energy.111101/14/1901/14/19AC generatorAC generatorCompute EMFCompute EMFIt is only generated in BC It is only generated in BC and DA wiresand DA wiresEMF generated in BC and EMF generated in BC and DA would beDA would beThus, total EMF isThus, total EMF isIf the loop is rotating with If the loop is rotating with Bvv sin AABCDBlv^= =BC DAE E2 2 sinBlv Blv q^= =E2 sin 2 sin2aBlv t Bl tw w w� �= =� �� �Eas v=r=a/2121201/14/1901/14/19AC generator (cont)AC generator (cont)Generalize the result to N loopsGeneralize the result to N loopswhere we also noticed that where we also noticed that A=laA=laNote: Note: is reached when is reached when t=90t=90˚˚ or 270 or 270˚˚ sinNBA tw w=ENBAw=maxEEMF generated by the AC generator131301/14/1901/14/19DC generatorDC generatorBy a clever change to the rings and brushes of the ac By a clever change to the rings and brushes of the ac generator, we can create a dc generator, that is, a generator, we can create a dc generator, that is, a generator where the polarity of the emf is always generator