A small bar magnet is moved through a coil at constant speed from one end to the other. Which of the following series of observations will be seen on the galvanometer G attached across the coil?
Three positions shown describe :
(a) the magnet's entry
(b) magnet is completely inside and
(c) magnet's exit.

Solution

Physics sir · 4 steps

Watch the flux, not the magnet: it rises on entry, stays steady inside, and falls on exit.

1. What G shows. The induced emf is e = -dφ/dt. So the needle moves only while the flux through the coil is changing, and the sign of the change decides which way it moves.

   Position	Flux through coil	Needle on G
   (a) entering	increasing	deflects one way
   (b) fully inside	constant, dφ/dt = 0	zero
   (c) leaving	decreasing	deflects the opposite way

2. Lenz's law. On entry the induced current opposes the growing flux. On exit it tries to keep the flux from falling, so it flows the other way. The two deflections are opposite.
3. Equal size. The speed is constant, so the flux rises on entry as fast as it falls on exit. The two deflections are therefore equal in size.
4. So. The needle deflects one way, returns to zero, then deflects equally the other way. That is option (c).

The idea to keep. A galvanometer responds to the rate of change of flux. Rising and falling flux give opposite currents, and steady flux gives zero, even while the magnet is still moving.

Answer from the NTA final answer key. Solution by PrepWiser.