CBSE Class 10 Science Sources of Energy Notes

2. Key Concepts and Components

2.1 Fossil Fuels

Fossil fuels are formed from the remains of ancient organisms buried under the earth over millions of years. They include coal, petroleum, and natural gas

•         Coal: Formed from ancient plant matter; used in thermal power plants. Calorific value: approximately 25,000-33,000 kJ/kg

•         Petroleum: Liquid fossil fuel refined into petrol, diesel, kerosene. Calorific value: approximately 45,000 kJ/kg

•         Natural Gas: Mainly methane (CH4); cleanest fossil fuel. Calorific value: approximately 55,000 kJ/kg

Disadvantages of fossil fuels: They are non-renewable, cause air pollution, contribute to acid rain and global warming, and will eventually be depleted.

2.2 Thermal Power Plant

Thermal power plants burn coal or other fuels to heat water, producing steam that drives turbines connected to generators to produce electricity. The efficiency of thermal power plants is low because a large amount of heat energy is wasted.

Fuel burns --> Heat --> Steam --> Turbine --> Generator --> Electricity

2.3 Hydroelectric Power

Hydroelectric power harnesses the potential energy of water stored at height. When water flows down from a dam, its kinetic energy turns turbines to generate electricity. It is a renewable, pollution-free source of energy.

•         Advantages: Renewable, no air pollution, long lifespan of dams

•         Disadvantages: Displacement of people, loss of biodiversity, affects river ecosystems

2.4 Biomass Energy

Biomass refers to organic material from plants and animals used as fuel. It includes wood, agricultural waste, cow dung, and municipal solid waste.

•         Biogas: Produced by anaerobic decomposition of organic matter in a biogas plant. Mainly contains methane (CH4) and is an excellent cooking fuel. The slurry left behind is a good manure.

•         Wood: Burning wood releases CO2 and causes deforestation; inefficient

•         Charcoal: Obtained by burning wood in limited oxygen; higher calorific value than wood

2.5 Wind Energy

Wind energy is harnessed using windmills. The kinetic energy of wind rotates the blades of the windmill, which drives a generator to produce electricity. A group of windmills is called a wind energy farm.

•         Minimum wind speed required: 15 km/h (approximately) for consistent power

•         Advantages: Pollution-free, renewable, low running cost

•         Disadvantages: Requires large land area, depends on weather, threat to birds

2.6 Solar Energy

Solar energy is energy from the sun. It is harnessed using:

•         Solar Cells (Photovoltaic Cells): Convert sunlight directly into electricity using the photovoltaic effect. Made from silicon semiconductor materials.

•         Solar Panel: A group of solar cells connected together. Used in calculators, street lights, satellites, and homes.

•         Solar Cooker: Uses a black-coated box with a glass lid and mirror reflectors to trap solar radiation and cook food. Temperature can reach 100-140 degrees Celsius.

•         Solar Water Heater: Uses solar collectors to heat water for domestic use

Box-type solar cooker: The box is painted black inside to absorb maximum heat. A glass lid prevents heat loss by the greenhouse effect. Mirror reflectors increase the amount of solar radiation entering the box.

2.7 Energy from the Sea

•         Tidal Energy: Generated by the rise and fall of ocean tides. Tidal barrages (dams) are built at sea inlets to harness this energy.

•         Wave Energy: Generated by the motion of waves on the ocean surface. Devices trap wave motion to generate electricity.

•         Ocean Thermal Energy Conversion (OTEC): Uses the temperature difference between warm surface water and cold deep ocean water to generate electricity. Requires a temperature difference of at least 20 degrees Celsius.

2.8 Geothermal Energy

Geothermal energy is heat energy from the interior of the Earth. It occurs naturally as hot springs, geysers, and fumaroles. The steam from underground reservoirs can be used to drive turbines and generate electricity.

•         Countries using geothermal energy: Iceland, New Zealand, United States, Philippines

•         Advantages: Renewable, available 24/7, small land footprint

•         Disadvantages: Limited to specific geographic locations, risk of releasing toxic gases

2.9 Nuclear Energy

Nuclear energy is released during nuclear reactions. Two types of nuclear reactions are used:

•         Nuclear Fission: A heavy nucleus (like Uranium-235 or Plutonium-239) is split into smaller nuclei by bombarding with neutrons, releasing enormous energy. This is used in nuclear reactors and nuclear bombs.

