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Unformatted text preview: Purpose: in this lab we will t ry to measure the magnetic force generated by a copper coil and ___ the torque generated by the magnetic force to ___ generated by the gravitational force. Int roduction: 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 tu rns in the solenoid is calculated Methods: The torque balance is shown schematically in Figure 1. The balance arm is a thin rectangular insulator upon which a 100-tu rn t r iangular 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 t g, about an axis through the attachment point of the main weight, and will be counterbalanced by the magnetic torque t m produced when current f lows in the 100-turn coil in the presence of the horizontal magnetic field near the center of the Helmholtz coils. 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: t m = m B = N I A B, (1) where m 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 = 8m 0N H I H / (53/2 R), (2) where N H is the total number of windings in a single Helmholtz coil, I H the current through the coils, and m 0. the magnetic permeability of the vacuum. B is adjusted by varying I H (in magnitude and f low 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. Data ...
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