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 → Current Electricity ↓


Factors affecting resistance


Resistance is a fundamental concept in the study of electricity and magnetism, and understanding the factors that affect resistance is important in many fields, from designing electronic circuits to power engineering. Resistance is the opposition that a substance offers to the flow of electric current. The resistance of a substance depends on many factors. Let's learn about each factor affecting resistance in detail.

1. Material of conductor

Different materials have different capacities to conduct electric current. This capacity is characterized by a property called resistivity. Conductors such as copper that easily allow electricity to pass through them have low resistivity. On the other hand, materials such as rubber, which do not easily allow electricity to pass through them, have high resistivity.

R = ρ × (L / A)

Where:

  • R is the resistance.
  • ρ (rho) is the resistivity of the material.
  • L is the length of the conductor.
  • A is the cross-sectional area of the conductor.

For example, if you have copper and aluminum wires of the same length and cross-sectional area, copper will have lower resistance because it has lower resistivity than aluminum.

2. Length of the conductor

The length of a conductor directly affects its resistance. The longer the conductor, the greater its resistance. This is because the electrons have to travel a greater distance through a longer conductor, causing them to face more resistance.

Small wire long wire

Consider two pieces of wire, one 1 m long and the other 2 m long, made of the same material and having the same thickness. Assuming all other factors are the same, the resistance of the longer wire will be twice the resistance of the shorter wire.

3. Cross-sectional area of the conductor

The cross-sectional area of a conductor also affects its resistance. A larger cross-sectional area allows more electrons to pass simultaneously, thus reducing the resistance.

thick wire thin wire

If you hold the material and length of the two conductors constant, a thicker wire, such as a power cable, will have a lower resistance than a thinner wire, such as that used in electronic components.

4. Temperature

The resistance of a conductor changes with temperature. Generally, as the temperature increases, the resistance of a conductor also increases. This is because at higher temperatures, the atoms in the conductor vibrate more rapidly, leading to more frequent collisions with the moving electrons.

R = R₀(1 + α(T - T₀))

Where:

  • R is the resistance at temperature T
  • R₀ is the original resistance at the reference temperature T₀.
  • α is the temperature coefficient of resistance.
  • T is the current temperature.
  • T₀ is the reference temperature.

In a practical scenario, the filament of the bulb gets brighter and hotter, and thus its resistance also increases as the temperature increases over time.

Examples and applications

Understanding the factors that affect resistance is important in practical applications. For example, electrical engineers consider these factors when designing electronics, ensuring that wires have the proper resistance so they can work safely and effectively without overheating.

Let's look at a practical example. Imagine you are designing an extension cord for a household appliance. The material, length, and thickness of the wire must be carefully selected. High-resistance material can overheat and potentially cause a fire if it is unable to withstand the current flowing through it.

For another example, consider why power lines are made from aluminum or copper. These materials are chosen because they have low resistance, which reduces power loss when electricity travels long distances from power stations to homes.

Additional information

In addition to the primary factors mentioned, other factors can affect resistance in more specific situations. For example, the presence of impurities in the conductor material, the frequency of the alternating current (in AC circuits), and the skin effect at high frequencies all contribute to variations in resistance. These factors, while more advanced, reflect the complexity and breadth of considerations that must be taken into account in electrical engineering.

Conclusion

In short, the resistance of an electrical conductor is determined by several factors, including the resistivity of the material, the length of the conductor, the cross-sectional area, and the operating temperature. By mastering these concepts, one can understand not only the theoretical aspects of electrical resistance but also their practical applications. This knowledge is vital in the fields of electronics and electrical engineering, helping to design efficient systems that transmit electricity safely and effectively.


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Ohm's Law and Resistance
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Factors affecting resistance

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