•         Nuclear Fusion: Two light nuclei (like hydrogen isotopes - deuterium and tritium) combine to form a heavier nucleus, releasing even greater energy. This powers the sun and stars. Practical fusion reactors are still under development (ITER project).

235U + 1n --> 141Ba + 92Kr + 3(1n) + Energy (Fission reaction)

Nuclear Reactor Components: fuel rods, moderator (heavy water or graphite), control rods (cadmium), coolant (water), and protective shielding.

•         Advantages: Very high energy output from small amount of fuel, no CO2 emission

•         Disadvantages: Radioactive waste is dangerous and hard to dispose, risk of accidents (Chernobyl, Fukushima), very expensive to build

3. Core Formulas with Derivations

3.1 Calorific Value

Definition: Amount of heat energy released by complete combustion of 1 kg of fuel.

Calorific Value (CV) = Heat energy released (Q) / Mass of fuel (m)

Unit: kJ/kg or MJ/kg

3.2 Efficiency of Energy Conversion

Efficiency tells us what fraction of input energy is converted to useful output energy.

Efficiency (%) = (Useful energy output / Total energy input) x 100

3.3 Power Output of a Solar Cell

Power from a solar cell or panel is given by:

Power (P) = Voltage (V) x Current (I)   [in Watts]

3.4 Einstein's Mass-Energy Equivalence (Nuclear Energy)

Einstein's equation gives the energy released in nuclear reactions:

E = mc^2   where m = mass defect (kg), c = speed of light (3 x 10^8 m/s)

Even a tiny mass converts to an enormous amount of energy because c^2 is extremely large (9 x 10^16 m^2/s^2).

3.5 OTEC Temperature Difference Requirement

Minimum temperature difference (Delta T) = 20 degrees Celsius (between surface and deep water)

4. Solved Examples

Example 1: Calorific Value Calculation

Question: 500 g of wood releases 10,000 kJ of heat on complete combustion. Calculate its calorific value.

Solution:

Mass of fuel = 500 g = 0.5 kg

Heat energy released = 10,000 kJ

Calorific Value = Q / m = 10,000 / 0.5 = 20,000 kJ/kg

Answer: The calorific value of the wood sample is 20,000 kJ/kg.

Example 2: Efficiency Calculation

Question: A solar panel receives 500 J of solar energy and produces 100 J of electrical energy. What is its efficiency?

Solution:

Useful energy output = 100 J

Total energy input = 500 J

Efficiency = (100 / 500) x 100 = 20%

Answer: The efficiency of the solar panel is 20%.

Example 3: Nuclear Energy - Einstein's Equation

Question: Calculate the energy released when 0.001 kg of mass is converted to energy in a nuclear reaction.

Solution:

E = mc^2

E = 0.001 x (3 x 10^8)^2

E = 0.001 x 9 x 10^16

E = 9 x 10^13 J = 90,000 GJ

Answer: 9 x 10^13 J of energy is released from 0.001 kg of mass.

Example 4: Comparing Fuels

Question: LPG has a calorific value of 55,000 kJ/kg and wood has a calorific value of 17,000 kJ/kg. How much wood would be needed to produce the same energy as 1 kg of LPG?

Solution:

Energy from 1 kg LPG = 55,000 kJ

Mass of wood needed = 55,000 / 17,000 = 3.24 kg (approximately)

Answer: Approximately 3.24 kg of wood is needed to match the energy output of 1 kg of LPG. This shows why LPG is a more efficient fuel.

Example 5: Solar Cooker vs Traditional Cooking

Question: Explain why a solar cooker has a glass lid and the inside of the box is painted black.

Solution:

Black interior: Black surfaces are good absorbers of radiation. The black coating absorbs maximum solar energy and converts it to heat, raising the temperature inside the box.

Glass lid: Glass allows short-wavelength solar radiation to enter the box. The hot surfaces inside emit long-wavelength infrared radiation, which cannot pass through glass. This is the greenhouse effect - heat is trapped inside, raising the temperature significantly.

