☰ Contents · Physics

Surface tension problems and lab work

Lessons 36–37 · 2 lessons · P. Habibullayev, A. Boydedayev, A. Bahromov, K. Suyarov, J. Usarov, M. Yuldasheva. Physics Grade 9, revised and expanded 3rd edition. G‘afur G‘ulom Publishing House, Tashkent, 2019
36

Problem solving

Textbook: pp. 100–102
GoalSolve problems on surface tension and capillary phenomena: F = σl, W = σS, h = 2σ/(ρgr).
New words
surface layer · sirt qatlamsoap bubble · sovun pufagidrop weight · tomchi og‘irligitwo-sided film · ikki tomonlama plyonka
Explanation

In this topic we link three formulas: the surface tension force F = σl, the surface energy W = σS and the capillary height h = 2σ/(ρgr). First decide which surface is involved: a soap bubble and a soap film have two surfaces (outer and inner), so the force and energy are multiplied by 2. Then convert the quantities to SI: cm and mm to metres, g to kilograms. When a drop breaks off, mg = σ·πd gives the drop mass, and then the number of drops is N = M/m. When a bubble expands, the energy change is ΔW = 2σ·4π(R₂² – R₁²). Finally check that the answer makes sense: the height is greater in a narrower tube, and σ of a soap solution is smaller than that of water.

Worked examples
Alcohol (σ = 0.022 N/m, ρ = 800 kg/m³) in a capillary of radius 0.5 mm: h = 2·0.022/(800·10·0.0005) = 0.011 m = 11 mm.
A soap bubble of radius 2 cm (σ = 0.04 N/m, two surfaces): W = 2σ·4πR² = 2·0.04·4·3.14·0.0004 ≈ 4.0·10⁻⁴ J.
Class activity

Calculation task: if 10 g of water is dripped from a dropper of inner diameter 2 mm, about how many drops form? First find the mass of one drop. Then, with an adult, weigh 5 drops with a dropper on a scale and compare with your calculation.

Practice
1
Why is the soap bubble energy multiplied by 2?
2
A soap film (σ = 0.04 N/m) pulls on the 6 cm movable side of a frame. Find the force.
3
At least how much work against surface tension is needed to blow a soap bubble (σ = 0.04 N/m) of radius 3 cm?
4
Why is the height in alcohol smaller than in water (same radius)?