Electric Flux Through an Open Surface
Electric flux is Phi = integral E dot dA. For a uniform field over a flat surface, Phi = E dot A = EA cos theta.
Why this shows up in the exam
Vector-area calculations · Flux through tilted planes · Flux through selected open faces
Learn the idea
Flux counts the normal component of electric field through an oriented area. Only the part of E piercing a surface contributes; a field sliding along it gives zero flux. The area vector fixes the sign.
🧠 Memory hook: Flux uses the field component through, not along, the surface.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- Phi = integral E dot dA — general electric-flux definition
- Phi = E dot A — uniform field through a flat surface
How to approach it
- 1Write the area vector
- 2Take E dot A or integrate
- 3Check the sign and N m²/C units
Common slip-ups that cost marks
- •Using sin theta when theta is measured from the normal
- •Ignoring area-vector orientation
- •Applying q/epsilon₀ to an open surface without symmetry
🌟 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.
Two point charges 1 microC and 2 microC are 1 m apart in vacuum. Take k = 9 x 10^9 SI. Find the force magnitude.
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