Question

On lecture 18, (time 41:50) Why does the north pole recede if it is being atracted to the south pole of the coil? Thank you very much!

Answer 1

Enough energy is imparted to the top when it's north pole is being attracted as it is approaching the South pole kicker solenoid that it can go past the solenoid a little bit. The instant that the north pole passes the kicker solenoid the transistor that controls the current in the solenoid turns off the solenoid. The base lead of a transistor requires very little current to operate and control a much larger current but it is a polarity sensitive semi conductor junction and the instant the current falls below a critical value even before an actual polarity reversal takes place due to the North pole starting to move away from the solenoid through inertia, the transistor halts conduction to the solenoid so the magnet spins around until the next pole arrives. Remember the direction a magnet moves relative to a coil of wire determines the direction of the EMF in the coil.