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# chapter20 - Chapter 20 Induced Voltages and Inductance...

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Chapter 20 Induced Voltages and Inductance

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Michael Faraday 1791 1867 Great experimental scientist Invented electric motor, generator and transformers Discovered electromagnetic induction Discovered laws of electrolysis
Faraday’s Experiment – Set Up A current can be produced by a changing magnetic field First shown in an experiment by Michael Faraday A primary coil is connected to a battery A secondary coil is connected to an ammeter

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Faraday’s Experiment The purpose of the secondary circuit is to detect current that might be produced by the magnetic field When the switch is closed, the ammeter reads a current and then returns to zero When the switch is opened, the ammeter reads a current in the opposite direction and then returns to zero When there is a steady current in the primary circuit, the ammeter reads zero
Faraday’s Conclusions An electrical current is produced by a changing magnetic field The secondary circuit acts as if a source of emf were connected to it for a short time It is customary to say that an induced emf is produced in the secondary circuit by the changing magnetic field

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Magnetic Flux The emf is actually induced by a change in the quantity called the magnetic flux rather than simply by a change in the magnetic field Magnetic flux is defined in a manner similar to that of electrical flux Magnetic flux is proportional to both the strength of the magnetic field passing through the plane of a loop of wire and the area of the loop
Magnetic Flux, 2 You are given a loop of wire The wire is in a uniform magnetic field The loop has an area A The flux is defined as Φ B = B A = B A cos θ θ is the angle between B and the normal to the plane B

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Magnetic Flux, 3 When the field is perpendicular to the plane of the loop, as in a, θ = 0 and Φ B = Φ B, max = BA When the field is parallel to the plane of the loop, as in b, θ = 90 ° and Φ B = 0 The flux can be negative, for example if θ = 180 ° SI units of flux are T . m² = Wb (Weber)
Magnetic Flux, final The flux can be visualized with respect to magnetic field lines The value of the magnetic flux is proportional to the total number of lines passing through the loop When the area is perpendicular to the lines, the maximum number of lines pass through the area and the flux is a maximum When the area is parallel to the lines, no lines pass through the area and the flux is 0

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Electromagnetic Induction An Experiment When a magnet moves toward a loop of wire, the ammeter shows the presence of a current (a) When the magnet is held stationary, there is no current (b) When the magnet moves away from the loop, the ammeter shows a current in the opposite direction (c) If the loop is moved instead of the magnet, a current is also detected
Electromagnetic Induction Results of the Experiment A current is set up in the circuit as long as there is relative motion

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## This note was uploaded on 04/12/2011 for the course PHYS 1410 taught by Professor Staff during the Spring '08 term at North Texas.

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chapter20 - Chapter 20 Induced Voltages and Inductance...

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