Lesson Sources and induction · Induction and spikes
Lenz’s law
Lenz’s law: an induced voltage or current opposes the change that created it. Push a magnet toward a coil → induced current makes a field that repels the magnet. Pull it away → induced field attracts back. Inductors therefore fight current changes — rising current meets opposing EMF; falling current gets a “keep going” kick. That opposition explains generator loading, relay timing, and why opening an inductive circuit can arc.
Induced current makes its own field
When induced voltage drives current in a closed coil, that current creates a magnetic field around the coil — same rule as any current.
Lenz noticed: the field from induced current always acts to oppose the motion (or flux change) that caused the induction.
Magnet pushed toward coil → induced current polarity produces a field that pushes back on the magnet — you feel mechanical resistance; work you do becomes electrical energy in the circuit.
Motion reversed → induced polarity reverses
Pull magnet away from coil → flux through coil decreases → induced current reverses so its field tries to hold flux from collapsing — attracts the magnet back.
Any time you reverse direction of flux change, induced polarity flips to keep opposing the change.
Static magnet inside coil, no motion: field surrounds coil but no changing flux → no induced voltage, no induced current.
Statement to memorize
Lenz’s law: The direction of an induced EMF or current is such that it opposes the change in magnetic flux that produced it.
Pair with Fleming’s generator rule for magnitude and direction of induced quantities under motion.
Inductors oppose change of current
A coil with steady DC current has steady flux — no opposition beyond wire resistance.
Increase current → flux expands → cutting action induces EMF against the applied voltage → current cannot jump instantly (next lesson’s exponential rise).
Decrease or interrupt current → collapsing flux induces EMF that tries to maintain current in the old direction → arc at opening contacts or spike across semiconductors (lesson 09).
Phrase for the field:
Inductors oppose a change in current — not the existence of current.
Energy and mechanical load on generators
Hand-crank a generator: harder when loaded because induced current in armature creates fields that oppose rotation — Lenz at work converting mechanical effort to electrical output.
Regenerative braking in modern drives uses the same principle deliberately: motor becomes generator; opposing EMF slows the load.
Understanding opposition prevents surprise when a small PM alternator “locks up” under a dead short — induced currents produce maximum opposing field.
Collapsing field — opposite polarity, same rule
When switch opens on an energized coil, flux collapses instead of expanding. Induced EMF polarity reverses relative to the rise case so current tries to continue in the same direction it had before the switch opened.
That is still opposing the change — the change was “current trying to fall.”
Protective devices (freewheel diode, MOV, RC snubber) give that current a safe path — covered next in spikes and RL timing lessons.
Quick contrast table
| Situation | Flux change | Induced field action |
|---|---|---|
| Magnet approaches coil | Increasing flux linkage | Opposes approach (repel) |
| Magnet retreats | Decreasing linkage | Opposes retreat (attract) |
| Current rising in coil | Expanding self-flux | EMF bucks applied V |
| Current falling / opened | Collapsing self-flux | EMF aids old current direction |
Field case
Situation. Tech rapidly slams a relay contact closed on a large DC holding coil. Contact welds on first energize; driver transistor fails on de-energize the same week.
Two Lenz stories: Closing fast still faces rising-current opposition (less severe than opening). Opening throws collapsing-flux spike across the driver with no snubber — kilovolt-scale transient destroys solid-state switch.
Fix: Flyback diode or MOV rated for coil energy; don’t “fix” weld by hammering contacts without addressing inrush/snubbing.
In the field
Symptom
Heavy arc when dropping relay; hard to push magnet into test coil
Where to look
Coil suppression missing, contact rating, semiconductor across coil
Likely causes
- Open path on coil interrupt
- no diode/MOV
- Lenz EMF finds air gap
What to measure
- Scope spike on open
- coil L and R
- presence of snubber
What not to do
- Open inductive DC with bare contacts near ICS
Checklist
- I state Lenz’s law in plain language
- I explain magnet approach vs retreat induced polarity
- I connect Lenz to “inductors oppose current change”
- I distinguish steady current vs changing current in a coil
- I predict opposing mechanical force on loaded generator
- I link collapsing field to same-direction current attempt