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Electromagnetic Induction 01-21-2011

Electromagnetic Induction 01-21-2011 - HB MS 1...

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HB, MS 01-21-2011 1 Electromagnetic Induction Equipment SWS, RLC circuit board, box with 2 coils and iron rod, magnet, 2 voltage sensors (no alligator clips), 2 leads (35 in.), bubble wrap to catch dropped magnet, Fluke multimeter Reading Review graph and scope displays, signal generator, and power amplifier II. 1 Introduction The phenomenon of electromagnetic induction was discovered by Joseph Henry in New York in 1830 and by Michael Faraday in England in 1831. Faraday’s name is commonly associated with the phenomenon (Faraday induction), although in this century Henry’s contribution has been recognized by assigning his name as the unit for inductance. In the 1820’s it was known that an electric current produces a magnetic field, and Henry and Faraday were trying to reverse the process and produce a current with a magnetic field. The result is elusive as a steady magnetic field will not cause a current to flow in a circuit. It is a time changing magnetic field that “induces” a current to flow. The changing magnetic field produces an electric field which is not conservative. The line integral of the electric field around a loop or circuit is not zero and is called an electromotive force or EMF. The EMF can drive a current in a circuit. The unit of EMF is the volt (V). In this experiment an emf will be induced in one coil by moving a permanent magnet into and out of the coil, moving a second coil with a current in it near the first coil, and changing the current in the second coil. Some of your results will be qualitative. You should describe and explain what you observe. Bear in mind that the strength of the induced emf depends on the rate of change of the magnetic field. When an emf is induced in a coil the current that results will depend on the resistance of the circuit which the coil is part of. A voltage will also appear across the coil which can be measured by standard means. Because the voltage across the coil used in this experiment will be measured by a high impedance (resistance) device, the amount of current that flows will be small. 2 Theory The flux Φ of the magnetic field is defined in a similar way to the flux of the electric field. See Fig. 1. A magnetic field ~ B passes through a differential area d ~ A . The differential element of magnetic flux associated with d ~ A is d Φ = ~ B · d ~ A . It is only the component of ~ B that is perpendicular to the surface area that contributes to the flux. For a finite area the magnetic flux is Φ = R ~ B · d ~ A . The line integral of the electric field around the boundary of the area is called the EMF. The positive directions of d ~ A and the line integral must be consistent with the right hand rule where the thumb points in the direction of d ~ A . Faraday’s law says that for a single circuit around the boundary of the area the EMf is given by EMF= - d Φ /dt . The non-conservative electric field giving rise to the emf exists whether there is a wire around the boundary or not. But if there is a wire a current will flow if the circuit is “complete.” The current will be limited by the resistance of the circuit. If the loop of wire has two ends

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