Comprehensive Revision Notes — Sources of Energy
1. Introduction
Energy is the ability to do work. Human societies rely on a variety of energy sources to meet domestic, industrial and transportation needs. Broadly, energy sources are classified as conventional (non-renewable) and non-conventional (renewable). The NCERT syllabus focuses on understanding these kinds, their availability, environmental effects and the need for conservation.
2. Classification of Energy Sources
Non-renewable (Conventional): Fossil fuels (coal, petroleum, natural gas) and nuclear fuels. These are finite and take millions of years to form.
Renewable (Non-conventional): Solar, wind, hydro, tidal, geothermal and biomass. These are replenished naturally and are more sustainable in the long term.
3. Fossil Fuels — Coal, Petroleum & Natural Gas
Coal: Formed from plant remains under high pressure over millions of years. Used in thermal power plants and industries. Advantages: high energy density, established technology. Disadvantages: air pollution (SO2, NOx), ash, mining impacts, greenhouse gases.
Petroleum (Crude oil): Mixture of hydrocarbons used for fuels (petrol, diesel, kerosene) and as a raw material in petrochemicals. Advantages: transport fuel, high energy. Disadvantages: oil spills, carbon emissions, finite reserves.
Natural Gas: Primarily methane (CH4) — cleaner than coal/oil, used for cooking, electricity generation and as feedstock. Lower CO2 per unit energy but still fossil-derived.
4. Renewable Energy Sources
Solar Energy
Solar energy is abundant and available across India. Photovoltaic (PV) cells convert sunlight directly into electricity; solar thermal converts sunlight to heat for water heating or power generation. Advantages: clean, reduces dependence on fossil fuels. Challenges: intermittency (night, clouds), initial cost, storage needs.
Wind Energy
Wind turbines convert kinetic energy of wind into electrical energy. Best in coastal and plateau regions. Advantages: low emissions, scalable. Limitations: site-dependent, variable output, visual/noise concerns.
Hydroelectricity
Hydropower uses flowing water to generate electricity. Large dams store potential energy; small hydro projects use river flow. Benefits: reliable baseload in many places, storage in reservoirs. Drawbacks: displacement, ecological impacts, sedimentation.
Biomass
Biomass includes wood, agricultural residues and dung used for cooking and small energy needs. Can be converted to biogas via anaerobic digestion. Sustainable when managed well but can cause indoor pollution if used inefficiently.
Geothermal and Tidal
Geothermal taps Earth's internal heat; tidal uses sea tides. India has limited geothermal deployment but tidal has potential in select coastal areas. Both are site-specific and require investment.
5. Nuclear Energy
Nuclear power uses nuclear fission of uranium or thorium to generate heat and electricity. Advantages: very high energy density, low CO2 emissions during operation. Concerns: radioactive waste management, safety risks (accidents), high capital cost and long lead times.
6. Environmental Impacts & Sustainability
Burning fossil fuels increases greenhouse gas concentrations causing global warming. Air and water pollution affect health and ecosystems. Renewable sources have lower direct emissions but can have local impacts (land use for solar farms, habitat changes from dams). Key idea: sustainable energy systems reduce emissions, use resources efficiently, and account for ecological and social effects.
7. Energy Conservation and Efficiency
Energy conservation means reducing energy use through behaviour change (switching off lights, using public transport). Efficiency means using less energy for the same service (LED bulbs, energy-efficient appliances). Conservation and efficiency are the quickest and cheapest ways to reduce demand and environmental impact.
- Use LED lights and energy-efficient fans.
- Insulate homes and use natural ventilation.
- Use public transport, cycle or carpool.
- Promote rooftop solar and rainwater harvesting at home/school.
8. Energy Flow & Conversion — Simple Examples
Every energy application involves conversion from one form to another. Examples:
- Chemical energy (coal) → Thermal energy → Mechanical energy (turbine) → Electrical energy.
- Solar radiation → Electrical energy (PV panels) → Electrical energy used in homes.
- Potential energy (water in a dam) → Kinetic energy (flowing water) → Mechanical → Electrical.
9. Quick Revision: Important Points to Remember
- Classify energy sources into renewable and non‑renewable with two examples each.
- List 2 advantages and 2 disadvantages of fossil fuels and renewable sources.
- Understand the working of solar cells, wind turbines and a thermal power plant in brief.
- Describe how energy conservation differs from energy efficiency.
- Know environmental impacts: air pollution, greenhouse effect, land and water changes.
10. CBSE Exam — Question Types & How to Answer
- For 1‑mark questions: give crisp definitions or single examples.
- For 3‑mark questions: include 2–3 points with a short diagram if relevant.
- For 5‑mark questions: explain processes, list advantages/disadvantages and add a labelled diagram or a small table.
11. Sample Questions (Practice)
- Define renewable and non‑renewable sources with examples. (2 marks)
- Give two environmental disadvantages of burning fossil fuels. (2 marks)
- Explain how solar panels convert sunlight into electricity. (3 marks)
- List advantages and disadvantages of nuclear energy. (5 marks)
- Describe two ways to conserve energy at home. (3 marks)
12. Useful Mnemonics & Memory Aids
To remember renewable sources: "S H W B T G" — Solar, Hydro, Wind, Biomass, Tidal, Geothermal. For fossil fuels remember "C P G" — Coal, Petroleum, Gas.
13. Final Checklist Before Board Exams
- Revise definitions, diagrams and differences.
- Practice writing two short advantages/disadvantages for each source.
- Make a one-page mind map of the chapter.
- Attempt previous year questions and sample papers focusing on this chapter.