Motional EMF in a Moving Conductor
For a straight rod of length l moving with velocity v in a uniform field B, the simple Blv form applies only when B, l, and v are mutually perpendicular.
Why this shows up in the exam
Rail generators · Flow meters · Aircraft wing voltage in Earth's field
Learn the idea
Charges in a moving conductor feel magnetic force and separate, creating emf. As a rod cuts magnetic field lines, positive and negative charges are pushed toward opposite ends. Their separation creates a voltage across the rod.
🧠 Memory hook: B-l-v: mutually perpendicular gives Blv.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- emf = B l v — perpendicular rod, field, and velocity
- F_m = q(v x B) — magnetic force that separates charge
How to approach it
- 1Draw v, B, and the rod
- 2Use v cross B for positive charge
- 3Apply Blv if perpendicular
Common slip-ups that cost marks
- •Using Blv without checking geometry
- •Confusing rod speed with charge drift speed
- •Choosing polarity without using v cross B
🌟 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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Motional EMF
Motional EMF is the voltage induced in a conductor moving through a magnetic field, depending on the speed, length, and orientation of the conductor.