Lesson 7 of 13 · 7 min
Electric flux
NCERT §1.9
Even on a clear day the air near the ground carries a downward field of roughly 100 N/C. How much of that field passes through Tara's 10 m² roof?
The lesson in notes
In short
Electric flux through a small area element is Δφ = E·ΔS = E ΔS cos θ, where θ is the angle between E and the area vector ΔS. It is proportional to the number of field lines crossing the element.
An area is a vector: its magnitude is the area and its direction is along the normal to the surface. For a closed surface the outward normal is always used.
Flux is largest when the surface faces the field squarely (θ = 0), and zero when the field lines run along the surface (θ = 90°).
Flux is a scalar. Its SI unit is N m² C⁻¹ (equivalently V m).
The total flux through a surface is the sum of E·ΔS over all its small elements, exact in the limit of vanishing elements (an integral).
Unlike water through a ring, nothing physically flows in electric flux; the name comes from the analogy.