Electrostatics – Stage 1
Chapter 1: Electric Charges & Fields – Page 6
1. What is Electric Flux?
Electric flux measures how much electric field passes through a given surface.
Physical Meaning: Electric flux tells us how strongly an electric field penetrates a surface.
2. Mathematical Definition of Electric Flux
ΦE = 𝐄 · 𝐀 = EA cosθ
- 𝐄 → Electric field
- 𝐀 → Area vector (normal to surface)
- θ → Angle between 𝐄 and 𝐀
3. Area Vector (Very Important for JEE)
- Magnitude = Area of surface
- Direction = Normal (perpendicular) to surface
- For closed surface → outward normal
JEE Rule: Always take area vector perpendicular to surface, NOT along surface.
4. Special Cases of Electric Flux
| Angle θ | Orientation | Flux |
| θ = 0° | Field perpendicular to surface | Maximum (Φ = EA) |
| θ = 90° | Field parallel to surface | Zero (Φ = 0) |
| θ = 180° | Opposite direction | Negative maximum |
5. Electric Flux Through Different Surfaces
(a) Open Surface
- Has a boundary (e.g., flat sheet)
- Flux depends on orientation
(b) Closed Surface
- No boundary (sphere, cube)
- Total flux depends only on charge enclosed
Preview: This leads directly to Gauss’s Law.
6. Sign of Electric Flux
- Positive flux → Field lines leaving surface
- Negative flux → Field lines entering surface
Exam Trap: Flux depends on orientation, not just area.
7. Flux Due to Point Charge (Conceptual)
- Flux through a closed surface depends only on enclosed charge.
- Independent of shape and size.
Φ = q / ε₀
(Result of Gauss’s Law)
8. Common JEE Mistakes
- Taking wrong area vector direction
- Confusing field lines with flux
- Assuming flux always non-zero
- Forgetting cosine factor
Stage 1 – Page 6 Takeaway
- Flux is dot product of E and A
- Orientation is crucial
- Foundation for Gauss’s Law
- Highly conceptual JEE topic
Next → Stage 1, Page 7: Gauss’s Law – Statement, Proof & Applications
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