Question
A bar magnet is moved towards a circular coil. As the north pole approaches the coil, what is the direction of the induced current in the coil (as seen by the approaching magnet)? Also state the principle that governs this.
This exact question appeared in CBSE 2024 Board Exam for 2 marks.
Solution — Step by Step
Lenz’s Law says: the induced current will flow in a direction such that its magnetic effect opposes the change in flux that caused it.
“Oppose the change” — not oppose the flux, but oppose the change. This distinction matters.
The north pole is approaching the coil. So the magnetic flux through the coil is increasing (field lines from the north pole are entering the coil face that faces the magnet).
We need to oppose this increase — meaning the coil must create its own magnetic field pointing away from the approaching magnet.
If the coil needs to produce a field pointing away from the approaching north pole (i.e., pointing outward toward the magnet), we use the right-hand thumb rule.
Curl the fingers of your right hand — if the field points toward you (out of the coil face facing the magnet), the current flows anticlockwise as seen from the magnet’s side.
The induced current flows anticlockwise as seen by the approaching north pole.
This makes the face of the coil facing the magnet behave like a north pole — repelling the approaching magnet and opposing the increase in flux.
Why This Works
Lenz’s Law is really just energy conservation in disguise. If the induced current helped the magnet instead of opposing it, we’d get acceleration for free — the magnet would go faster, inducing more current, which would pull it faster… perpetual motion. Physics doesn’t allow that.
So nature always ensures the induced effect fights the cause. When flux increases, induced current opposes the increase. When flux decreases (magnet pulled away), induced current tries to maintain it — the coil face now becomes a south pole to attract the receding magnet.
The negative sign is Lenz’s Law — it tells you the EMF opposes the change in flux.
Alternative Method
Instead of thinking about flux, use the force argument directly:
The approaching magnet must feel a repulsive force (otherwise we get free energy). A north pole is repelled only by another north pole. So the coil face facing the magnet must act as a north pole.
For a coil face to be a north pole (field emerging from it), current must be anticlockwise when viewed from that face — same answer, different reasoning path.
In exam time pressure, the force argument is faster: “magnet approaches → coil repels → identify which pole the coil face becomes → use right-hand rule.” Three steps, done.
Common Mistake
Confusing “oppose the flux” with “oppose the change in flux.”
Many students write: “the induced current opposes the magnetic flux.” Wrong. The coil doesn’t try to zero out the flux — it tries to prevent the change. If a north pole approaches, the coil creates a north pole (opposing increase), not a south pole (which would be opposing the flux itself). This error costs full marks on a 2-mark board question.
Quick Revision
| Situation | Coil’s response | Induced face |
|---|---|---|
| North pole approaching | Opposes increase → repels | North pole |
| North pole receding | Opposes decrease → attracts | South pole |
| South pole approaching | Opposes increase → repels | South pole |
| South pole receding | Opposes decrease → attracts | North pole |
For JEE Main, Lenz’s Law questions often combine this with calculating induced EMF using . Get the direction right first, then plug into Faraday’s formula for the magnitude.