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Lecture_12

# Lecture_12 - Magnetic Field A compass needle turns and...

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A compass needle turns and points in a particular direction … there is something which interacts with it Magnetic field ( B ): whatever it is that is detected by a compass Compass: similar to electric dipole Magnetic Field Magnetic fields are produced by moving charges Current in a wire: convenient source of magnetic field The electron current ( i ) is the number of electrons per second that enter a section of a conductor.

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Conclusions: • The magnitude of B depends on the amount of current • A wire with no current produces no B B is perpendicular to the direction of current B under the wire is opposite to B over the wire Oersted effect: discovered in 1820 by H. Ch. Ørsted How does the field around a wire look like? The Magnetic Effects of Currents Hans Christian Ørsted (1777 - 1851)
Principle of superposition: wire Earth net B B B + = What can you say about the magnitudes of B Earth and B wire ? What if B Earth were much larger than B wire ? The Magnetic Effects of Currents The moving electrons in a wire create a magnetic field

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A current-carrying wire is oriented N-S and laid on top of a compass. The compass needle points 27 o west. What is the magnitude and direction of the magnetic field created by the wire B wire if the magnetic field of Earth is B Earth = 2 × 10 -5 T (tesla). wire Earth net B B B + = θ tan Earth wire B B = 27 tan 10 2 5 × × = T wire B T 5 10 1 × wire B Exercise
Biot-Savart Law for a Single Charge r r q E ˆ 4 1 2 0 πε = Electric field of a point charge: Moving charge makes a curly magnetic field: 2 0 ˆ 4 r r v q B × = π μ B units: T (tesla) = kg s -2 A -1 Jean-Baptiste Biot (1774-1862)

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