Define surface tension and surface energy. Show that surface tension is equal to the surface energy per unit area. Write its SI unit and dimensions.

How to lay out the answer. Follow the frame, then write it in your own words.

1 · Define surface tension. T = F/L: the force per unit length acting along the surface, at right angles to a line drawn on it.
2 · Explain surface energy. Surface molecules feel a net inward pull. The work done to bring molecules to the surface is stored as surface energy.
3 · Draw the frame. A rectangular wire frame with a sliding wire of length L and a soap film; mark the force F and the shift Δx.
4 · Force on the wire. The film has two surfaces, so F = 2TL.
5 · Work to stretch the film. W = F × Δx = 2TLΔx, and the new area A = 2LΔx. So W = TA.
6 · Result and units. T = W/A, the surface energy per unit area. SI unit N m⁻¹ (= J m⁻²); dimensions [MT⁻²].
7 · Real life. Detergents lower T, so water spreads into cloth fibres and lifts out oily dirt.
8 · Close in one line. Surface tension is both a force per unit length and an energy per unit area.

Scoring tip: Say 'the film has two surfaces' before you write F = 2TL. Missing the factor 2 is the most common slip.