Lenz's Law — Prelims Strategy

Prelims Strategy

To excel in NEET questions on Lenz's Law, a systematic approach is essential. First, always clearly identify the external magnetic field's direction. Second, determine how the magnetic flux through the loop or conductor is changing – is it increasing or decreasing?

And in which direction? For example, 'flux into the page is increasing' or 'flux out of the page is decreasing'. Third, apply Lenz's Law: the induced current will create its own magnetic field to *oppose* this change.

If flux is increasing in a direction, the induced field is in the opposite direction. If flux is decreasing in a direction, the induced field is in the same direction. Fourth, use the Right-Hand Thumb Rule (for coils) or Fleming's Right-Hand Rule (for motional EMF in straight conductors) to determine the direction of the induced current that would produce this opposing magnetic field.

For numerical problems involving motional EMF, first calculate the EMF magnitude using $BLv$, then use Lenz's Law to find the current direction. Be wary of trap options that suggest the induced current aids the change or that no current is induced.

Always remember the underlying principle of energy conservation.