PHYA22 Chapter 34.doc

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Physics and Astrophysics
Brian Wilson

Chapter 34 Electromagnetic InductionInduced CurrentsFaradays Law Faraday found that there is a current in a coil of wire if and only if the magnetic field passing through the coil is changingoOpening or closing the switch creates a momentary currentoPushing the magnet into the coil or pulling it out creates a momentary currentoPushing the coil into the magnet or pulling it out creates a momentary currentThere is no current if the field through the coil is not changing so its not the magnetic field itself that is responsible for the current but the changing magnetic fieldInduced Current the current in a circuit due to a changing magnetic field not caused by a batteryMotional emfAn induced current can be created by two different waysoBy changing the size or orientation of a circuit in a stationary magnetic fieldoBy changing the magnetic field through a stationary circuitThe magnetic force on the charge carriers in a moving conductor creates an electric field EvB inside the conductorThe motion of the wire through a magnetic field induces a potential difference DVvlB between the ends of the conductor and the potential difference depends on the strength of the magnetic field and on the wires speed through the fieldMotional emf of a Conductor with velocity perpendicular to a magnetic field B evlBIvlBR where R is the total resistanceThere is no magnetic force on a charge moving parallel to a magnetic fieldForce required to pull the wire with a constant speed v FFIlBpullmag22vlBR222Power provided to the circuit by pulling on the wire PFvvlBRinputpull2222PIRvlBRdissipatedThe rate at which work is done on the circuit exactly balances the rate at which energy is dissipated thus energy is conservedMagnetic force is always poopsite to the wires direction of motionGenerator a device that converts mechanical energy to electric energyoPulling or pushing the wire through the magnetic field at speed v creates a motional emf e in the wire and induces a current IeR in the circuitoTo keep the wire moving at constant speed a pulling or pushing force must balance the magnetic force on the wire This force does work on the circuitoThe work done by the pulling or pushing force exactly balances the energy dissipated by the current as it passes through the resistance of the circuitEddy Currents spreadout current whirlpools in a solid metaloA pulling force must be exerted to pull the loop out of the magnetic fieldoEddy currents are induced when a metal sheet is pulled through a magnetic fieldoThe magnetic force on the eddy currents is opposite in direction to v
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