Introduction to Surface Tension

Surface tension is a key concept in NEET Physics under the topic of properties of liquids. It is the force acting along the surface of a liquid that minimizes its surface area. This phenomenon arises due to cohesive forces between liquid molecules and explains several real-life effects such as capillary rise, formation of droplets, and floating of light objects on a liquid surface.

StudentBro notes provide clear, structured explanations of surface tension concepts, formulas, and applications for NEET aspirants.

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Cohesive and Adhesive Forces

• Cohesive forces: Forces of attraction between similar molecules (liquid-liquid). Responsible for surface tension.

• Adhesive forces: Forces of attraction between different molecules (liquid-solid). Responsible for wetting and capillary action.

• NEET often includes questions on why water rises in a capillary or why mercury forms convex meniscus.

Understanding the difference between these forces is fundamental for solving surface tension problems.

Surface Energy

• Surface tension can be defined as energy per unit area required to increase the surface of a liquid.

• Formula: Surface Tension (T) = Surface Energy / Area

• It is expressed in N/m or J/m².

NEET questions often ask for calculations of work done in creating new surface or energy associated with a liquid surface.

Surface Tension Force

• Surface tension also represents a force acting tangentially along the surface per unit length:
  F = T × L, where L is the length along which force acts

• Explains phenomena like floating of needle on water or capillary rise.

StudentBro notes include step-by-step derivation and examples for NEET numerical problems.

Angle of Contact

• Angle formed between tangent to the liquid surface at the point of contact and the solid surface: θ

• Determines wetting behavior:

  • θ < 90°: Liquid wets the surface (water on glass)

  • θ > 90°: Liquid does not wet (mercury on glass)

• NEET may ask conceptual questions or involve angle of contact in capillarity formulas.

Capillary Action

• Phenomenon: Rise or fall of liquid in a narrow tube due to surface tension and adhesion

• Capillary rise formula: h = 2T cos θ / (ρ g r)

  • T = surface tension

  • θ = angle of contact

  • ρ = density of liquid

  • r = radius of tube

• Explains suction in plants, oil in thin tubes, and meniscus formation

• NEET numerical problems often involve calculating height of liquid in capillary tubes.

Excess Pressure in a Liquid Drop and Bubble

• Liquid drop: Pressure inside is greater than outside

  • Formula: ΔP = 2T / r

• Soap bubble: Pressure inside is greater than outside by two surfaces

  • Formula: ΔP = 4T / r

• Important for NEET numerical questions on drops, bubbles, and curved liquid surfaces.

Variation of Surface Tension with Temperature

• Surface tension decreases with increase in temperature due to weakened cohesive forces.

• NEET often includes conceptual questions on temperature dependence and boiling effects.

Applications of Surface Tension

• Formation of droplets and bubbles

• Capillary rise in thin tubes and plant vessels

• Floating of light objects on liquid surfaces

• Wetting of surfaces by liquids

• NEET may ask numerical or conceptual questions based on real-life phenomena.

StudentBro notes provide detailed illustrations to make these applications clear.

Experimental Determination of Surface Tension

1. Capillary Rise Method: Measures height of liquid in a narrow tube

2. Drop Weight Method: Measures mass of liquid drops formed at the tip of a capillary

3. Torsion Balance Method: Measures force needed to detach a wire from liquid surface

NEET aspirants benefit from understanding experiments as they are often tested conceptually.

Tips for NEET Preparation on Surface Tension

1. Memorize key formulas for surface tension, capillary rise, and excess pressure.

2. Practice problems involving drops, bubbles, and meniscus formation.

3. Understand cohesive and adhesive forces and their effects.

4. Visualize capillary action and angle of contact for conceptual clarity.

5. Solve previous NEET questions to strengthen numerical problem-solving speed.

StudentBro notes are structured to implement these tips effectively.

Advantages of StudentBro NEET Physics Notes

• Covers surface tension, cohesive and adhesive forces, surface energy, capillary action, and excess pressure in drops and bubbles

• Step-by-step examples and diagrams included

• Structured for easy revision and conceptual clarity

• Focused on NEET syllabus and high-yield problems

These notes ensure aspirants can confidently tackle surface tension questions in NEET exams.

Conclusion

The chapter Surface Tension is a vital part of NEET Physics under liquid properties. Mastery of cohesive and adhesive forces, surface energy, capillarity, angle of contact, and excess pressure is essential for solving conceptual and numerical problems.

StudentBro NEET Physics notes provide structured, clear, and exam-focused guidance, enabling aspirants to confidently solve surface tension problems and excel in NEET exams.