Question 1

What is Lenz's law?

Accepted Answer

Lenz's law states that the direction of an induced current is always such that it opposes the change in magnetic flux that produced it. If a magnet approaches a coil, the induced current makes the coil's near face repel the magnet; if the magnet recedes, the coil attracts it. It is the directional rule hidden in the minus sign of Faraday's law, ε = −N dΦ/dt.

Question 2

Why is there a minus sign in Faraday's law?

Accepted Answer

The minus sign encodes Lenz's law: the induced EMF drives a current whose magnetic field opposes the change in flux causing it. It is not an arbitrary convention — it is required by conservation of energy. Without it, the induced current would reinforce the change, accelerating the magnet and creating electrical energy from nothing.

Question 3

What happens when a magnet is held stationary inside a coil?

Accepted Answer

Nothing, electrically. The flux through the coil is at its maximum, but because it is not changing, dΦ/dt = 0 and the induced EMF is exactly zero. EMF depends on the rate of change of flux, not on the amount of flux — a favourite trap in NEET questions.

Question 4

How is Lenz's law related to conservation of energy?

Accepted Answer

If the induced current aided the change in flux instead of opposing it, the magnet would accelerate on its own and the coil would deliver free electrical energy — a perpetual motion machine, violating energy conservation. By opposing the change, the coil forces the agent moving the magnet to do work against an opposing force, and that mechanical work is exactly the electrical energy generated in the circuit.

Question 5

What is motional EMF?

Accepted Answer

When a conducting rod of length l moves with velocity v through a magnetic field B, its charges experience a force q(v × B) that separates them, producing an EMF ε = (v × B)·l. Only in the special case where the rod, its velocity and the field are mutually perpendicular does this reduce to the familiar ε = Bvl. The general vector form is what should be used in any other geometry.

Lenz's Law Lab — The Coil Pushes Back

Practice

Learned it? Now prove it under the clock.