Electric Flux & Gauss’s Law — A/L Physics

What must I remember about Electric Flux & Gauss’s Law?

• Electric Flux (Φ): Measure of the total number of electric field lines passing through a given surface: Φ = ∫ E · dA = E A cos θ (units N·m²·C⁻¹ or V·m).
• Gauss's Law Statement: The total outward electric flux passing through ANY closed hypothetical Gaussian surface equals the net charge enclosed by the surface divided by permittivity of free space: ∮ E · dA = Q_enclosed / ε0.
• Symmetry Applications of Gauss's Law: 1) Infinite line of charge: E = λ / (2πε0 r); 2) Infinite plane sheet of charge: E = σ / (2ε0); 3) Conducting sheet / plate surface: E = σ / ε0; 4) Spherical shell: E = Q / (4πε0 r²) for r ≥ R, and E = 0 for r < R.
• Electrostatic Shielding (Faraday Cage): Since electric field inside an enclosed conducting cavity with zero enclosed charge is strictly zero, sensitive electronic instruments placed inside are perfectly shielded from external electrostatic fields.

Test yourself (1 minute)

1. Choice of Gaussian Surface

   Must possess spatial symmetry matching the charge distribution (spherical, cylindrical, or planar pillbox) such that E is either constant and normal to the surface, or parallel to the surface (zero flux).

2. Charge Outside Gaussian Surface

   Charges located outside the closed surface contribute zero net flux through the surface (field lines entering must exit).

Practise Electric Flux & Gauss’s Law questions today

Must know

• Field between two oppositely charged parallel conducting plates: Charges migrate to facing internal surfaces; inside the gap E = σ / ε0, outside the plates E = 0.

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 6: Electrostatic Field

What should I study first?

What comes next?

Next in the syllabus

Where does Electric Flux & Gauss’s Law fit in the syllabus?

Unit 6.4 · Year 1 · Term 3