Concepts of Physics

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

Magnetic Field Driven Train


This experiment is a direct demonstration of the force law \(\mathrm{d}\vec{F}=i\mathrm{d}\vec{l}\times\vec{B}\).


DC power supply, two parallel copper wires pasted on a fiber base with marks A and B. Aluminium foil, pen body/refill, speaker magnet.


  1. Put the speaker magnet on the working table with a flat surface resting on the table. Place the fiber base on the magnet so that the two wires are symmetrically placed on the magnet. Connect with the DC power supply. Do not switch on.
  2. Wrap the aluminium foil on the pen body/refill in such a way that the total length is almost same as that of the body. Start with placing the a triangular corner of the given foil on the pen body and start wrapping. As you wrap, it will go on up the body and at no place the thickness will be more 2-3 layers.
  3. Press the final loose end of this roll. This will fix it firmly.
  4. Gently pull the pen body out of the rod. Take care that the roll itself retain the cylindrical structure.
  5. Place the roll on the two copper wires, perpendicular to them and symmetrically about the center of the speaker magnet.
  6. Now switch on the power supply. Look at the roll. Which direction did the roll go? Towards the mark A or B on the base?
  7. Place the roll at different positions on the track (wires) and repeat. Write your observations.
  8. Invert the magnet and repeat step-6. Which direction did the roll go? Towards the mark A or B on the base?
  9. From these observations identify which side of the speaker magnet is north.