Grade 9
  1. Mechanics  1.1. Motion  1.1.1. Types of motion  1.1.2. Distance and displacement  1.1.3. Speed and Velocity  1.1.4. Acceleration  1.1.5. Graphical representation of motion  1.1.6. Equations of motion  1.1.7. Uniform and non-uniform motion  1.1.8. Free fall and acceleration due to gravity  1.1.9. Relative speed  1.1.10. Circular motion  1.2. Laws of force and motion  1.2.1. The concept of force  1.2.2. newton's first law of motion  1.2.3. Newton's second law of motion  1.2.4. Newton's third law of motion  1.2.5. Inertia and types of inertia  1.2.6. Motion  1.2.7. Conservation of momentum  1.2.8. Application of Newton's laws in daily life  1.2.9. Types of Forces (Contact and Non-Contact)  1.2.10. Friction and its effects  1.3. Gravitational force  1.3.1. Newton's law of universal gravitation  1.3.2. Importance of Gravitational Force  1.3.3. Acceleration due to gravity  1.3.4. Free fall and weightlessness  1.3.5. Mass and weight  1.3.6. Variation of g with height and depth  1.3.7. Kepler's laws of planetary motion  1.3.8. Satellites and their orbits  1.4. Work, Energy and Power  1.4.1. Concept of work  1.4.2. Positive and negative actions  1.4.3. Work done by constant and variable force  1.4.4. Energy and its forms  1.4.5. Kinetic energy and potential energy  1.4.6. Law of conservation of energy  1.4.8. Commercial unit of energy  1.4.9. Work–energy theorem  1.5. Simple machines  1.5.1. Types of simple machines  1.5.2. Mechanical advantage  1.5.3. Machine efficiency  1.5.4. Levers and their types  1.5.5. Pulleys and Inclined Planes  1.5.6. Gears and their uses
  2. Properties of matter  2.1. States of matter  2.1.1. Solids, liquids and gases  2.1.2. Properties of different states of matter  2.1.3. Change in state of matter  2.1.4. Plasma and Bose–Einstein condensate  2.2. Density and pressure  2.2.1. Concept of density and relative density  2.2.2. Pressure in solids, liquids and gases  2.2.3. Pascal's law and its applications  2.2.4. Atmospheric pressure and its variations  2.2.5. Surface tension and capillarity  2.3. Buoyancy and Archimedes' principle  2.3.1. Buoyant force  2.3.2. Archimedes principle  2.3.3. Applications of Archimedes' Principle  2.3.4. floating and sinking objects  2.3.5. Theory of Flotation and Archimedes' Principle
  3. Heat and Thermodynamics  3.1. Temperature and heat  3.1.1. Concept of heat and temperature  3.1.2. Temperature measurement  3.1.3. Temperature scales  3.2. Heat transfer  3.2.1. Conduction of heat  3.2.2. Convection of heat  3.2.3. heat radiation  3.2.4. Applications of heat transfer  3.2.5. Thermal conductivity  3.3. Thermal expansion  3.3.1. Expansion of solids, liquids and gases  3.3.2. Abnormal expansion of water  3.3.3. Applications of Thermal Expansion  3.4. Specific heat capacity and latent heat  3.4.1. Concept of specific heat capacity  3.4.2. Measurement and applications of specific heat  3.4.3. Latent heat of fusion and vaporization  3.4.4. Heat Engines and Efficiency
  4. Waves and sound  4.1. Waves and their types  4.1.1. Mechanical and electromagnetic waves  4.1.2. Longitudinal and Transverse Waves  4.1.3. Properties of Waves  4.1.4. Wavelength, frequency and speed of the wave  4.1.5. Superimposition of waves  4.2. Sound waves  4.2.1. Nature and transmission of sound  4.2.2. Speed of sound in different mediums  4.2.3. Reflection of sound and echo  4.2.4. Sonar and ultrasound  4.2.5. Resonance and pulsation  4.3. Sound characteristics  4.3.1. Intensity, loudness and quality of sound  4.3.2. Doppler effect in sound
  5. Lighting and Optics  5.1. Reflection of light  5.1.1. Laws of reflection  5.1.2. Reflection from a plane mirror  5.1.3. Reflection from a spherical mirror  5.1.4. Image formation by concave and convex mirrors  5.1.5. Applications of Reflection  5.2. Refraction of light  5.2.1. Laws of refraction  5.2.2. Refractive index  5.2.3. Refraction through a glass slab  5.2.4. Refraction in water and air  5.2.5. Lenses and their types  5.2.6. Image formation by convex and concave lenses  5.2.7. Total internal reflection and its applications  5.3. Dispersion and scattering of light  5.3.1. Spectrum of white light  5.3.2. Prisms and Dispersion  5.3.3. Tyndall effect and blue sky
  6. Electricity and Magnetism  6.1. Electric charge and static electricity  6.1.1. The concept of electric charge  6.1.2. Charging by friction, contact and induction  6.1.3. Electroscope and its working  6.2. Current Electricity  6.2.1. The concept of electric current  6.2.2. Ohm's Law and Resistance  6.2.3. Factors affecting resistance  6.2.4. Series and Parallel Circuits  6.2.5. Heating effect of electric current  6.2.6. Electric power and energy  6.3. Magnetism  6.3.1. Natural and artificial magnets  6.3.2. Properties of magnets  6.3.3. Earth's magnetism  6.3.4. Magnetic field and field lines  6.3.5. Electromagnets and their applications
  7. Modern Physics  7.1. structure of the atom  7.1.1. Atomic Structure and Subatomic Particles  7.1.2. Electrons, Protons, and Neutrons  7.1.3. Rutherford and Bohr model  7.2. Radioactivity  7.2.1. Types of radioactive emissions  7.2.2. Half-life and radioactive decay  7.2.3. Uses and Hazards of Radioactivity
  8. Space science and astronomy  8.1. The universe and its components  8.1.1. Stars and galaxies  8.1.2. Planets and Solar System  8.2. Satellites  8.2.1. Natural and artificial satellites  8.2.2. Use of satellites

