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 → Electricity and Magnetism ↓


Electric charge and static electricity


Electricity and magnetism are fundamental forces of nature, and they play a vital role in our daily lives. The key concepts under these forces include electric charge and static electricity. Understanding these concepts is very important to understand how many electrical devices we use. Let's understand these ideas in a detailed and easy to understand manner.

Electric charge

Electric charge is a fundamental property of matter. This property causes matter to experience a force when placed in an electromagnetic field. There are two types of electric charge: positive and negative.

Positive and negative charge

Electrons (negatively charged particles) orbit the atom's nucleus. Protons, which reside in the nucleus, have a positive charge. Normally, an atom has an equal number of protons and electrons, making it electrically neutral. However, an imbalance in this number results in a net electrical charge.

Quick check: What happens with a charge imbalance?

When electrons are gained or lost, the atom becomes charged:

  • If electrons are added then the atom becomes negatively charged.
  • If the electrons are removed the atom becomes positively charged.

Static electricity

Static electricity refers to the accumulation of electrical charge on the surface of an object. This occurs because some materials allow electrons to flow between them more easily than others, allowing the transfer of electrons to occur.

Example of static electricity

A classic demonstration of static electricity is rubbing a balloon on your hair. When you rub the balloon, electrons transfer from your hair to the balloon. This causes your hair to become positively charged and the balloon to become negatively charged, causing your hair to be attracted to the balloon and stand up.

How does static electricity work?

Static electricity is generated due to friction. When two substances come into contact and then separate, electrons are transferred. If one substance holds on to its electrons more tightly than the other, an imbalance in charge is created.

Condition: Two objects come into contact Result: Transfer of electrons occurs Cause: Imbalance of electric charges Effect: Static electricity is produced

Observing charge interactions

It is important to understand how electric charges interact with each other. Opposite charges attract each other, while like charges repel each other.

+-

In the picture above, we see that a positive charge is being attracted to a negative charge. It is this attraction that holds objects with different charges together.

Experiment: Observation of static electricity

Materials Required: Balloon, woolen cloth, small pieces of paper.

  1. Inflate the balloon and tie the end.
  2. Rub the balloon vigorously against the woolen cloth.
  3. Bring the balloon close to the small pieces of paper.

See how the pieces of paper stick to the balloon. This happens because the balloon becomes charged and exerts a force on the neutral pieces of paper, attracting them.

Law of conservation of charge

Electric charges are conserved. The total charge in an isolated system remains constant. Charges cannot be created or destroyed, but they can be transferred from one object to another. For example, when a rod is rubbed with a cloth, electrons move from one to the other, but the total charge in the rod-cloth system remains constant.

Initial charge of Rod = 0 Initial charge of Cloth = 0 Final charge of Rod + Final charge of Cloth = 0

Conductors and insulators

Substances are classified as conductors or insulators based on their ability to allow the flow of electrical charge.

  • Conductors: These are materials that allow electrons to move freely through them. Metals such as copper and aluminum are good conductors.
  • Insulators: These materials do not allow electrons to move freely. For example rubber, glass and wood.
MetalWoodWire

The diagram above shows the comparison of different materials and their ability to conduct electrical charge. Metal is a good conductor, while wood is an insulator, and wire is used to conduct electricity due to its metal core.

Charging methods

There are different ways to charge an object:

Charging by friction

This happens when two different materials are rubbed together, transferring charge from one material to the other. For example, rubbing a glass rod with silk transfers electrons from the glass to the silk.

Charging by conduction

This involves touching a charged object to a neutral object. Electrons are transferred between the two, resulting in both having the same charge.

Charging by induction

This is charging without direct contact. When a charged object is placed near a neutral object, it causes a redistribution of charges in the neutral object. If the neutral object is grounded, electrons will either leave it or enter it, causing it to charge in the opposite direction from the induced charge.

Electric field

The electric field describes the force per unit charge experienced by a small positive test charge placed in the field. The direction of the field is the direction of the force exerted on the test charge.

Visual example of an electric field

Below is a diagram of a positive charge surrounded by electric field lines:

+

Electric field lines radiate outward from the positive charge, showing the direction in which a positive test charge will move if it is placed in the field.

Conclusion

Electric charge and static electricity are fundamental concepts in the study of physics. They form the basis for understanding how objects interact electrically and how static effects occur in our daily environment. By understanding these concepts, we can better appreciate and use the various technological advancements that rely on electrical and magnetic principles.


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Electric charge and static electricity

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