Answer: The black interior maximizes heat absorption, and the glass lid traps heat inside by preventing infrared radiation from escaping.

5. Applications and Special Cases

5.1 Biogas Plant - Construction and Working

Biogas plant (also called a digester) consists of a mixing tank, underground digester (dome/tank), and gas outlet. Cattle dung mixed with water forms a slurry which is fed into the digester. Bacteria decompose the organic matter anaerobically (without oxygen) to produce biogas (mainly CH4 + CO2).

•         Biogas burns without smoke, making it ideal for cooking

•         The leftover slurry is nitrogen-rich and used as organic manure

•         Reduces dependence on wood and fossil fuels in rural areas

5.2 Solar Energy in India

India receives abundant sunshine throughout the year, making it ideal for solar energy. The Indian government has launched major initiatives:

•         National Solar Mission: Target of 100 GW of solar capacity by 2022

•         Solar Street Lights: Widely deployed in remote and rural areas

•         Solar Pumps: Used for irrigation in agriculture

•         Rooftop Solar: Encouraged for homes and commercial buildings through subsidies

5.3 Nuclear Power in India

India has several operational nuclear power plants:

•         Tarapur (Maharashtra): India's first nuclear power plant

•         Rawatbhata (Rajasthan): Uses CANDU-type reactors

•         Kalpakkam (Tamil Nadu): Features India's first prototype fast breeder reactor

•         Kudankulam (Tamil Nadu): Largest nuclear power station in India (with Russian collaboration)

5.4 Charcoal vs Wood - Why Charcoal is Better

Charcoal is obtained by heating wood in limited supply of oxygen (destructive distillation). Charcoal has a higher calorific value (~33,000 kJ/kg) than wood (~17,000 kJ/kg). It burns without smoke and does not produce irritating fumes, making it more useful for cooking and industrial purposes.

5.5 Hydrogen as Future Fuel

Hydrogen has the highest calorific value of all fuels (~150,000 kJ/kg) and burns to produce only water. It is being explored as a clean fuel of the future. Challenges include safe storage and production costs. Hydrogen fuel cells are already used in space technology.

6. Formula and Concept Summary

Calorific Value:

CV = Heat Energy Released (Q) / Mass of Fuel (m)   [Unit: kJ/kg]

Efficiency of Energy Conversion:

Efficiency (%) = (Useful Output Energy / Total Input Energy) x 100

Electrical Power:

P = V x I   [Watts = Volts x Amperes]

Einstein Mass-Energy Equivalence:

E = mc^2   [m in kg, c = 3 x 10^8 m/s, E in Joules]

OTEC Requirement:

Minimum temperature difference = 20 degrees Celsius

Calorific Values of Common Fuels (Approximate):

•         Wood: 17,000 kJ/kg

•         Coal: 25,000-33,000 kJ/kg

•         Charcoal: 33,000 kJ/kg

•         Petrol / Diesel: 45,000 kJ/kg

•         LPG (Liquefied Petroleum Gas): 55,000 kJ/kg

•         Biogas: 35,000-40,000 kJ/kg

•         Hydrogen: 150,000 kJ/kg

7. Key Theorems and Properties

7.1 Law of Conservation of Energy

Statement: Energy cannot be created or destroyed; it can only be converted from one form to another. The total energy of an isolated system remains constant.

Application: In all energy conversion devices (solar cells, turbines, engines), input energy = useful output energy + energy lost as heat. No device is 100% efficient.

7.2 Properties of a Good Fuel

•         High calorific value: More energy per unit mass

•         Low moisture content: Wet fuels waste energy evaporating water

•         Moderate ignition temperature: Not too high (hard to ignite) or too low (fire hazard)

•         Clean combustion: Minimal ash, soot, or toxic gases

•         Easy availability and low cost

7.3 Renewable vs Non-Renewable Energy

•         Renewable energy: Can be replenished naturally in a short time (solar, wind, tidal, biomass, geothermal). These are sustainable and environmentally friendly.

•         Non-renewable energy: Formed over millions of years and will run out (coal, petroleum, natural gas, nuclear fuels). These cause pollution and cannot be replenished on human timescales.

