NCERT Solutions Class 10 Science Chapter 12 Sources of Energy

Mastering NCERT Solutions Class 10 Science Chapter 12 Sources of Energy is essential for students preparing for their CBSE board examinations and competitive foundation tests. This chapter bridges fundamental physics, chemistry, and environmental science, exploring the classification, working mechanisms, efficiency, and ecological impact of various conventional and non-conventional energy sources. With detailed step-by-step explanations, comprehensive in-text solutions, and complete textbook exercise answers, these NCERT Science Class 10 solutions provide conceptual clarity and exam-ready presentation techniques.

Chapter Overview

In our daily lives, energy is required to perform physical labor, cook food, provide lighting, power transport systems, and run industries. In Chapter 12, "Sources of Energy," the NCERT syllabus systematically evaluates how energy is harnessed from natural reservoirs and converted into usable forms, primarily electrical energy.

The key themes covered across the chapter include:

• Characteristics of Good Energy Sources & Fuels: High calorific value, low smoke/residue, easy availability, safe storage, and economic transport.
• Conventional Sources of Energy:
  • Fossil Fuels: Coal, petroleum, and natural gas; combustion products and pollution hazards (acid rain, greenhouse effect).
  • Thermal Power Plants: Fuel combustion heating water to generate high-pressure steam that drives turbines.
  • Hydro Power Plants: Conversion of potential energy of stored water into kinetic energy and then electricity; associated ecological and rehabilitation challenges.
  • Biomass and Biogas: Cow-dung cakes vs. fixed-dome biogas plants (anaerobic digestion yielding up to 75% methane).
  • Wind Energy: Harnessing kinetic energy of moving air via wind turbines; requirements like minimum wind speed of 15 km/h.
• Alternative or Non-Conventional Sources of Energy:
  • Solar Energy: Solar cookers (box-type with plane mirrors and glass sheets), solar water heaters, and photovoltaic solar cells made of silicon.
  • Energy from the Sea: Tidal energy (gravitational pull of the moon), Wave energy, and Ocean Thermal Energy Conversion (OTEC, operating on a 20°C temperature gradient).
  • Geothermal Energy: Harnessing heat trapped in underground hot spots.
  • Nuclear Energy: Nuclear fission (e.g., Uranium-235 bombarded by low-energy neutrons) and nuclear fusion; energy release governed by Einstein's mass-energy equation: E = Δm × c².
• Environmental Consequences & Sustainability: Evaluating the carbon footprint, pollution levels, and classifying sources into renewable (inexhaustible) and non-renewable (exhaustible).

NCERT In-Text Questions and Step-by-Step Solutions

In-Text Questions (Set 1)

Question 1: What is a good source of energy?

Answer:

A good source of energy is one that satisfies the following criteria:

1. High Output: It does a large amount of work per unit volume or per unit mass (high calorific value / energy density).
2. Accessibility: It is easily accessible and abundantly available.
3. Convenience: It is easy to store and transport safely without excessive loss or hazard.
4. Economic Viability: It is economical and cost-effective to extract and utilize.
5. Environmental Safety: It produces minimal smoke, hazardous pollutants, or toxic residues upon combustion.

Question 2: What is a good fuel?

Answer:

A good fuel is defined by the following characteristics:

• High calorific value (releases a substantial quantity of heat per unit mass).
• Moderate ignition temperature (neither too high, which makes ignition difficult, nor too low, which poses spontaneous fire hazards).
• Moderate rate of combustion ensuring a controlled and steady heat supply.
• Low residue (ash) and safe combustion products (negligible poisonous gases such as CO, SO₂, and NO_x).
• Safe and cost-effective handling, storage, and transportation.

Question 3: If you could use any source of energy for heating your food, which one would you use and why?

Answer:

Selected Source: Liquefied Petroleum Gas (LPG) or Microwave/Induction (Electricity from renewable sources).

Reasoning:

1. LPG has a very high calorific value (~50 kJ/g) and burns with a smokeless blue flame, leaving zero particulate residue or ash.
2. The heat output can be instantly regulated using a burner control knob.
3. It avoids the severe indoor air pollution and respiratory hazards associated with burning wood or dried cow-dung cakes.

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In-Text Questions (Set 2)

Question 1: What are the disadvantages of fossil fuels?

