Introduction to Surface Tension

Surface Tension is the property of a liquid due to intermolecular forces that allows its surface to resist external forces. It plays a crucial role in capillarity, formation of droplets and bubbles, and pressure differences across curved surfaces.

This chapter is important for JEE Main as it integrates concepts of forces, energy, and fluid mechanics into practical problems, making it a scoring topic if understood conceptually and numerically.

Direct Links to Download JEE Main - Chapter Wise Previous Year Papers Physics (PDF)

► Click “Download Here” next to your subject to access the free PDF.

Importance of Surface Tension in JEE Main

Studying surface tension helps students:

• Understand molecular interactions at liquid surfaces

• Solve problems on droplets, bubbles, and capillary rise

• Apply surface energy concepts in fluid mechanics

• Relate surface tension to pressure difference and curvature

JEE Main often tests numerical, conceptual, and application-based questions, making this chapter both interesting and scoring.

Molecular Basis of Surface Tension

• Molecules inside liquid experience equal cohesive forces in all directions

• Molecules at the surface experience unbalanced inward force, causing surface contraction

• Surface tension (T) is defined as force per unit length along the surface: T = F / L

• Units: N/m

Applications for JEE Main:

• Capillarity in thin tubes

• Formation of droplets and bubbles

• Calculation of surface forces

Surface Energy

• Work done to increase surface area is stored as surface energy

• Surface energy per unit area: E_s = T × ΔA

• Units: J/m²

• Applications:

  • Formation of new surfaces in liquids

  • Spreading of liquids on solids

  • Problems on energy required to form droplets or bubbles

Pressure Difference Across Curved Surfaces

1. Spherical droplet:

   • ΔP = 2T / r, where r = radius of droplet

2. Spherical bubble:

   • ΔP = 4T / r, as bubble has inner and outer surfaces

Applications in JEE Main:

• Calculating pressure inside drops and bubbles

• Soap bubble and droplet problems

• Surface tension forces in small-scale fluid systems

Capillarity

• Rise or fall of liquid in a narrow tube due to surface tension: h = 2T cos θ / (ρ g r)

  • ρ = liquid density, r = radius of tube, θ = contact angle

• Applications:

  • Capillary rise in tubes

  • Movement of liquids in porous materials

  • Biological systems like water transport in plants

JEE Main numericals often involve height of capillary rise or depression and effect of radius or surface tension.

Cohesive and Adhesive Forces

• Cohesive forces: between like molecules (maintain droplet shape)

• Adhesive forces: between liquid and solid (affect wetting and capillarity)

Understanding these forces is essential for solving JEE Main problems on capillary rise and liquid behavior on surfaces.

Angle of Contact

• Angle between liquid surface and solid surface at the point of contact

• Determines wettability:

  • θ < 90° → wetting, θ > 90° → non-wetting

• Used in capillarity calculations and predicting liquid behavior in tubes

Applications in Daily Life and Engineering

• Soap bubbles and balloons

• Capillary action in plants and porous materials

• Liquid spreading on surfaces

• Microfluidic systems and biomedical applications

Understanding these applications helps students visualize concepts for JEE Main.

Common Mistakes Students Make

• Confusing surface tension (T) and surface energy (E_s)

• Using wrong formula for pressure difference in droplets vs bubbles

• Ignoring the contact angle in capillarity problems

• Misapplying curvature or radius in numerical problems

Avoiding these mistakes ensures high accuracy in JEE Main numericals.

Weightage of Surface Tension in JEE Main

• 1–2 questions per exam

• Mostly conceptual or short numericals

• Moderate difficulty but high scoring for students with conceptual clarity

Preparation Tips for JEE Main Students

• Memorize formulas for surface tension, pressure difference, capillarity, and surface energy

• Solve numericals on droplets, bubbles, and capillary rise

• Draw diagrams for capillary tubes, droplets, and bubbles

• Practice unit conversions (N/m, J/m²)

• Understand angle of contact and forces involved

Regular practice ensures accuracy, speed, and confidence in exams.

Why Study Surface Tension from Studentbro.in

Studentbro.in provides:

• Step-by-step explanations for surface tension problems

• Solved examples on droplets, bubbles, capillary rise, and surface energy

• Conceptual clarity for advanced JEE numericals

• Chapter-wise preparation for effective exam learning

This ensures students can tackle both theoretical and numerical surface tension problems efficiently.

Conclusion

Surface Tension is a fundamental Class 11 Physics chapter that explains molecular interactions, liquid surfaces, and capillarity. Mastery of this chapter allows students to solve numericals on droplets, bubbles, capillary rise, and pressure difference with confidence. With structured guidance and practice from Studentbro.in, students can excel in JEE Main Physics and handle surface tension questions effectively.