Magnetic Flux
For a uniform field and a flat surface, magnetic flux is the dot product of the magnetic field and area vector. The area vector is perpendicular to the surface.
Why this shows up in the exam
Sizing generator coils · Magnetic sensors · Understanding MRI pickup coils
Learn the idea
Flux measures how much magnetic field passes through a surface. Imagine magnetic field lines as rain and the loop as a window. More rain passes through when the window is larger, the field is stronger, or the window faces the rain.
🧠 Memory hook: Flux likes the normal: use the angle with the area vector, not with the plane.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- Phi = B A cos(theta) — theta is the angle between B and the area normal
- [Phi] = weber = tesla metre² — SI unit of magnetic flux
How to approach it
- 1Draw the area normal
- 2Write B A cos(theta)
- 3Check whether theta is with the normal
Common slip-ups that cost marks
- •Using the angle with the plane directly
- •Ignoring the sign of the dot product
- •Adding fluxes as scalars when their area normals differ
🌟 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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