Direction of the current: two quick cases

Two quick cases train the eye to find the direction of the induced current using Lenz’s law alone, without calculations.

1. Fixed loop, magnet approaching with N pole downward

$\Phi_B$ through the loop increases downward. To oppose it, the induced current must generate a $B$ field pointing upward inside the loop: seen from above, the current flows anticlockwise. Result: the loop “sees” the magnet as another magnet with N pole pointing upward, and the two repel each other.

2. Magnet moving away

Same geometry, but now the downward $\Phi_B$ decreases. The induced current reverses direction (clockwise seen from above); the field it generates inside the loop has the same direction as the vanishing flux: it tries to retain it. The loop now attracts the magnet.

The moral is simple and always holds: whether the magnet approaches or moves away, Lenz hinders it. The loop reacts with a force that makes the relative motion harder, never easier.

Links

Topics: Electromagnetic induction Concepts: Lenz’s law Skills: Right-hand rule

Related exercises: Worked exercise — loop entering and leaving a field · Problem — The magnet in the copper tube · Problem — Loop entering a field (qualitative)