Answer:

1. Air Pollution: Burning coal and petroleum releases acidic oxides of carbon (CO₂, CO), nitrogen (NO₂), and sulfur (SO₂), which cause severe respiratory illnesses and lead to acid rain that damages crops, aquatic ecosystems, and historical monuments.
2. Global Warming & Greenhouse Effect: Enormous quantities of carbon dioxide (CO₂) accumulate in the atmosphere, trapping infrared radiation and accelerating global climate change.
3. Exhaustibility: Fossil fuels take millions of years to form under high pressure and temperature. At current rates of consumption, their known reserves will be depleted in a few generations.
4. Solid Waste Generation: Coal combustion produces massive volumes of fly ash and bottom slag, requiring expansive landfills and risking soil contamination.

Question 2: Why are we looking at alternate sources of energy?

Answer:

• Depletion of Fossil Reserves: Known reserves of petroleum, coal, and natural gas are finite and rapidly exhausting due to rising industrialization and population growth.
• Mitigating Environmental Degradation: Alternate sources (solar, wind, hydro, geothermal) produce little to no direct greenhouse gases or toxic pollutants.
• Energy Security and Decentralization: Renewable resources are widely distributed and can supply power to remote and rural areas without requiring long-distance transmission grids.

Question 3: How has the traditional use of wind and water energy been modified for our convenience?

Answer:

Traditionally, kinetic energy from flowing water and wind was used directly for localized mechanical tasks, such as turning waterwheels to grind grain or operating windmills to draw water from wells. Today, these have been modernized into large-scale electrical generation systems:

• Hydroelectric Modernization: High-head dams store river water into massive reservoirs, converting water's potential energy into kinetic energy as it rushes through penstocks to rotate high-efficiency hydro-turbines connected to electric generators.
• Wind Energy Modernization: Modern aerodynamic wind turbines convert kinetic energy of moving air into rotatory mechanical energy, which is geared up to drive alternators. Clustered arrays of wind turbines across large land areas form wind energy farms feeding electricity directly into the national grid.

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In-Text Questions (Set 3)

Question 1: What kind of mirror—concave, convex or plain—would be best suited for use in a solar cooker? Why?

Answer:

A concave mirror (or a parabolic reflector) is best suited for focusing solar cookers, while a plane mirror is preferred in box-type solar cookers.

Reasoning:

• A concave mirror converges parallel sunlight rays onto a single focal point, creating extremely high local temperatures suitable for fast cooking and baking.
• In standard box-type cookers, a flat plane mirror acts as an adjustable reflector that directs extra incoming solar radiation onto the glass-covered cooking compartment to optimize heat capture without causing pinpoint scorching.

Question 2: What are the limitations of the energy that can be obtained from the oceans?

Answer:

1. Tidal Energy: Sites suitable for building tidal dams across narrow sea openings are extremely limited globally, and the difference in water level between high and low tides must be substantial to generate commercial power.
2. Wave Energy: Strong, steady wave motion is concentrated only in specific geographic coastal zones, and wave-energy capture devices require expensive engineering to withstand corrosive seawater and severe ocean storms.
3. Ocean Thermal Energy (OTEC): Continuous plant operation requires a strict minimum temperature difference of 20°C between warm surface water and deep cold water (at depths up to 2 km). Capital setup costs are high, and conversion efficiency remains relatively low.

Question 3: What is geothermal energy?

Answer:

Geothermal energy is the thermal energy trapped within the molten rocks deep inside Earth's crust. Geological changes push molten rocks upward into specific regions called hot spots. When underground water contacts these hot spots, it converts into high-pressure steam. This steam is tapped through drilled extraction wells and routed through pipes to drive turbines that power electric generators.

Question 4: What are the advantages of nuclear energy?

Answer:

• Tremendous Energy Density: Fission of 1 atom of Uranium-235 produces approximately 10 million times more energy than the combustion of 1 carbon atom from coal.
• Zero Direct Greenhouse Gas Emissions: Normal nuclear power plant operations emit no carbon dioxide, sulfur dioxide, or particulate matter into the atmosphere.
• Continuous Base-Load Generation: Unlike solar and wind, a nuclear reactor provides uninterrupted power independent of seasonal and weather conditions.
• Low Fuel Volume: Tiny quantities of enriched uranium fuel can sustain reactor power generation for months without refueling.

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In-Text Questions (Set 4)

Question 1: Can any source of energy be pollution-free? Why or why not?

