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 ↓


Ohm's Law and Resistance


Understanding the flow of electricity is essential to understanding how many devices work. In the fascinating world of electricity and magnetism within physics, we learn about concepts such as current, resistance, and voltage. Two fundamental ideas that help us understand this flow of electricity are Ohm's law and resistance.

Introduction to Ohm's Law

Ohm's law is a fundamental principle of electricity. It is named after Georg Simon Ohm, a German physicist who first formulated it in the 1820s. This law helps us understand how voltage, current, and resistance are related. According to Ohm's law, the current flowing through most conductors is directly proportional to the voltage across them. This relationship is usually expressed by the formula:

        I = V / R

Where:

  • I is the current in amperes (A).
  • V is the voltage in volts (V).
  • R is the resistance in ohms (Ω).

It is important to understand each component:

Voltage (V)

Voltage, also known as electric potential difference, is the driving force that causes electric charges to flow through a circuit. It is similar to the pressure in a water pipeline; the higher the pressure, the more water will flow. In terms of electricity, voltage is measured in volts (V).

Current(I)

Electric current is the flow of electrical charge. It can be compared to the flow of water through a pipe. It is measured in amperes (A). Higher current means more electrical energy is flowing through the circuit.

Resistance(R)

Resistance is the property that restricts the flow of electric current. It is like the obstacles in a water pipe that slow down the flow of water. Resistance is measured in ohms (Ω) and depends on the material, temperature, and shape of the conductor.

Visualization of Ohm's Law

To understand Ohm's law, let's consider a simple circuit. Assume we have a battery, a resistor and a conductor wire. Here is a graphical representation:

Battery Obstructions Wire

In the diagram above, the resistors are labeled, and the lines represent the conductor wires. A battery is also included to supply voltage.

Applying Ohm's Law

Ohm's law is applied to calculate any variable, if the other two variables are known. For example:

Example 1: Suppose the resistance of a light bulb (resistor) in a circuit is R = 5 Ω and the battery provides a voltage of V = 10 V. What is the current flowing in the circuit?

        I = V / R
I = 10V / 5Ω 
I = 2A

Here, we find that a current of 2 amperes flows in the circuit.

Example 2: If the resistance of a resistor in a circuit is R = 10 Ω and the desired current is I = 3 A, how much voltage is required?

        V = I * R
V = 3A * 10Ω
V = 30V

To obtain this current the circuit will require a voltage of 30 volts.

Understanding Resistance

Resistance is an important factor in electronic circuits. Every component in a circuit, whether it is a simple light bulb or a complex electronic device, will have some resistance. Resistance determines how much current will flow with a given voltage. Let's learn more about resistance:

Factors affecting resistance

  • Materials: Different materials have different resistivities. For example, metals such as copper have low resistance and are excellent conductors of electricity, while rubber has high resistance and is a good insulator.
  • Length: The longer the conductor, the greater the resistance. This can be compared to a long pipeline that offers more resistance to the flow of water.
  • Cross-sectional area: A conductor with a larger cross-sectional area will have lower resistance. This resembles a wide pipeline through which more water can flow easily.
  • Temperature: The resistance of most conductors increases as temperature increases, although this is not the case for all materials.

Resistance Formula

The resistance of a material can be calculated using the following formula:

        R = ρ * (L / A)

Where:

  • R is the resistance in ohms (Ω).
  • ρ (rho) is the resistivity of the material in ohm-meters (Ω m).
  • L is the length of the conductor in meters.
  • A is the area of the cross section in square metres (m²).

Let's look at an example:

Example: Consider a copper wire with length L = 2 m and cross-sectional area A = 0.5 mm². The resistivity of copper is approximately ρ = 1.68 * 10^-8 Ω·m. What is its resistance?

First, convert the area to m²: 0.5 mm² = 0.5 * 10^-6 m².

        R = ρ * (L / A) 
R = 1.68 * 10^-8 * (2 / (0.5 * 10^-6)) 
R = 1.68 * 10^-8 * (4 * 10^6) 
R = 1.68 * 4 * 10^-2 
R ≈ 0.0672Ω

The resistance of the copper wire is approximately 0.0672 ohms.

Ohm's Law in Real Life

Ohm's law isn't just a theoretical concept; it has practical applications in everyday life. It helps engineers design electrical systems, ensures that our homes are wired safely, and allows countless devices to function:

  • Home wiring: Electricians use Ohm's law to make sure circuits in homes are designed to handle specific appliances. Knowing the voltage supplied and the resistance of the wire, they can calculate the current to make sure it doesn't exceed safe limits.
  • Electronic devices: Devices are designed with specific resistances so that they can work efficiently with the intended power supply. Understanding Ohm's Law ensures that these devices do not overheat or malfunction.
  • Power transmission: Power companies use this concept to reduce the loss of electricity during transmission from power plants to our homes. Increasing the voltage reduces the current, which results in less energy being lost as heat.

Limitations of Ohm's Law

Although Ohm's law is a fundamental concept in electrical engineering, it still has its limitations:

  • Temperature dependence: Ohm's law assumes that resistance remains constant, but in reality, resistance can change with temperature.
  • Non-ohmic materials: Many materials do not obey Ohm's law. These non-ohmic devices have current-voltage characteristics that do not form a straight line. Examples include semiconductor devices such as diodes and transistors.

In these examples, the relationship between voltage, current, and resistance becomes more complex.

Conclusion

Ohm's Law is a cornerstone in the study and application of electricity. By understanding the relationship between voltage, current, and resistance, students, engineers, and electricians can predict how electricity will behave in different materials and configurations. Resistance plays a critical role in determining how much current flows through a conductor and must be carefully managed to ensure safety and efficiency in electrical systems. Whether designing circuits for high-tech devices or wiring a household bulb, Ohm's Law is the key to unraveling the mysteries of electricity.


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Ohm's Law and Resistance

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