Concepts of Physics

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

The Mother Coil!


The electromagnetic induction is used in many applications like motor, transformer, electromagnets etc. This demonstration show these concepts in an interesting and transparent manner.


The mother coil is constructed by giving a large number of turns of enamelled copper wire. The core of the coil is made from a bunch of bicycle spokes. A bell switch is given because coil should not be made on for a longer duration. The coil is usually connected to 220 V AC supply.


  1. The first demonstration is to feel the magnetic field around the core when switch is made on. Take an iron nail and take it close to the core. Where is the magnetic field maximum and where it is minimum?
  2. Put an aluminium ring over the coil. The ring jumps when switch is made on. Why?
  3. Touch the aluminium ring with hand. Is it hot or cold? Now, put the ring over the coil and don't allow it to jump by pressing it with a wooden stick. Switch on the coil for few seconds. Touch the ring again. Is it hot or cold? Why?
  4. Take an additional coil (secondary coil) with a torch bulb connected to it. Slide this coil over the bicycle spokes. What happened to the bulb? Why? Why does intensity varies with distance of this coil from the mother coil?
  5. Take a cylindrical magnet and hold it in such a way that it is free to rotate. Bring this magnet close to the bottom of mother coil. The magnet will start rotating. Why?
  6. We can ask students to measure the induced voltage in secondary coil with different number of turns in it. Also, how does induced voltage varies with separation between primary and secondary coils.

Extension: Connect a \(100\,\mathrm{\Omega}\) and \(1000\,\mathrm{\Omega}\) resistance in series and form a loop by using a connector of appropriate size. Place the loop on the mother coil. Use a multimeter to measure the AC voltage across one of the resistor. Now take multimeter to other side and measure the voltage again. Why there is a difference in measured voltage?


The house hold power supply is AC with rms voltage 220 V and frequency 50 Hz. Thus, current through the coil varies with time. The time varying current produces the time varying magnetic field.

Most of the demonstrations can be explained by Faraday's law of electromagnetic induction. In case of aluminium ring, the magnetic flux through the ring varies with time. This induces emf and hence current (called eddy current) in the ring. This, current carrying ring is placed in the magnetic field of the mother coil. The jumping of aluminium ring is caused by magnetic force acting on current carrying ring.

Heating of the ring is related to heat loss due to eddy current.

The lighting of bulb in secondary coil demonstrate the concept of transformer. The flux linkage between the primary and the secondary coils is more when two coils are close to each other.

The rotation of cylindrical magnet is related to the motors.


Hazard: Be careful when working with 220 V AC. It may be fatal.