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 ↓


Electric power and energy


Electrical power and energy are important concepts in the study of electricity and magnetism. When dealing with electric currents, it is essential to understand how energy is used and transferred. In simple terms, electrical power refers to the rate at which electrical energy is consumed or produced by a component in a circuit, while electrical energy refers to the total amount of energy consumed or produced over a time period.

Understanding electricity

Before diving into electrical power and energy, let's briefly understand what electricity is. Electricity is the movement of electrons through a conductor, such as a wire. This movement is driven by an electric field created by a potential difference, or voltage, between two points. The flow of electrons is known as electric current and is measured in amperes (A).

Electric power

Electric power is the rate at which energy is transferred or transformed by an electric circuit. It is measured in watts (W). One watt is equal to one joule of energy per second. The formula to calculate electric power is:

P = V × I

Where:

  • P is the power in watts (W)
  • V is the voltage in volts (V)
  • I is the current in amperes (A)

Let us consider an example. If a light bulb is connected to a 240V power supply and the current flowing through it is 0.5A, then the power consumed by the bulb is:

P = 240V × 0.5A = 120W

This means that the bulb consumes 120 watts of power.

Electrical energy

Electrical energy is the total energy consumed by an electrical device over a period of time. It is measured in joules (J) or kilowatt-hours (kWh). The formula to calculate electrical energy is:

E = P × t

Where:

  • E is energy in joules (J) or kilowatt-hours (kWh)
  • P is the power in watts (W)
  • t is the time in seconds (s) or hours (h)

Consider the example of the light bulb given above. If it is kept on for 3 hours, the energy consumed will be:

E = 120W × 3h = 360Wh = 0.36kWh

This means that the bulb consumes 0.36 kilowatt hours of electrical energy when run for 3 hours.

Understanding power and energy with visual examples

Battery Bulb

A simple circuit with a battery and a light bulb. The battery provides the voltage, and current flows through the circuit to power the bulb.

Efficiency of electrical appliances

Not all electrical energy is converted into useful work. Some of it is wasted as heat. The efficiency of an electrical device is the ratio of useful electrical output to the total electrical input, expressed as a percentage:

Efficiency (%) = (Useful Power Output / Total Power Input) × 100

For example, if a refrigerator uses 200W of electricity but only 150W is used for cooling and the rest is dissipated as heat, the efficiency would be:

Efficiency = (150W / 200W) × 100 = 75%

Practical Example

Example 1: Toaster

Consider a toaster that operates on a voltage of 120V and draws a current of 5A. Find the electrical power consumed and the energy used by the toaster in 10 minutes.

First, calculate the power:

P = V × I = 120V × 5A = 600W

Now, calculate the energy consumed in 10 minutes (600 seconds):

E = P × t = 600W × 600s

Convert to kilowatt-hours for easy reference:

E = 0.6 kW × (600/3600)h = 0.1 kWh

Example 2: Ceiling fan

A ceiling fan has a power rating of 60W. How much energy will it consume if it runs continuously for 24 hours?

Calculate energy in kilowatt-hours (kWh):

E = P × t = 60W × 24h = 1440Wh = 1.44kWh

Optional units

In some cases, alternative units are used to describe power and energy:

  • Horsepower is a unit of power used primarily in reference to engine and motor output. 1 horsepower is equal to 746W.
  • Calories (cal) are sometimes used as a unit of energy, especially in nutrition. 1 calorie = 4.184 joules.

Power stations and energy sources

Electrical energy can come from a variety of sources such as fossil fuels, nuclear, wind and solar. Power stations, also called power plants, convert these resources into electrical energy. Electricity can be used to power homes, industries and other applications.

The energy efficiency of these stations can vary depending on the methods and technology used. For example, fossil fuel stations have low efficiency due to energy losses as heat, while modern renewable plants may be more efficient due to lower losses in generation.

Conclusion

Understanding electrical power and energy is important in our daily lives because it governs how we use electricity and manage our consumption efficiently. From small household appliances to large-scale industrial applications, knowing how to calculate and apply these concepts can aid in energy conservation and cost savings.

Using the formulas and examples given, one can easily determine the power and energy usage of various electrical devices. This understanding is fundamental to physics and its application in real-world scenarios.


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