Lab 6 - Magnetic Forces 2

Lab 6 - Magnetic Forces 2 - PHY134 - Classical Physics II...

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PHY134 - Classical Physics II Laboratory Magnetic Forces - 2 In this experiment a simple current balance will be used to measure the force on a current- carrying wire in a magnetic field. The magnetic field is produced by a solenoid, and from the magnitude of the force on the wire the number of turns in the solenoid will be calculated. Equipment 5 A Power Supply, Helmholtz Coils 500 mA Power Supply, 5A Current shunt, 2 Digital Multimeters Electronic weight scale, 100-turn triangular coil with balance arm. Method The torque balance is shown schematically in Figure 1. The balance arm is a thin rectangular insulator upon which a 100-turn triangular coil is mounted. The coil is balanced by a weight resting on the pan of electronic scale. A small counterweight (a short piece of wire) is hung from the hook near the small coil to provide a gravitational torque τ g , about an axis through the attachment point of the main weight, and will be counter-balanced by the magnetic torque m produced when current flows in the 100-turn coil in the presence of the horizontal magnetic field near the center of the Helmholtz coils.
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The torque on a coil of N windings, of area A and carrying a current I , mounted in a magnetic field B (assumed constant over the coil's area) is given by: τ m = μ × B = NI A × B , (1) where μ is the total magnetic moment of the coil. A pair of Helmholtz coils produces the magnetic field B . The distance between the Helmholtz coils is approximately equal to the radius R of the coils, and the B field near the center is very uniform with magnitude given by: B = 8 μ 0 N H I H / (5 3/2 R ), (2) where N H is the total number of windings in a single Helmholtz coil, I H the current through the coils, and 0 . the magnetic permeability of the vacuum. B is adjusted by varying I H (in magnitude and flow direction!) until the magnetic torque precisely counter-balances the gravitational torque from the piece of wire used as weight, as indicated by the electronic scale. Q1. In what direction does the gravitational torque point? Use the right-
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This note was uploaded on 09/26/2008 for the course PHY 132 taught by Professor Rijssenbeek during the Spring '04 term at SUNY Stony Brook.

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Lab 6 - Magnetic Forces 2 - PHY134 - Classical Physics II...

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