Physics Lecture 8

Physics Lecture 8 - i. use Right Hand Rule to get direction...

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Lecture 8: How to make electricity and light I. Faraday’s Law of Induction a. Induction – generating electrical currents with a changing magnetic flux b. Ε = dΦ/dt direction is found with Lenz’s Law i. Approx. ΔΦ/Δt c. Ohm’s Law i. Ε = IR d. Special Cases i. Uniform B, loop in a plane 1. Ε = dΦ/dt = d(NBAcosθ) a. To have E: i. Vary B ii. Vary A iii. Vary θ 2. it is the change in flux that matters not Φ itself e. EXAMPLE: Coil of 100 turns, resistance of 5Ω, radius of 5 cm. Field strength is B=.5T decrease the field to zero in 3 seconds. What is the current i. E = DΦ/dt = 100*π(.05m)^2 (.5/3s) = .13 V ii. I = E/R = .13/5 = 0.026A f. If given B(t) = NBAcos(θ) i. EMF = dΦ/dt = NAcos(θ)dB/dt g. If angle is changing = θ= ωt i. =NBA d/dt(cos(ωt)) II. Lenz’s Law a. There are two magnetic fields i. B ext => causes Faraday’s Effect ii. B int => field produced by the induced current b. If the flux is increasing then the internal field is opposite the external c. If the flux is decreasing the internal is in the same direction as the external
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Unformatted text preview: i. use Right Hand Rule to get direction of I III. jumping ring Demo a. increase the magnetic field – ring jumps upward i. the field goes up through the ring creating a magnet with a south pole at the bottom IV. Eddy Currents and Magnetic Braking a. Swirling currents in metal plate b. Magnetic force opposes velocity to create braking V. Maxwell’s Equations and Electromagnetic Waves a. Faraday changing B produces E b. Maxwell changing E produces B i. Equations are in integral form (not responsible for them) ii. Possible to make a wave of electric and magnetic fields iii. V propegation of EM wave = 3.0 x 10 8 m / s 1. LIGHT IS AN ELECTROMAGNETIC WAVE iv. Led to the invention of radio waves UNIT 2: Induction, Waves & Light Faraday’s Effect – induced emf Ε = dΦ/dt Φ m = NB dot A = NBAcosθ Ohm: E = IR Lenz’s Law Induced current flows to set up induced field (B ind ) to oppose the change in Φ m A θ B...
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This note was uploaded on 04/07/2008 for the course PHYS 212 taught by Professor Ladd during the Spring '08 term at Bucknell.

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