Physics · Electromagnetic Induction · NEET
Current needs a CHANGING flux, not just flux. While the magnet moves closer, more field lines pass through the coil each second, so flux keeps rising and an EMF is induced. Once the magnet stops (even inside the coil), the flux becomes a fixed number — it is not changing — so dΦ/dt = 0 and the galvanometer reads zero. This is exactly what NCERT's Faraday experiment shows: the deflection lasts only as long as the magnet is in motion.
Only RELATIVE motion matters. You get the same current whether you push the magnet toward a fixed coil, or push the coil toward a fixed magnet. In both cases the flux through the coil changes at the same rate. Physics only cares that the magnet and coil are moving relative to each other, not which one you label as 'moving'.
Induced EMF equals the RATE of change of flux (EMF = -N dΦ/dt). A faster push changes the flux in less time, so dΦ/dt is larger and EMF is bigger. A stronger magnet means more field lines to begin with, so each bit of motion changes the flux more. Both increase dΦ/dt, so both give a bigger galvanometer deflection.
The changing flux is the real cause; motion is just one easy way to change it. Faraday proved this with a third experiment: two coils held completely still, where switching a current on/off in one coil (changing its field) induced a current in the other coil with no motion at all. Motion is not required — a changing flux is.
Pushing the magnet in makes flux INCREASE; pulling it out makes flux DECREASE. The induced current always flows in the direction that opposes the change (Lenz's law), so an increasing flux and a decreasing flux drive current in opposite directions. That is why the galvanometer needle swings the other way when you reverse the motion.
A strong bar magnet is moving towards solenoid-2 from solenoid-1 (the two solenoids lie along the same axis). The directions of the induced current in solenoid-1 and in solenoid-2, respectively, are along:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Relative motion changes the magnetic flux through the coil, and a changing flux induces an EMF (Faraday's law). No change in flux means no current.
No, not by itself. A stationary magnet gives a constant flux, so dΦ/dt = 0 and there is no induced EMF. You must change something — move the magnet, move the coil, or change the field.
No. Faraday's third experiment used two stationary coils; switching the current in one coil on or off changed its magnetic field and induced current in the other coil. Any changing flux works — motion is just the most common way.
Going in increases the flux; coming out decreases it. By Lenz's law the induced current opposes the change, so increasing and decreasing flux produce currents in opposite directions.
Here the whole coil experiences a changing flux because the source (magnet) moves. Motional EMF is a special case where a conductor of length l moves with speed v through a steady field, giving EMF = Bvl. Both trace back to a changing flux.
