Sources of Energy

Why this chapter matters for UPSC: This was Chapter 14 of NCERT's Class X Science in the 2020-21 edition (pp. 242-255). It is not in the current book: Reprint 2026-27 has 13 chapters and no chapter on sources of energy. Its ideas still sit under a large share of GS3 and GS1 questions: what makes a fuel good, how a thermal, hydro, wind, solar or nuclear plant turns energy into electricity, why every source disturbs the environment in some way, and how to tell a renewable source from an exhaustible one. Learn the chapter's principles first, then attach the dated Indian figures given below; most of the numbers printed in the chapter have since moved.

Contemporary hook: On 31 August 2026 non-fossil sources made up 54.88% of India's installed electricity capacity of 5,54,544 MW, and solar alone 30.30%, according to the Central Electricity Authority (CEA). The chapter's own box had put all of India's nuclear stations at less than 3% of capacity; the CEA share for nuclear on that date is 1.58%. In April 2026 the 500 MWe Prototype Fast Breeder Reactor at Kalpakkam attained first criticality. The physics in this chapter explains each of these facts; the boxes below date them.

🧠 First Principles — Read This First

1. Energy is never lost, but it becomes less usable. Hot water left in a room cools from 348 K (75 °C) to 298 K (25 °C); the energy has spread into the surroundings in a form we can hardly use again. That is why we keep needing sources of energy even though energy is conserved (p. 242).
2. A good source does much work for little mass or volume. NCERT's test: a large amount of work per unit volume or mass, easy access, easy to store and transport, and, most importantly, economical (p. 243).
3. Most sources are the Sun's energy, stored or flowing. Wood, coal, wind, ocean thermal energy and the rain that refills a reservoir all trace back to the Sun. Geothermal energy (heat from inside the earth), nuclear energy (energy locked in nuclei) and tides (mainly the Moon's pull) do not (pp. 246-252, 254).
4. Most power plants spin a turbine. Steam from burning fuel or from a reactor, falling water, or wind turns a turbine, and the turbine turns a generator. The solar cell is the exception: it turns sunlight straight into electricity, with no moving parts (pp. 244-251).
5. No source is perfectly clean. Every source disturbs the environment in some way. NCERT's precise wording is that one source is cleaner than another, not that it is clean (p. 253).
6. Renewable means regenerated as fast as it is used. Fossil fuels took millions of years to form and will run out: they are exhaustible. Bio-mass is renewable only if we replace the trees we cut; sunlight, wind and flowing water arrive afresh (pp. 244, 253-254).

PART 1 — Quick Reference

Table 1: What makes a good fuel and a good source of energy (14.1)

Source: NCERT, Science Class X, 2020-21 edition, ch. 14, sections 14.1, 14.2.1 and 14.4, Activities 14.1-14.2 (pp. 242-244, 253).

Table 2: Conventional sources of energy (14.2)

Source: NCERT, Science Class X, 2020-21 edition, ch. 14, sections 14.2.1-14.2.4 (pp. 244-248).

Table 3: Inside a biogas plant (14.2.4, Fig. 14.4)

Source: NCERT, Science Class X, 2020-21 edition, ch. 14, section 14.2.4 (pp. 246-247).

Table 4: Alternative or non-conventional sources of energy (14.3)

Source: NCERT, Science Class X, 2020-21 edition, ch. 14, sections 14.3.1-14.3.4, Activity 14.5 and Do You Know boxes (pp. 249-252).

Table 5: Numbers printed in the chapter, and how to use them now

Source: NCERT, Science Class X, 2020-21 edition, ch. 14 (pp. 242-254); CEA, "Installed Capacity (in MW) of the country as on 31.08.2026"; NASA Goddard, "Solar Irradiance Science".

Table 6: Where the energy ultimately comes from (Activity 14.7)

Source: NCERT, Science Class X, 2020-21 edition, ch. 14, sections 14.2.3-14.3.4, Activity 14.7, summary and Exercise 3 (pp. 246-255); NCERT, Science Class X, Reprint 2026-27, ch. 4, box "Formation of coal and petroleum" (p. 70).

Table 7: Renewable and exhaustible (14.5)

Source: NCERT, Science Class X, 2020-21 edition, ch. 14, sections 14.4-14.5, Activity 14.9 and Exercises (pp. 253-255).

Table 8: Exercises and the 2020-21 key

Source: NCERT, Science Class X, 2020-21 edition, ch. 14, Exercises (pp. 254-255) and Answers (p. 282).