Answer:

Strictly speaking, no source of energy is completely 100% pollution-free across its entire lifecycle.

While solar panels, wind turbines, and geothermal systems produce zero emissions during actual operation, the manufacturing, extraction of raw materials (silicon purification, mining of rare earths, steel and concrete production), transportation, installation, and end-of-life disposal cause environmental disruption and industrial emissions.

Question 2: Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel than CNG? Why or why not?

Answer:

Yes, hydrogen is a cleaner fuel than CNG (Compressed Natural Gas).

Reasons:

1. Combustion Reaction of Hydrogen:
   2H2(g) + O2(g) → 2H2O(l) + Heat
   The only byproduct formed is harmless water vapor, leaving zero carbon residue.
2. Combustion Reaction of CNG (Methane):
   CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + Heat
   CNG releases carbon dioxide (CO₂), a greenhouse gas, along with trace amounts of unburned hydrocarbons and carbon monoxide.

Question 3: Name two energy sources that you would consider to be renewable. Give reasons for your choices.

Answer:

1. Solar Energy: The Sun has radiated energy at its present rate for nearly 5 billion years and will continue doing so for another 5 billion years through nuclear fusion of hydrogen into helium. It is naturally replenished continuously.
2. Wind Energy: Wind patterns arise from uneven solar heating of Earth's surface and atmospheric pressure gradients. As long as solar radiation reaches Earth, winds will be regenerated indefinitely.

Question 4: Give the names of two energy sources that you would consider to be exhaustible. Give reasons for your choices.

Answer:

1. Coal: Formed from decomposed plant matter buried under deep sedimentary rock millions of years ago (carbonization). Current extraction rates far outpace the geological formation timescale.
2. Petroleum (Crude Oil): Formed from marine organisms decaying in anoxic seabed layers over geological eras. Once existing reservoirs are pumped dry, they cannot be replenished on a human timescale.

NCERT Textbook Exercise Solutions

Exercise Question 1: A solar water heater cannot be used to get hot water on:

(a) a sunny day
(b) a cloudy day
(c) a hot day
(d) a windy day

Answer: (b) a cloudy day

Explanation: A solar water heater relies directly on incident solar radiation. Cloud cover obstructs and scatters sunlight, preventing the absorber plate from reaching the temperatures needed to heat water effectively.

Exercise Question 2: Which of the following is not an example of a bio-mass energy source?

(a) wood
(b) gobar-gas
(c) nuclear energy
(d) coal

Answer: (c) nuclear energy

Explanation: Biomass refers to organic matter derived from plants and animals (such as wood, agricultural residues, and cow-dung in gobar-gas). Nuclear energy is generated via the nuclear fission of heavy radioactive isotopes (e.g., Uranium-235) or fusion of light nuclei, having no connection to biological organic matter.

Exercise Question 3: Most of the sources of energy we use represent stored solar energy. Which of the following is not ultimately derived from the Sun’s energy?

(a) geothermal energy
(b) wind energy
(c) nuclear energy
(d) bio-mass

Answer: (c) nuclear energy (and (a) geothermal energy)

Explanation: Nuclear energy originates from internal subatomic nuclear forces binding nucleons inside atomic nuclei formed during stellar nucleosynthesis, not modern solar radiation. (Note: Geothermal energy is also generated by internal radioactive decay and primordial heat from Earth's core; among the textbook options, nuclear energy is the primary standard NCERT key).

Exercise Question 4: Compare and contrast fossil fuels and the Sun as sources of energy.

Answer:

Parameter	Fossil Fuels (Coal, Petroleum)	The Sun (Solar Energy)
Availability	Exhaustible and non-renewable; finite reserves that will deplete in the near future.	Inexhaustible and renewable; will continue radiating for ~5 billion years.
Pollution	Causes heavy air pollution, emitting CO2, SO2, NOx, and ash.	Pollution-free during direct operational use.
Cost & Storage	Can be stored, transported, and combusted on demand at any time of day or night.	Intermittent (unavailable at night or on cloudy days); requires battery storage.
Efficiency / Density	High energy density in concentrated form.	Diffused energy; requires large collector surface areas to harvest.

Exercise Question 5: Compare and contrast bio-mass and hydro electricity as sources of energy.

