Physics 1161 Pre Lecture 16 Generators and Motors

Physics 1161: Pre. Lecture 16 Generators and Motors • Textbook Section 23 -6

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

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

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:

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

Generators and EMF is voltage! eside 1 = v B L sin(q) v = r eside 1 = r B L sin(q) eside 2 = r B L sin(q) eloop = eside 1 + eside 2 = 2 r B L sin(q) 2 r. L = A eloop = A B sin(q) eloop = A B sin( t) 1 • w v 2 w. AB x q v r e t -w. AB

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

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! x Flux is minimum – Zero lines thru loop EMF is maximum • x – Just before: lines enter from top. – Just after: lines enter from bottom. – Big change!

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