[NOTES/EM-09002]-Understanding Electromagnetic Induction

Recall the flux rule When conductor moves in a magnetic field, the electrons due to their motion experience a magnetic force and an e.m.f. is generated. Another situation when e.m.f. is induced is when a conductor is placed in a time varying magnetic field. The induced e.m.f. produced in a circuit, is frequently computed using the flux rule. This rule gives e.m.f. as the rate of change of flux linked with the circuit. \[ \mathcal{E}= - \frac{d \Phi}{dt}.\] This flux rule is not always give the correct answer.

1. Trouble with flux rule --- Example I

While the flux rule works correctly in many situations, in which it gives a wrong answer, and in some cases it is not applicable. As an example consider a metallic disc with a metallic rod passing through the centre, The disc rotates about the rod. A point on the rim is connected to one of the end points of a rod as shown in the figure.  The flux rule does not offer any explanation at the microscopic level about the flow of current, viz. what makes the electrons move?

2. Exceptions to the Flux Rule --- Example-II

 Consider a conducting disc which rotates about an axis passing through it center. A permanent magnet is held as shown in the figure. As the disc rotates about its own axis, the flux linked with the disc does not change but there are induced currents. A detailed of discussion of exception to the flux rule is given in lectures by Feynman.

REF: R. P. Feynman, Lectures on Physics, Vol-II, Addison Wesley Publishing Co.