Mutual Inductance
Mutual inductance M relates the current change in one circuit to the induced emf in the other. For passive reciprocal coils, M12 equals M21.
Why this shows up in the exam
Transformers · Wireless chargers · Ignition coils
Learn the idea
Changing current in one coil induces emf in a nearby coil. The first coil creates changing magnetic flux; the second coil intercepts part of it. Better magnetic coupling means larger induced emf.
🧠 Memory hook: Self is same coil; mutual is mate coil.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- emf₂ = -M dI₁/dt — emf induced in coil 2
- M = k sqrt(L₁ L₂) — coupling coefficient relation, 0 <= k <= 1
How to approach it
- 1Identify source and receiving coils
- 2Find dI/dt in the source
- 3Use M and apply Lenz polarity if required
Common slip-ups that cost marks
- •Using the turns ratio when M is requested
- •Assuming perfect coupling
- •Confusing mutual inductance with magnetic permeability
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A conducting rod of length 2 m moves at 3 m/s perpendicular to a 4 T magnetic field. Find the motional emf.
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