Table 9: NCERT lines to read with care

Source: NCERT, Science Class X, 2020-21 edition, ch. 14 (pp. 245-255) and Answers (p. 282); NCERT, Science Class X, Reprint 2026-27, ch. 4 (p. 70); NCERT, India: Physical Environment, Class XI, Reprint 2026-27, ch. 3 (p. 21); CEA installed capacity, 31.08.2026; NPCIL plant list; THDC, Tehri Hydro Power Project; NASA Goddard, "Solar Irradiance Science".

PART 2 — Concepts & Narrative

What is a good source of energy? (14.1)

The chapter opens with a puzzle. If energy can be neither created nor destroyed, why do we worry about running out of it? Activity 14.1 answers it with a beaker of hot water. Water at 348 K (75 °C) left in a room at 298 K (25 °C) cools down; its energy has not vanished, but it has spread into the surroundings, where we cannot easily use it again. In NCERT's words, energy "in the usable form, is dissipated to the surroundings in less usable forms". So what we really need is a steady supply of energy in a usable form.

Activity 14.2 then asks which fuel is best for a given job, through three questions: how much heat does it release on burning, does it produce a lot of smoke, and is it easily available? From these the chapter builds its definition of a good source of energy: one that would do a large amount of work per unit volume or mass, be easily accessible, be easy to store and transport, and, "perhaps most importantly, be economical".

Fossil fuels and thermal power plants (14.2.1-14.2.2)

The conventional sources are the ones people have long used. In ancient times, wood was the most common source of heat energy, and the energy of flowing water and wind was used for limited activities. Then came coal. NCERT puts the change plainly: "The exploitation of coal as a source of energy made the industrial revolution possible." The growing demand for energy that followed was largely met by the fossil fuels, coal and petroleum, and our technologies were developed for using them.

Fossil fuels have two problems. The first is supply. They were formed over millions of years, and there are only limited reserves, so they are non-renewable sources of energy. The second is pollution. Burning coal or petroleum releases oxides of carbon, nitrogen and sulphur, and NCERT notes that these are acidic oxides, which cause acid rain that affects water and soil. Carbon dioxide is also a greenhouse gas. The chapter suggests that the pollution can be reduced to some extent by increasing the efficiency of the combustion process and by using techniques that cut the escape of harmful gases and ash.

Activity 14.3 builds a working model of a power plant. A table-tennis ball with three slits holds three semicircular fins; it spins on a wire axle in a jet of water or steam from a pressure cooker, and turns a cycle dynamo that lights a bulb. The point is that the simplest turbine has only one moving part, a rotor-blade assembly, and that the moving fluid spins it.

A thermal power plant does this on a large scale. Large quantities of fossil fuel are burnt every day to heat water into steam, and the steam runs the turbines that generate electricity. NCERT adds a reason for siting: "The transmission of electricity is more efficient than transporting coal or petroleum over the same distance. Therefore, many thermal power plants are set up near coal or oil fields." The name thermal power plant comes from the fact that fuel is burnt to produce heat, which is then converted into electrical energy.

Hydro power plants (14.2.3)

The kinetic energy of flowing water, and the potential energy of water at a height, were traditional sources of energy. Hydro power plants convert the potential energy of falling water into electricity. Since there are very few waterfalls that could be used for this, hydro plants are built with dams. A high dam blocks a river and collects water in a large reservoir; as the level rises, the kinetic energy of the flowing water is stored as potential energy. Water from the high level is carried through pipes to a turbine at the bottom of the dam.

NCERT calls hydro power renewable: the reservoir is refilled each time it rains, so we need not worry about using it up the way we will use up fossil fuels. It then lists the costs of large dams:

• dams can be built only in a limited number of places, preferably in hilly terrain;
• large areas of agricultural land and human habitation are submerged;
• large ecosystems are destroyed when submerged;
• the submerged vegetation "rots under anaerobic conditions and gives rise to large amounts of methane which is also a green-house gas";
• the displaced people need satisfactory rehabilitation.

NCERT names the opposition to the Tehri Dam and to the Sardar Sarovar project on the river Narmada as examples. (On the river under Tehri, see Table 9.)

Improvements in technology: bio-mass and biogas (14.2.4)

Wood and cow-dung cakes are the traditional fuels of rural India. "Since these fuels are plant and animal products, the source of these fuels is said to be bio-mass." But they burn poorly: they give much smoke and little heat. Better technology improves them. When wood is burnt in a limited supply of oxygen, the water and volatile matter in it are driven off and charcoal is left behind. "Charcoal burns without flames, is comparatively smokeless and has a higher heat generation efficiency."