7.4 Photovoltaic Effect

Photovoltaic effect: When light falls on certain semiconductor materials (like silicon), it frees electrons, generating an electric current. This is the working principle of solar cells. Each solar cell produces a small voltage (about 0.5 V), and many cells are connected in series to form a solar panel with higher output.

7.5 Nuclear Chain Reaction

Chain reaction: In nuclear fission, when a Uranium-235 nucleus splits, it releases 2-3 neutrons. These neutrons can split other Uranium nuclei, releasing more neutrons, and so on. A controlled chain reaction is used in reactors. An uncontrolled chain reaction leads to a nuclear explosion.

Critical mass: The minimum amount of fissile material needed to sustain a chain reaction. Control rods made of cadmium absorb excess neutrons to control the reaction rate in reactors.

8. Common Mistakes and Exam Tips

8.1 Common Mistakes

•         Confusing Fission and Fusion: Fission = splitting of heavy nucleus; Fusion = joining of light nuclei. Fusion releases more energy but requires extremely high temperatures.

•         Thinking solar energy is free: Solar energy itself is free, but the initial cost of solar panels and installation is very high.

•         Assuming all renewable sources are pollution-free: Biomass burning (wood) causes air pollution. Dams cause environmental displacement.

•         Forgetting that calorific value is per kg: Many students forget the unit is kJ/kg, not just kJ.

•         OTEC confusion: It is NOT the temperature of the water that matters, but the DIFFERENCE in temperature between surface and deep water (minimum 20 degrees Celsius).

•         Biogas composition: Many students say biogas is only methane. It is mainly methane (50-75%) plus carbon dioxide and small amounts of other gases.

8.2 Important Exam Tips

•         Know calorific values of common fuels by heart - values are frequently asked in numerical questions.

•         Draw and label diagrams: solar cooker, biogas plant, solar cell panel, nuclear reactor.

•         Distinguish clearly between conventional and non-conventional energy sources with examples.

•         E = mc^2 - know how to apply this formula with correct units. Remember c = 3 x 10^8 m/s.

•         For long-answer questions on nuclear energy, cover both advantages and disadvantages.

•         Box-type solar cooker: Always explain the role of black paint AND glass lid separately in exam answers.

•         Know the names of nuclear power plants in India with their locations - frequently asked in board exams.

9. Practice Questions

1 Mark Questions (MCQ / Very Short Answer)

Q1. What is the calorific value of a fuel? In what unit is it measured?

Q2. Which of the following has the highest calorific value? (a) Wood (b) Coal (c) Hydrogen (d) LPG

Q3. Name the process by which the sun produces energy.

Q4. What is the minimum wind speed required for a windmill to generate electricity?

Q5. What is the main component of biogas?

Q6. State Einstein's mass-energy equivalence relation.

3 Mark Questions (Short Answer)

Q7. What are the characteristics of a good source of energy? Explain any three characteristics.

Q8. Explain the construction and working of a box-type solar cooker. What are its limitations?

Q9. Differentiate between nuclear fission and nuclear fusion. Give one example of each.

Q10. What is biogas? How is it produced? What are the advantages of using biogas over firewood?

Q11. List three advantages and three disadvantages of establishing hydroelectric power plants.

5 Mark Questions (Long Answer)

Q12. Explain the various non-conventional sources of energy in detail. For each source, describe the working principle, one advantage, and one disadvantage. Include solar, wind, tidal, geothermal, and biomass energy.

Q13. What is nuclear energy? With a neat labelled diagram, explain the working of a nuclear reactor. What are the safety precautions and environmental concerns associated with nuclear power?

Q14. Compare conventional and non-conventional energy sources under the following headings: availability, environmental impact, cost, and future prospects. Why should we shift towards non-conventional energy sources?

Q15. Explain the working of a biogas plant with the help of a labelled diagram. How does it benefit the rural population economically and environmentally? What is the composition of biogas?

Q16. What is solar energy? Explain with diagrams how a solar cooker and a solar water heater work. What are the advantages and limitations of using solar energy? How are solar cells used to generate electricity?