Key relationship
Ffilm=2γl; ΔU=2γlΔx; wetting term=γcosθ.
Physics · Class 11
Stretch a two-sided liquid film
Mechanical Properties of Fluids
Creating more liquid surface requires energy. A soap film has two faces, so its force and energy contain a factor of two. Contact angle controls the sign of the wetting term and distinguishes rise from depression in capillary systems.
Open the interactive experiment →Stretch a two-sided liquid film
திரவப் பரப்பை அதிகரிக்க ஆற்றல் தேவை. மெல்லிய படலத்தில் இரண்டு பரப்புகள் உள்ளதால் விசை 2γl.
Key notes, relationships and model limits
Remember
Creating more liquid surface requires energy. A soap film has two faces, so its force and energy contain a factor of two. Contact angle controls the sign of the wetting term and distinguishes rise from depression in capillary systems.
Common trap
A film’s two surfaces require twice the area change of a single liquid surface.
Model limits
Thin film has two surfaces and constant surface tension. Contact angle separately indicates the wetting sign; no detailed molecular meniscus is modeled.
What you will learn
Relate surface energy to pulling force and contact-angle sign.
Try the experiment
Try both scenarios, change one control, and predict the result before checking the graph and readouts.
Think of it this way
Change one input while keeping the others fixed, as you would in a controlled experiment.
Worked example
Moving that wire 0.02 m creates energy ΔU=0.0144×0.02=2.88×10⁻⁴ J. At 90° the wetting term is zero.
Why this matters
The model connects stretch a two-sided liquid film with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
What pull comes from a two-sided film with γ=0.072 N/m and l=0.1 m?
- 0.0072 N
- 0.0144 N
- 0.144 N
Read the explanation
F=2×0.072×0.1=0.0144 N.
Course connection: Class XI Chapter 9. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.