18 - Fields from Currents

18 - Fields from Currents - PHYS 212 Fall 2009 Fields Due...

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PHYS 212 Fall 2009 Fields Due to Currents 10/5/09
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Summary F = q E + q v × B Force on a charge Force on a wire F = i L × B Torque on a current loop τ = μ × B μ = Ni A μ p Electric Dipole μ Magnetic Dipole Q - Is a current loop a dipole magnet?
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Torque Example A coil of wire has an area of 2.0E-4 m 2 , consists of 100 loops or turns, and contains a current of 0.045 A. The coil is placed in a uniform magnetic field of 0.15 T. (a) Determine the magnetic moment of the coil. (b) Find the maximum torque that the magnetic field can exert of the coil. μ = Ni A μ = 100(0.045 A )(2.0 E 4 m 2 ) = 9.0 E 4 Am 2 (a) Magnetic moment is normal ( ) to the coil Maximum torque is when magnetic moment is perpendicular to B τ = μ × B = μ B sin θ (b) τ = (9.0 E 4 Am 2 )(0.15 T ) = 1.4 E 4 Nm Am 2 ( ) ( T ) = Am 2 ( ) ( N C m s ) = Am 2 ( ) ( N C s ( ) m ) = Nm Units:
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Magnetic Fields from Currents Discovered by Hans Christian Oersted (1777-1851) A current carrying wire creates a magnetic field (deflects a compass) Compass “maps out” field lines - circular Right Hand Rule 2: Curl fingers in a half circle. Point thumb in the direction of the current, and fingers will point in the direction of the magnetic field.
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