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GeneratorsGenerators and Motors Textbook Section 23-6 Physics 1161: PreLecture 16

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Review: Two uses of RHR’s Force on moving charge in Magnetic field –Thumb: v (or I) –Fingers: B –Palm: F on + charge Magnetic field produced by moving charges –Thumb: I (or v for + charges) –Fingers: where you want to know B –Palm: B Palm: out of page. B I F + v +++ Thumb: out Fingers: up Palm: left. x

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Review: Two uses of RHR’s Force on moving charge in Magnetic field –Thumb: v (or I) –Fingers: B –Palm: F on + charge Magnetic field produced by moving charges –Thumb: I (or v for + charges) –Fingers: curl along B field Palm: out of page. B I F + v +++ I

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Review: Induction Lenz’s Law –If the magnetic flux ( B ) through a loop changes, an EMF will be created in the loop to oppose the change in flux –EMF current (V=IR) additional B-field. Flux decreasing => B-field in same direction as original Flux increasing => B-field in opposite direction of original Faraday’s Law –Magnitude of induced EMF given by:

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Review: Rotation Variables v, , f, T Velocity (v): –How fast a point moves. –Units: usually m/s Angular Frequency ( ): –How fast something rotates. –Units: radians / sec r v v v = r f = / 2 T = 1 / f = 2 / Frequency ( f ): –How fast something rotates. –Units: rotations / sec = Hz Period (T): –How much time one full rotation takes. –Units: usually seconds

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Generators and EMF rL = A side 1 = r B L sin( ) side 2 = r B L sin( ) loop = side 1 + side 2 2 r B L sin( ) loop = A B sin( ) loop = A B sin( t) v v x r 1 2 t AB AB EMF is voltage! side 1 = v B L sin( ) v = r

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ACT: Generators and EMF x = A B sin( ) x x 1 2 3 At which time does the loop have the greatest emf (greatest / t)? 1) Has greatest flux, but = 0 so = 0. 2) (Preflight example) 30 so AB/2. 3) Flux is zero, but = 90 so = AB. θ

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Comparison: Flux vs. EMF Flux is maximum –Most lines thru loop EMF is minimum –Just before: lines enter from left –Just after: lines enter from left –No change! Flux is minimum –Zero lines thru loop EMF is maximum –Just before: lines enter from top. –Just after: lines enter from bottom. –Big change! x x

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Generators and Torque v v x r = A B sin( ) Recall: = A B I sin( ) = A 2 B 2 sin 2 ( )/R Torque, due to current and B field, tries to slow spinning loop down. Must supply external torque to keep it spinning at constant Voltage! Connect loop to resistance R use I=V/R: I = A B sin( ) / R

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