Answer:

Parameter	Biomass Energy (Biogas / Wood)	Hydroelectricity
Source Origin	Organic waste, agricultural residues, and animal dung.	Gravitational potential energy of stored water in high-altitude dams.
Pollution Level	Direct burning of wood causes smoke; biogas burns cleanly with minimal emissions.	Completely clean and emission-free during electricity generation.
Ecological Impact	Biogas plants aid in hygienic waste management and produce rich organic slurry (manure).	Dam construction submerges vast forests, alters river ecology, and creates social displacement.
Infrastructure Cost	Low to moderate setup cost; easily constructed in decentralized rural settings.	Requires huge capital investment, specific geographic terrain, and long transmission lines.

Exercise Question 6: What are the limitations of extracting energy from: (a) the wind? (b) waves? (c) tides?

Answer:

1. (a) Wind Energy Limitations:
   • Wind energy farms require high initial capital investment and large contiguous land areas (~2 hectares per 1 MW output).
   • Windmills can operate only in areas where wind blows steadily at a minimum speed of 15 km/h throughout the greater part of the year.
   • Tower structures, blades, and generators suffer continuous wear, tear, and corrosion from exposure to moisture and high-speed winds.
2. (b) Wave Energy Limitations:
   • High mechanical efficiency is achievable only in coastal belts experiencing exceptionally strong and persistent sea waves.
   • Wave-energy converters are expensive to build and require specialized marine engineering to withstand corrosion and violent storm surges.
3. (c) Tidal Energy Limitations:
   • Tidal power generation relies on building dams across narrow coastal inlets, which are geographically rare.
   • Power generation is intermittent, fluctuating directly with the lunar tidal cycle (high tide vs. low tide).

Exercise Question 7: On what basis would you classify energy sources as: (a) renewable and non-renewable? (b) exhaustible and inexhaustible? Are the options given in (a) and (b) essentially the same?

Answer:

1. (a) Renewable and Non-renewable:
   • Renewable: Energy sources that are replenished continuously by natural cycles and cannot be permanently depleted (e.g., solar, wind, biomass if cultivated sustainably).
   • Non-renewable: Sources that have accumulated over millions of years and cannot be replenished within human timescales once exhausted (e.g., coal, crude oil, natural gas).
2. (b) Exhaustible and Inexhaustible:
   • Exhaustible: Energy reservoirs that have finite quantities on Earth and will eventually run out through continuous consumption (e.g., fossil fuels, nuclear mineral ores).
   • Inexhaustible: Energy reserves that are unlimited in supply and will not get exhausted regardless of consumption rates (e.g., solar radiation, tidal movement, geothermal flow).
3. Are the two classifications essentially the same?
   Yes, in practical applications they are largely synonymous. A renewable source of energy is inherently inexhaustible because nature continuously replenishes it, whereas a non-renewable energy source is exhaustible because its stock is fixed and cannot regenerate quickly.

Exercise Question 8: What are the qualities of an ideal source of energy?

Answer:

An ideal source of energy possesses the following six fundamental qualities:

1. High Specific Energy / Calorific Value: It delivers maximum output of useful work per unit mass or volume.
2. Controllable & Steady Rate: It supplies energy at an adjustable, uniform rate without violent explosions or sudden dropouts.
3. Clean & Eco-Friendly: It produces negligible smoke, greenhouse gases, or non-biodegradable waste.
4. Safety in Handling: It has a safe ignition threshold and poses minimal transport or explosion risks.
5. Economic Affordability: It has a low production, transmission, and operational maintenance cost.
6. Storage Convenience: It can be stored compactly without degradation or hazard over long durations.

Exercise Question 9: What are the advantages and disadvantages of using a solar cooker? Are there places where solar cookers would have limited utility?

Answer:

Advantages of Solar Cookers:

• Consumes zero recurring fossil fuel or electricity, leading to zero fuel expenses.
• Environmentally clean; creates no smoke, soot, or greenhouse gas emissions.
• Slow, gentle cooking preserves essential heat-sensitive nutrients and vitamins.
• Negligible risk of burning or charring food due to controlled interior temperatures (~100°C to 140°C).

Disadvantages of Solar Cookers:

• Cannot be used at night, in rainy conditions, or on cloudy days.
• Cooking takes longer compared to gas stoves or induction heaters.
• Cannot perform high-temperature cooking tasks such as deep-frying, baking chapatis, or searing.
• The reflector mirror must be manually reoriented at regular intervals to track the shifting position of the Sun.

