First of all, you probably figured out that the two LEDs were both connected in parallel, directly to the coil, but in opposite directions. This would explain why they do not light up together. (LEDs are diodes, so they allow current to flow in only one direction).

Let’s move on to the EMI part of the explanation.

Firstly, Faraday’s Law says that if the magnetic flux linkage of a coil Φ changes, an emf ε directly proportional to the rate of change of Φ will be induced in the coil. (Faraday’s Law: ε = -dΦ/dt)

When the magnet was pushed into the coil, Φ increased, thus inducing an emf and current in one direction in the coil, lighting up the red LED. When the magnet was pulled out of the coil, Φ decreased, thus inducing an emf and current in the coil in the other direction, thus lighting up the green LED.

To analyze such situations, it is usually useful to sketch the variation of Φ with time. We can then tell the induced emf ε from the gradient of the Φ-t graph. (Faraday’s Law: ε = -dΦ/dt)

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