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# Lec21 - Lecture 21 Lecture 21-1 Transformer • AC voltage...

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Unformatted text preview: Lecture 21 Lecture 21-1 Transformer • AC voltage can be stepped up or down by using a transformer . • AC current in the primary coil creates a time-varying magnetic flux through the secondary coil via the iron core. This induces EMF in the secondary circuit. Ideal transformer (no losses and magnetic flux per turn is the same on primary and secondary). (With no load) 1 2 1 2 B turn d V V dt N N ε Φ = = = step-up step-down 1 2 1 2 N N V V < ⇒ < 1 2 1 2 N N V V > ⇒ > 11 22 IV IV = conservation of energy proportional to average power Lecture 21 Lecture 21-2 Maxwell’s Equations (so far) Gauss’s law inside S Q EdA ε = ∫ ca ca i I Gauss’law for magnetism S BdA = ∫ ca ca i I Faraday’s law B C d Edl dt Φ =− ∫ caa i I Ampere’s law * C Bdl I μ = ∫ caa i I Lecture 21 Lecture 21-3 Parallel-Plate Capacitor Revisited S S Bdl I μ ∂ = ∫ ca a i I For surface S 1 , I s = I , but for surface S 2 , I s = 0 ?? Wait, LHS is the same (because C is the same)! You could make this work if a fictitious current I d is added to I s in such a way that I d is zero for S 1 but is equal to I for S 2 . E dQ dV AdV I C dt dt d dt dE d A dt dt...
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