physic 難題 24

Q14
Induced emf = 100 x 0.01 x 0.1sin(30) T/s = 0.05 v

Q15-16
For the induced current in coil A flowing clockwise, the upper side becomes a South Pole and the lower side becomes a North Pole.

By Lenz's Law, the approaching magnet would be the South Pole at the bottom so as to give a repulsion to the magnet.

When the bar magent is moving away, the lower end of the magnet should be the North pole so as to prevent the magent from moving away.

Hence, the answers are options A and C

Q17.
The primary coil is connected to a d.c. battery, there is no induced emf at the secondary coil. No current flows through the resistor.

Q18.
The variation of B field is not clearly seen. You can apply Faraday's Law of electromagentic induction to calculate the induced emf first, then apply Ohm's Law to find the current.