JEE/NEET Physics · Electromagnetic Induction series · Part 2 of 6 · All parts →
- Lenz: induced current OPPOSES the change that made it — the minus sign of Faraday
- Approaching N-pole → coil’s near face becomes N (repels the approach)
- Withdrawing N-pole → near face becomes S (attracts it back)
- Lenz’s law is energy conservation in disguise — ‘free energy’ would otherwise appear
- Right applications: braking, damping, and every motor-as-generator
Push a magnet toward a coil and the coil pushes back. Pull it away and the coil pulls it back. Induced currents always fight the change that created them — nature’s reflex that keeps the energy books honest. Part 2 of the Electromagnetic Induction series.
- The opposition rule
- Two scenarios, one reflex
- Why opposition? Energy!
- Magnetic braking
- Solved examples
- Common mistakes
- This physics in your daily life
- Practice set
- Recap
The Opposition Rule
The induced current’s magnetic field always acts to oppose the change in flux that produced it. Flux increasing? Induced field points against it. Flux decreasing? Induced field points to prop it up. The coil is a conservative: it resists both fortune and loss.
Two Scenarios, One Reflex
| Your action | Coil’s response | Result |
|---|---|---|
| N-pole approaches | near face turns N | repulsion — you’re resisted |
| N-pole withdraws | near face turns S | attraction — you’re held back |
| Flux rising through loop | induced B opposes rise | counter-flux |
| Flux falling | induced B supports it | propping-flux |
Why Opposition? Energy!
Suppose the induced current HELPED the change instead: the moving magnet would speed up, inducing more current, speeding it further — runaway energy from nowhere. Lenz’s minus sign is energy conservation enforced at the field level: you must do work against the opposition, and that work becomes the induced electrical energy. No opposition, no free lunch — and no free lunch anyway.
Magnetic Braking
Drop a magnet through a copper tube: induced eddy currents oppose its motion both falling and (if pulled) rising — it drifts like through honey. Trains and roller-coasters use this exact drag as wear-free brakes.
Solved Examples
Opposition to approach → left face becomes N (repels); current flows anticlockwise viewed from the left.
✔
Answer: Left face = N
Opposition to departure → left face becomes S (attracts back): current reverses to clockwise (from the left view).
✔
Answer: Reversed
From gravity’s work: the magnet exits slower and lower in potential-energy terms than free-fall — mechanical energy converted via the opposition force.
Lenz guarantees the accounting closes.
✔
Answer: Gravity’s work, minus the magnet’s lost KE
- Induced current ‘aids’ intuition. Never — opposition always; if your answer aids the change, flip it.
- Confusing which flux. Lenz opposes the CHANGE IN external flux, not the field itself: a decreasing field induces a SUPPORTING current.
- Lenz as a new law. It’s the minus sign of Faraday = energy conservation = Lenz: one fact, three names.
- Polarity guessing on the far face. Coil faces are N-S simultaneously: near face decided by Lenz, far face is automatically the opposite pole.
This Physics in Your Daily Life
- Roller-coaster and train magnetic brakes — eddy-current opposition as wear-free stopping: no pads, no friction, just Lenz.
- Induction stoves’ safety — pans heat, but the field ‘opposes’ only conductive loads: paper stays cold.
- Metal detectors at airports — induced opposing currents in metal objects re-radiate fields: security by Lenz reflex.
- Speedometers and electricity meters (old type) — eddy drag proportional to speed: analog readings by opposition.
- Regenerative braking in EVs — the motor as generator: opposition slows the car and charges the battery: Lenz paying you back.
A stubborn doorman: whatever you try to do to the flux, he pushes the other way — not out of malice, but because letting change proceed unopposed would unlock infinite energy. His resistance is exactly the tax that pays for the electricity you extract.
Magnet falling through a coil: gravity pulls down, Lenz pushes up — the magnet descends at a lazier rate, converting gravitational energy to induced current at exactly the rate of its slowing. A 0.5 J induced burst means 0.5 J less kinetic energy at exit: the books always close.
Draw the approaching N-pole and the coil: sketch induced current’s face as producing its own N toward the magnet — two N’s glaring at each other. Reverse the motion: the coil flips to S, now holding hands. Two little drawings cover every Lenz question ever set.
Practice set (answers hidden — try first)
(NEET-level) N-pole approaches a coil: the near face becomes
(JEE Main-level) Flux through a loop is decreasing: induced current flows to
(NEET-level) Lenz’s law is a statement of
(Concept) A magnet dropped through a copper tube falls
(JEE Main-level) The work you do pushing a magnet against Lenz-opposition becomes
- induced effects oppose the change
- approach → like pole (repel); exit → opposite (attract)
- Lenz = energy conservation = the minus sign
- magnetic braking by eddy drag
- work against opposition = induced energy
- 🔁 opposition principle
- 🔁 scenario table
- 🔁 energy conservation logic
- 🧠 Chant: ‘the coil fights the change’.
- 🧠 Two-face rule: ‘approach N→N; retreat N→S’.
- 🏠 Daily: train magnetic brakes = Lenz drag.
- 🏠 Daily: EV regen braking pays you back through Lenz.
Quick revision
- Lenz: induced current OPPOSES the change that made it — the minus sign of Faraday
- Approaching N-pole → coil’s near face becomes N (repels the approach)
- Withdrawing N-pole → near face becomes S (attracts it back)
- Lenz’s law is energy conservation in disguise — ‘free energy’ would otherwise appear
- Right applications: braking, damping, and every motor-as-generator
- Two scenarios, one reflex
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