Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Magnet Near Contactor Coil


Changing magnetic field creates electric field which drives current in conductors placed in the region. The present experiment is an interesting demonstration of this fact where a magnet keeps rotating about its axis when it is placed in an induced electric field.


A contactor coil, a cylindrical magnet, DC an AC power source.

Contactor coil: Contactor coil given to you is simply a finite solenoid with height of the order of width and rectangular cross-section. It has very large number of turns of fine wire so that you can connect it to 220 V for a longer period.


  1. Place the contactor coil on the working table and the cylindrical magnet outside it at 4-5 cm.
  2. Connect to the AC power supply. Switch it on for few tens of second. The magnet starts rotating about its own axis. Switch off the current. Whether it rotates clockwise or anticlockwise?
  3. Invert the magnet and again pass AC current in the coil. Whether the magnet rotates clockwise or anticlockwise?
  4. Now connect the coil to DC source. Does the magnet rotate? Does it get attracted or repelled from the coil?
  5. Invert the magnet and repeat step-4. Does the magnet rotate? Does it get attracted or repelled from the coil?
  6. Now put the magnet on the working table inside the contactor coil near its center. Connect the coil to AC source. What does the magnet do?
  7. Keeping the magnet at the same place, connect the coil to the DC source. What does the magnet do?