Lenz's Law and Current Direction
The minus sign in Faraday's law represents energy-conserving opposition to the change in magnetic flux. The right-hand grip rule converts the required induced field into current direction.
Why this shows up in the exam
Magnetic braking · Induction cooktops · Damping in measuring instruments
Learn the idea
The induced current opposes the change in flux that creates it. The coil behaves like a defender: it creates its own magnetic field to resist an increase or decrease of the original flux. It opposes the change, not necessarily the original field.
🧠 Memory hook: Lenz resists the change.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- emf = -d(Phi)/dt — the minus sign encodes Lenz's law
How to approach it
- 1State whether external flux rises or falls
- 2Choose the induced field that resists that change
- 3Use the right-hand grip rule for current
Common slip-ups that cost marks
- •Opposing the field instead of the change
- •Applying the right-hand rule before deciding the required induced field
- •Changing direction when only the loop orientation changes
🌟 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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