Grade 9 → Properties of matter → Density and pressure ↓


Surface tension and capillarity


Matter is everything that has mass and occupies space. The study of the properties of matter involves understanding the behavior and characteristics of the solid, liquid, and gas states. In this article, we will focus on two fascinating phenomena found primarily in liquids: surface tension and capillarity.

What is surface tension?

Surface tension is the elastic tendency of the surface of a liquid that forces it to acquire the least surface area possible. This phenomenon occurs when small objects appear to float on a liquid surface or when liquid droplets form beads on a smooth surface.

Imagine a layer of molecules on the surface of a fluid. These molecules are attracted to each other, a phenomenon known as fusion. The molecules below the surface are pulled equally in all directions, but the molecules on the surface are pulled only sideways and downwards. This imbalance results in a "skin" on the surface, which we recognize as surface tension.

downward force Side Force Side Force

Surface tension can be quantitatively expressed by the following formula:

Surface Tension (γ) = Force (F) / Length (L)

In this equation, γ is the surface tension, F is the force applied on the surface, and L is the length over which the force acts.

Examples of surface tension

  • Soap Bubbles: The spherical shape of a bubble is due to surface tension which minimises the surface area for a given volume.
  • Floating needle: If a needle is placed carefully it can be made to float on water as its surface tension supports it.
  • Raindrops: Raindrops are usually spherical because surface tension pulls them into this shape.
Circle

Factors affecting surface tension

Various factors can affect the surface tension of a liquid:

  • Temperature: Generally, as the temperature of a liquid increases, its surface tension decreases. This is because higher temperatures increase the kinetic energy of the molecules, which reduces the force of attraction.
  • Impurities: Adding impurities or surfactants (such as detergents) to a liquid can lower its surface tension. Surfactants reduce the cohesive forces of molecules in a liquid.

What is capillarity?

Capillarity or capillary action is the ability of a fluid to flow into narrow spaces without the aid of external forces such as gravity. This effect can be observed when a small tube is placed in a fluid, and the fluid is higher or lower inside the tube than the liquid level outside.

Capillarity is caused by a balance between cohesive forces (attraction between similar molecules) and adhesive forces (attraction between dissimilar molecules). In a narrow tube, if the adhesive forces between the liquid and the tube material are stronger than the cohesive forces within the liquid, the liquid will rise in the tube.

Liquid growth

Examples of capillarity

  • Thin tubes in plants: The movement of water and nutrients from the soil to the plant roots and throughout the plant uses capillary action.
  • Paper towel absorption: When one end of a paper towel is in water, the liquid moves upward due to capillarity.
  • Ink in the pen: Capillary action draws ink from the pen reservoir to the pen tip, where writing takes place.

Factors affecting capillarity

Several factors affect capillary action in a fluid:

  • Diameter of the tube: Smaller the diameter of the tube, the greater will be the rise of the liquid due to stronger adhesive forces relative to gravity.
  • Liquid properties: Liquids with low surface tension will exhibit a more pronounced capillary effect. In contrast, liquids with high surface tension resist capillarity.
  • Tube material: The interaction between the liquid and the tube material affects the height of the liquid.

The math behind capillarity

The height to which the liquid rises in a capillary tube can be given by the formula:

h = (2 * γ * cos(θ)) / (ρ * g * r)

Where:

  • h = the height to which the liquid rises
  • γ = surface tension of liquid
  • θ = contact angle (angle between the liquid surface and the solid surface)
  • ρ = density of the liquid
  • g = acceleration due to gravity
  • r = radius of the capillary tube

Surface tension and capillarity in nature

These phenomena play an important role in many natural processes:

  • Dew drops on leaves: Due to surface tension, dew forms in the form of spherical droplets on leaves, which facilitates water absorption in plants.
  • Water movement in soil: Capillarity helps water and nutrients move upward from the ground to plant roots, supporting plant life.

Conclusion

Surface tension and capillarity are key concepts in understanding the behavior of fluids. These phenomena are not only interesting but also important in nature and various applications. By studying these properties, we gain insight into the underlying forces that govern matter and its interactions.


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Surface tension and capillarity

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