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When the magnet was moving slowly, the LEDs did not light up because the magnetic flux linkage was changing too slowly. There was an induced emf, but it was too small (Faraday’s Law: ε = -dΦ/dt) to light up the LEDs. (About 1.5V is required to forward bias the LEDs)

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The LEDs also did not light up when the long magnet was moving vigorously in the coil. This is because since the entire magnetic flux of the magnet was already captured by the coil. So even though the magnet was moving rapidly, the magnetic flux linkage remains constant. Hence no emf nor current was induced in the coil. (Faraday’s Law: ε = -dΦ/dt)

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