Define electric flux and give its SI unit. State Gauss's theorem. A charge q is placed at the centre of a cube: find the flux through the cube and through each face.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define flux. Electric flux Φ = E·ΔS = EΔS cos θ, where θ is the angle between E and the outward normal.
2 · Give the unit. N m² C⁻¹ (or V m). Flux is a scalar.
3 · State Gauss's theorem. The flux through any closed surface is 1/ε₀ times the charge enclosed: ∮E·dS = q/ε₀.
4 · Draw the cube. [Cube with charge q at its centre; field lines going out through all six faces]
5 · Apply it. Whole cube: Φ = q/ε₀. By symmetry, each face gets q/(6ε₀).
6 · Close in one line. Only the enclosed charge decides the total flux, whatever the shape of the surface.
Scoring tip: Write 'closed surface' and 'charge enclosed' in your statement. Without those words, the statement of Gauss's theorem loses its mark.
