Old spec Tutorial 1 Problem 1 (ENGR5003)

Old spec Tutorial 1 Problem 1TutorialOld spec ENGR217Third5 min

ENGR217 (old spec) Thermo L6 Tutorial 1, Problem 1 official answers

A clean wire ring of internal diameter 30 mm is repeatedly dipped into a liquid with surface tension 0.075 N/m and withdrawn. Sometimes after withdrawing the ring from the liquid, a film of liquid has formed across the ring.

Formulas you may need

Surface-tension work: $\delta W = -\sigma_s\,dA$ (work done by the system), i.e. work $\sigma_s\,\Delta A$ must be done on a film to create area $\Delta A$ (learn this, Week 1 Lecture 2)
Circle area $A = \pi r^2$ (learn this)

(a)
How much work must be done to form the film?
work to form the film =J
(b)
In order to increase the chance of a film forming across the ring, should the surface tension be increased or decreased?