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 ↓


The concept of electric charge


Electric charge is a fundamental property of matter and plays a vital role in the fields of electricity and magnetism. It is the building block that describes how objects interact with each other electrically. Even though we cannot see electric charge, we can observe its effect on objects. Let's take a deeper look at what electric charge is and some basic concepts related to it.

What is electric charge?

Electric charge is a property of certain subatomic particles such as electrons and protons. When placed in an electric field, it exerts a force on these particles. There are two types of electric charge:

  • Positive charge
  • Negative charge

A simple rule is that opposite charges attract each other while like charges repel each other. A positive charge will attract a negative charge, and a negative charge will attract a positive charge. Conversely, a positive charge will repel another positive charge, and a negative charge will repel another negative charge.

Units and symbols

The SI unit of electric charge is the coulomb (C). Electric charge is often represented by the symbol Q Small charges are often measured in microcoulombs (μC) or nanocoulombs (nC).

For example, the charge of an electron is about -1.6 x 10-19 coulombs, and is expressed as:

    Charge of one electron = -1.6 x 10 -19 C

Subatomic particles and charge

Electric charge is a property that is inherent in certain subatomic particles:

  • Electrons: These have negative charge.
  • Protons: These have positive charge.
  • Neutrons: These have no charge (neutral).

The balance between protons and electrons in an atom determines its total charge. If there are more electrons than protons, the atom's total charge will be negative. Conversely, if there are more protons than electrons, the atom's total charge will be positive. Atoms that have no net charge are electrically neutral because they have equal numbers of protons and electrons.

Law of conservation of charge

The law of conservation of charge states that the total charge in an isolated system remains constant. Charge cannot be created or destroyed, but can only be transferred from one body to another.

Consider a simple example of rubbing a balloon on your hair. Before rubbing, both the balloon and hair are neutral. Rubbing them together causes electrons to transfer from your hair to the balloon, leaving the balloon negatively charged and your hair positively charged. Even though the charges have been transferred, the total amount of charge remains the same.

Coulomb's law

Coulomb's law states that the electric force between two charged objects depends on their charges and the distance between them. This law can be stated as:

The force acting between two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them.

The mathematical form of Coulomb's law is:

    F = k * (|Q1 * Q2| / r^2)

Where:

  • F is the magnitude of the force between the charges.
  • Q1 and Q2 are the amounts of charges.
  • r is the distance between the centres of the two charges.
  • k is the Coulomb constant, which is approximately equal to 8.99 x 10^9 N m^2/C^2.

Examples of electric force

To understand how Coulomb's law works, let's consider some examples:

-Why +Q

In the picture above there are two charges: one negative and one positive. According to Coulomb's law, there is a force of attraction between these charges.

Electric field

The electric field is formed around a charged object. It represents the space around the charged object where other objects with charge will experience the electric force. The strength of the electric field is determined by the amount of charge and the distance from the charge. The electric field is directed away from a positive charge and toward a negative charge.

The electric field E due to a point charge is given by the following formula:

    E = k * |Q| / r²

Where Q is the charge and r is the distance from the charge.

+Q

This diagram shows a positive charge with electric field lines pointing away from it, showing how a positive charge exerts an outward force on other charges.

Conductors and insulators

Materials can be classified based on how they allow electric charge to flow through them:

  • Conductors: These materials allow electrical charge to flow easily. Common conductors include metals such as copper and aluminum.
  • Insulators: These materials do not allow electrical charge to flow easily. For example, rubber, wood, and plastic.

Understanding how materials interact with electric charges is important in designing circuits and electronic devices.

Here is a simple example to explain the behaviour of conductors and insulators:

If we connect a metal wire (conductor) between the two terminals of the battery, charge will flow through the wire, completing the electric circuit. However, if we replace the metal wire with a rubber hose (insulator), charge will not flow, and the circuit will remain open.

Static electricity

Static electricity is a phenomenon in which electrical charge accumulates on the surface of a material. It typically occurs when two materials come into contact and then separate, causing the charge to transfer between them.

Consider the following everyday example:

When you walk on a carpeted floor and then touch a metal door handle, you may get a mild shock. This shock occurs because electrons from the carpet were transferred to your body. Your body became charged and when you touched the door handle, which is a conductor, the extra electrons moved to neutralize the charge difference.

Conclusion

Electric charge is a fundamental aspect of physics that explains a wide range of behavior in everyday life and modern technology. The principles of electric charge, electric fields, and forces are essential to understanding more complex topics in physics and engineering. By understanding these fundamental concepts, students are equipped to further explore the fascinating fields of electricity and magnetism.


Grade 9 → 6.1.1


U
username
0%
completed in Grade 9

← Prev (6.1)
Electric charge and static electricity
Next (6.1.2) →
Charging by friction, contact and induction

Comments 



The concept of electric charge

https://www.physicsflow.com/g9/6.1.1?pfal=en