Places with Limited Utility:

Solar cookers have very limited utility in regions with persistent cloud cover, high-latitude polar zones with short daylight hours, dense forest belts, and cold temperate countries that receive weak, diffused solar irradiance.

Exercise Question 10: What are the environmental consequences of the increasing demand for energy? What steps would you suggest to reduce energy consumption?

Answer:

Environmental Consequences:

1. Atmospheric Pollution & Acid Rain: Heavy combustion of coal and petroleum produces toxic sulfur dioxide and nitrogen oxides, forming corrosive acid rain.
2. Global Climate Change: Rising CO₂ emissions intensify the greenhouse effect, raising planetary temperatures, melting glaciers, and shifting weather patterns.
3. Habitat Destruction & Deforestation: Constructing mega-hydroelectric reservoirs and open-cast coal mines destroys rich forest ecosystems and disrupts biodiversity.
4. Nuclear Waste Hazard: Radioactive byproducts from nuclear power generation remain hazardous for thousands of years, posing long-term ecological risks if containment fails.

Actionable Steps to Reduce Energy Consumption:

• Switch off electrical appliances, lights, and fans when not in use; replace filament bulbs with energy-efficient LED fixtures.
• Promote public transportation, carpooling, and non-motorized transport (bicycles) over single-occupancy fossil fuel vehicles.
• Design energy-efficient buildings with ample natural daylighting and thermal insulation to reduce air conditioning and heating loads.
• Adopt 5-star BEE-rated energy-efficient appliances.
• Transition domestic heating systems to rooftop solar water heaters and solar panels.

Important Formulas, Principles, and Scientific Concepts

To score full marks in numerical and principle-based questions in CBSE Class 10 Science, understand these core scientific foundations:

1. Einstein's Mass-Energy Equivalence Principle

The energy released during nuclear fission or fusion originates from the conversion of a minute difference in rest mass (Δm) into pure energy:

E = Δm × c²

• E = Energy released (in Joules, J)
• Δm = Mass defect = (Total mass of reactants) − (Total mass of products) (in kg)
• c = Speed of light in vacuum ≈ 3 × 10⁸ m/s

Note: 1 atomic mass unit (1 u) converted entirely into energy produces approximately 931 MeV (Million Electron Volts) of energy, where 1 eV = 1.602 × 10^-19 J.

2. Working Principle of a Biogas Plant

The fixed-dome biogas plant operates via the anaerobic microbial decomposition of cow-dung, plant residues, and water slurry:

• Anaerobic Digestion: In the absence of dissolved oxygen, anaerobic methanogenic bacteria break down complex organic compounds over several weeks.
• Product Composition: Produces biogas comprising up to 75% Methane (CH₄), with the remaining fraction consisting of CO₂, H₂, and H₂S.
• Byproduct Utility: The spent slurry left behind is rich in nitrogen (N) and phosphorus (P), serving as excellent organic manure.

3. Ocean Thermal Energy Conversion (OTEC) Rule

For an OTEC plant to operate commercially:

T_surface − T_{deep (up to 2 km)} ≥ 20°C (or 20 K)

The warm surface water is used to boil a volatile working fluid with a low boiling point (such as liquid ammonia). The resulting high-pressure vapor drives an expansion turbine to spin a generator. Cold water pumped from ocean depths then condenses the vapor back into liquid for reuse in a closed Rankine cycle.

Common Mistakes and Exam Tips

Board Exam Relevance and Question Patterns

In the CBSE Class 10 Science board examinations, Chapter 12 carries consistent weightage across multiple question formats:

• 1-Mark Objective Questions / MCQs: Identifying non-renewable sources, components of biogas, and minimum speed for wind farms.
• 2-Mark Short Answer Questions: Distinguishing between exhaustible and inexhaustible sources, explaining why charcoal is a better fuel than wood, or stating the role of the black coating and glass sheet in solar cookers.
• 3-Mark Conceptual Questions: Explaining the working of a fixed-dome biogas plant with a neat labelled diagram, or detailing the operational mechanism and limitations of OTEC plants.
• 5-Mark Long Answer Questions: Comparative analysis of conventional vs. alternative energy sources, outlining environmental impacts and sustainability strategies.

More NCERT Solutions and Practice

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