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Lecture 25 - Induction. Faradays law

Lecture 25 - Induction. Faradays law - ACT Moving loop A...

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Lecture 25 Induction. Faraday’s law. Lenz’s law. ACT: Moving loop A closed loop with a bulb (no battery) is half-way in a region with a uniform magnetic field (shaded area). A. CW direction B. CCW direction C. Huh? If the loop is stationary, the magnetic force on the electrons in the wire is… zero, right? If the loop is pulled to the right, the bulbs lights up because of a current in the v • Velocity to the right F • Force points down I • Consider positive charge + Faraday’s experiment Faraday tried the opposite (keep the loop stationary, move the magnet) and obtained the same current! I v But the charges in the wire are not moving now, so the magnetic force on them should be zero! Hm… This “coincidence” bothered Einstein and eventually led to to the Special Theory of Relativity. Another induction example Now nothing moves, but the magnitude of the magnetic field decreases with time. Again, CCW current observed! I More hmm… ~ dB I dt Overall, it seems that current is induced when the magnetic field through the loop changes…

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Faraday’s Law Φ = G G B B da Magnetic flux through a surface: It turns out it’s not exactly when B changes… ε Φ = − B d dt
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