Surface tension of liquids and capillary phenomena
Answers are for parents and teachers.
1
A square wire frame with 5 cm sides is being lifted out of water; the surface film clings to it on both sides (σ = 0.073 N/m). Estimate the surface tension force.
l = 4·0.05 = 0.2 m; F = 2σl = 2·0.073·0.2 ≈ 2.9·10⁻² N
2
The water surface grows by 10 cm². Estimate ΔW (σ = 0.073 N/m).
7.3·10⁻⁵ J
3
By how many mm does water rise in a 1 mm radius capillary? (σ = 0.073; g = 10)
about 15 mm
4
Compare the action of water and mercury on glass: which wets and what is the meniscus like?
Water wets (concave meniscus), mercury does not (convex meniscus).
5
Why does soapy water clean dirt well?
Its σ is lower: it wets dirt and fabric well and penetrates them.