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Lecture

# Magnetic fields and forces, dipoles and fields, sources of the magnetic field, force on a moving charged particle

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School
Department
Physics
Course
PH 122
Professor
Brian Jones
Semester
Spring

Description
3 March Magnetic Fields & Forces Magnetic fields are produced by currents and permanent magnets. The smallest unit is the dipole, as we’ll see. Magnetic fields exert forces on moving charges. The resulting path is often a circle. Magnetic dipoles line up with magnetic fields. That’s why compasses point north. Dipoles & Fields An electric dipole rotates to point along the direction of the electric field. The magnetic field vectors point in the direction of compass needles. We represent the stronger magnetic field near the magnet by longer vectors. The Magnetic Field of the Earth The south pole of the earth’s magnet is not at the north geographic pole. Tilted Sources of the Magnetic Field Current loop Permanent magnet Atomic magnet Sources of the Magnetic Field: Field of a Current-Carrying Wire Compasses surrounding the wire – needles are tangent to a circle around the wire When the current direction is reversed, the compass needles turn to point in the opposite direction. Point your right thumb in the direction of the current. Curl your fingers around the wire to indicate a circle. Your fingers point in the direction of the magnetic field lines around the wire. Unit: Tesla (T) Magnetic field due to a long, straight, current-carrying wire μ0I B = 2πr μ 0 1.26 x 10 Tm/A Sources of the Magnetic Field: Field of a Current Loop There is a simple expression for the strength of the field at the center of the loop. Because the loop can be thought of as a wire bent into a circle, the expression for t
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