Surface Tension Class 11: Definition, Formula, Unit, Dimensions & Formula Sheet with Free PDF Download (Physics, JEE & NEET)
Surface Tension Class 11 Formula Sheet — Competishun
Surface Tension Class 11: Definition, Formula, Unit, Dimensions & Formula Sheet with Free PDF Download (Physics, JEE & NEET)
Surface Tension is a key topic in the Class 11 physics chapter Mechanical Properties of Fluids, and it appears regularly in JEE and NEET. It explains why water forms droplets, why insects can walk on water and how liquid rises in a thin tube, all through a simple property of the liquid surface.
This page gives you the complete Surface Tension formula sheet for Class 11 in one place: definition, formula, unit and dimensions, surface energy, angle of contact, capillary rise and excess pressure in drops and bubbles, with worked examples. Download the free PDF below and keep it handy for revision.
Download the Surface Tension Formula Sheet PDF
Get all Surface Tension formulas, unit, dimensions, surface energy, capillary rise and excess pressure in one clean PDF, free. Perfect for Class 11, JEE and NEET revision.
Download Free PDFWhat is Surface Tension?
Surface tension is the property of a liquid surface by which it behaves like a stretched elastic membrane. It arises from the cohesive forces between liquid molecules. A molecule at the surface is pulled inward, so the surface tends to shrink to the smallest possible area.
Key Terms You Must Know
| Term | Meaning |
|---|---|
| Cohesive force | Attraction between molecules of the same liquid |
| Adhesive force | Attraction between molecules of liquid and another surface |
| Surface energy | Work done to increase the surface area of a liquid |
| Angle of contact | Angle between the liquid surface and the solid at the point of contact |
| Capillarity | Rise or fall of liquid in a narrow tube |
| If adhesion > cohesion the liquid wets the surface (rises), if cohesion > adhesion it does not wet (falls). | |
Surface Tension Formula Sheet
| Quantity | Formula |
|---|---|
| Surface tension | T = F / L |
| SI unit | N/m (= J/m²) |
| Dimensional formula | [M¹L⁰T⁻²] |
| Surface energy | W = T × ΔA (work to increase area) |
| Surface energy per unit area | = T (numerically equal) |
| Capillary rise | h = 2T cosθ / (r ρ g) |
| Surface tension and surface energy per unit area are numerically the same for a liquid. | |
Excess Pressure (Drops & Bubbles)
| Case | Excess Pressure | Why |
|---|---|---|
| Liquid drop | P = 2T / R | One surface |
| Soap bubble | P = 4T / R | Two surfaces |
| Air bubble in liquid | P = 2T / R | One surface |
| A soap bubble has two surfaces (inner and outer), so its excess pressure is double that of a drop. | ||
Angle of Contact
- Acute angle (< 90°): Liquid wets the surface, meniscus is concave (e.g. water in glass)
- Obtuse angle (> 90°): Liquid does not wet the surface, meniscus is convex (e.g. mercury in glass)
- Depends on: Nature of the liquid and the solid, and their cohesive/adhesive forces
Factors Affecting Surface Tension
| Factor | Effect |
|---|---|
| Temperature | Surface tension decreases as temperature rises, becomes zero at critical temperature |
| Impurities (soluble) | Salt-like impurities generally increase surface tension |
| Surfactants / detergents | Decrease surface tension (helps cleaning) |
| Contamination | Oil or grease lowers surface tension |
| This is why hot water and detergent clean better, both lower the surface tension. | |
Worked Examples
| Problem | Solution |
|---|---|
| T if F=0.02 N, L=0.1 m | T = F/L = 0.02/0.1 = 0.2 N/m |
| Excess P in drop, T=0.072, R=0.001 | P = 2T/R = 2(0.072)/0.001 = 144 Pa |
| Excess P in soap bubble (same T,R) | P = 4T/R = 288 Pa |
| Surface energy, T=0.05, ΔA=0.2 | W = T×ΔA = 0.05×0.2 = 0.01 J |
| Remember a soap bubble gives double the excess pressure of a drop of the same radius. | |
Common Mistakes to Avoid
- Drop vs bubble: Drop is 2T/R, soap bubble is 4T/R (two surfaces)
- Unit confusion: N/m and J/m² are the same for surface tension
- Temperature effect: Surface tension decreases with temperature, not increases
- Capillary formula: Do not forget cosθ in h = 2T cosθ/(rρg)
Why This Topic Matters for JEE & NEET
- Regular questions: Excess pressure and capillary rise are common in JEE and NEET
- Formula based: Clear formulas make it a quick, scoring topic
- Conceptual links: Connects to fluid mechanics and pressure
- Easy marks: Once the drop vs bubble idea is clear, problems are fast
How to Use This Formula Sheet
- Fix the basics: T = F/L, unit N/m, dimension [MT⁻²]
- Learn excess pressure: 2T/R for drops, 4T/R for soap bubbles
- Remember capillary rise: h = 2T cosθ/(rρg)
- Revise from the PDF: Skim it before every physics test and mock
Get the Complete Formula Sheet PDF Free
Download the full Surface Tension Class 11 formula sheet and revise anytime, anywhere.
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Frequently Asked Questions — Surface Tension
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