Cow-dung, crop residue, vegetable waste and sewage decompose without oxygen to give biogas. Because cow-dung is the usual starting material, it is popularly known as gobar-gas. In the plant, a slurry of dung and water goes from a mixing tank into the digester, which is "a sealed chamber in which there is no oxygen". Anaerobic micro-organisms break down the slurry over a few days, giving methane, carbon dioxide, hydrogen and hydrogen sulphide. The gas collects in a tank above the digester and is drawn off by pipes.

"Bio-gas is an excellent fuel as it contains up to 75% methane." It burns without smoke, leaves no residue like ash, and has a high heating capacity. The slurry left behind is removed periodically and makes excellent manure, rich in nitrogen and phosphorus. NCERT sums up the gain: a biogas plant disposes of bio-waste and sewage safely while supplying both energy and manure.

Wind energy (14.2.4)

NCERT reminds the reader of Class IX: unequal heating of the landmass and water bodies by solar radiation sets the air moving and causes winds to blow. That kinetic energy was used in the past by windmills for mechanical work, such as lifting water from a well. Today it also generates electricity. A windmill is a structure like a large electric fan, mounted high on a rigid support; its rotation turns the turbine of a generator. One windmill gives little power, so many are set up over a large area as a wind energy farm, and their output is combined to supply electricity on a commercial scale.

Wind energy is environment-friendly, an efficient source of renewable energy, and needs no recurring expenses for the production of electricity. Its limits explain the GS1 question about why wind farms cluster in a few states:

• farms can be set up only where wind blows for the greater part of a year, and the wind speed should be higher than 15 km/h to keep the turbine at the required speed;
• a back-up facility (like storage cells) is needed for periods when there is no wind;
• a 1 MW generator needs about 2 hectares of land;
• the initial cost is high;
• the tower and blades face rain, Sun, storms and cyclones, so they need a high level of maintenance.

The chapter's Do You Know box on wind (Denmark, Germany, India's rank and potential, Kanyakumari) is undated; see Table 9 before using any of it.

Alternative or non-conventional sources (14.3)

As living standards rise, so does the demand for energy. Activity 14.4 asks students to compare how their grandparents went to school, fetched water and found entertainment with how they do these things now. The more energy we use, the more we need to look both for more efficient ways to use known sources and for new ones. Each new source needs devices designed for it.

Solar energy (14.3.1)

The Sun has been radiating energy at its present rate for nearly 5 billion years and will continue for about 5 billion years more. Only a small part of that energy reaches the outer layer of the earth's atmosphere, and nearly half of it is absorbed while passing through the atmosphere. The rest reaches the surface. NCERT's Do You Know box adds that India receives the energy equivalent to more than 5,000 trillion kWh a year, with a daily average of 4 to 7 kWh/m² under clear (cloudless) skies. It also gives the solar constant, the solar energy reaching unit area at the outer edge of the earth's atmosphere at right angles to the Sun's rays, at the average distance between the Sun and the earth, as about 1.4 kW/m² (1.4 kJ per second per square metre).

Activity 14.5 compares two identical flasks filled with water, one painted black and one white, left in the Sun: the black one warms more, because "A black surface absorbs more heat" than a white or reflecting one. Solar cookers and water heaters use this. Some cookers reach a higher temperature by using mirrors to focus the Sun's rays, and all are covered with a glass plate, which traps heat in the way the greenhouse effect does. Their limit is plain: they work only at certain times of the day.

Solar cells overcome this by converting sunlight into electricity. A typical cell develops 0.5 to 1 V and can produce about 0.7 W in sunlight; a large number joined in a solar cell panel deliver usable power. NCERT lists their advantages: they have "no moving parts, require little maintenance and work quite satisfactorily without the use of any focussing device", and they can be set up in remote hamlets where a transmission line would not be commercially viable. The drawbacks are cost and efficiency: silicon is abundant, but the special grade needed for cells is limited; manufacture is expensive; and the silver used for interconnection of the cells adds to the cost. Even so, satellites and space probes such as Mars orbiters, radio and TV relay stations in remote places, traffic signals, calculators and toys run on solar cells. Panels are mounted on inclined roofs to catch more sunlight.

Energy from the sea (14.3.2)

Three kinds of energy come from the sea.

