PHY028 · Fluids · Classes 11–12
Surface tension
Objective: To measure the surface tension of a liquid from the force needed to pull a film on a wire frame.
🧰 Apparatus
- U-shaped wire frame with a sliding wire
- Sensitive spring balance or torsion balance
- Dish of liquid (water, soap solution, alcohol)
- Vernier callipers
📐 Formulas and SI units
γ = F / (2L) (N/m)
excess pressure in a drop: 2γ/r
💡 Theory
The surface of a liquid behaves like a stretched skin. Surface tension γ is the force per unit length along a line in the surface. A film on a wire frame has two surfaces, so the pull on a wire of length L is F = 2γL. Soap lowers the surface tension of water.
🪜 Procedure
- Clean the frame and dip it into the liquid.
- Lift it slowly so a film forms.
- Read the force just before the film breaks.
- Subtract the weight of the frame.
- Calculate γ = F/(2L).
- Repeat with soap solution and compare.
📋 Observations Simulated values
✅ Result and conclusion
Water (γ = 0.073 N/m) on a 0.1 m frame pulls with 0.0146 N; soapy water needs only a third as much force.
🔬 In a real lab
Tiny amounts of grease change γ a lot, and readings are small (millinewtons), so a torsion balance gives better results.
⚠️ Safety
Mercury is shown as data only: never handle it in school labs. Wipe up soap solution spills.
❓ Quick check
1. Why does a film pull with 2γL, not γL?
2. Adding soap to water makes γ…
3. Unit of surface tension?
🏋️ Practice
EasyF = 0.005 N, L = 0.1 m (film). γ?
Answer: 0.025 N/m
MediumWhy are raindrops round?
Answer: Surface tension makes the smallest surface
ChallengeName an insect that walks on water.
Answer: Pond skater (water strider)