• Tidal energy. Because of "the gravitational pull of mainly the moon on the spinning earth", the level of water in the sea rises and falls; these are the high and low tides. A dam built across a narrow opening to the sea, with a turbine fixed at the opening, converts tidal energy to electricity. Suitable sites are limited.
• Wave energy. Waves raised by strong winds blowing across the sea carry kinetic energy that devices near the shore can trap to turn turbines. It is viable only where waves are very strong.
• Ocean thermal energy. Surface water is warmed by the Sun while deeper water stays cold. An OTEC plant can work if the surface water is 20 K (20 °C) or more warmer than water at depths up to 2 km. Warm water boils a volatile liquid like ammonia, the vapour runs a turbine, and cold water pumped up from the depths condenses it again.

NCERT's verdict covers all three: the energy potential from the sea is quite large, but efficient commercial exploitation is difficult.

Geothermal energy (14.3.3)

Geological changes push molten rock from the deeper hot regions of the earth's crust upwards, where it is trapped in regions called hot spots. Underground water that touches a hot spot turns to steam; where hot water finds an outlet at the surface, we see a hot spring. The steam trapped in the rocks can be piped to a turbine to generate electricity. Production costs are low, but there are very few commercially viable sites. NCERT names New Zealand and the United States of America as countries with geothermal power plants.

Nuclear energy (14.3.4)

In nuclear fission, a heavy nucleus such as uranium, plutonium or thorium, bombarded with low-energy neutrons, splits into lighter nuclei. If the mass of the original nucleus is a little more than the sum of the masses of the products, a tremendous amount of energy is released. NCERT's comparison: the fission of an atom of uranium gives 10 million times the energy produced by the combustion of an atom of carbon from coal. In a reactor, the fuel is part of a self-sustaining fission chain reaction that releases energy at a controlled rate, which raises steam to generate electricity.

The Do You Know box gives the physics: the mass difference Δm becomes energy by E = Δm c², first derived by Albert Einstein in 1905, where c is the speed of light in vacuum. Nuclear energies are measured in electron volts, with 1 eV = 1.602 × 10⁻¹⁹ J, and 1 atomic mass unit (u) is equivalent to about 931 MeV.

The hazards are serious. The major one is the storage and disposal of spent fuel, which keeps decaying and giving off harmful radiation; improper storage contaminates the environment. There is also a risk of accidental leakage of radiation. The high cost of a plant, the risk of contamination and the limited availability of uranium make large-scale use of nuclear energy prohibitive, in NCERT's view. The chapter also notes that nuclear energy was first used for destructive purposes: the physics of a weapon is similar to that of a controlled reactor, but the two are engineered quite differently.

Nuclear fusion. "Currently all commercial nuclear reactors are based on nuclear fission." Fusion joins light nuclei into a heavier one, most commonly hydrogen or its isotopes into helium, as in ²H + ²H → ³He + n. The product's mass is a little less than the sum of the original masses, so a large amount of energy is released, by the same Einstein equation. Fusion is the source of energy in the Sun and other stars. Forcing nuclei to fuse needs extreme conditions, millions of degrees of temperature and millions of pascals of pressure. The hydrogen bomb uses this: a fission bomb at its core, embedded in a substance containing deuterium and lithium, raises the temperature to 10⁷ K in a few microseconds, and the light nuclei fuse. NCERT calls fusion a safer process than fission for power, without going into detail.

Environmental consequences (14.4)

"Exploiting any source of energy disturbs the environment in some way or the other." The chapter therefore treats the choice of source as a trade-off between four things: the ease of extracting energy, the economics of extracting it, the efficiency of the available technology, and the environmental damage. It refuses to call any fuel simply clean: even of fuels like CNG, "it would be more exact to say that a particular source is cleaner than the other." Even the solar cell, whose operation may be pollution-free, caused some environmental damage when it was assembled. Research continues to make devices that last longer and do less damage over their whole life.

Two in-text questions test the idea. Can any source be pollution-free? Not over its whole life, by the chapter's own argument. Is hydrogen, used as a rocket fuel, cleaner than CNG? Burning hydrogen gives only water, whereas CNG gives carbon dioxide; but how the hydrogen was made decides how clean it really is, which is exactly the point the chapter makes about solar cells.

How long will an energy source last us? (14.5)

Sources that will be used up some day are exhaustible or non-renewable. If we manage bio-mass by replacing the trees we cut down for firewood, we can count on a constant supply of energy at a particular rate: such sources, which can be regenerated, are renewable. A note on p. 254 widens the definition: renewable energy is available in our natural environment as continuing or repetitive currents of energy, or is stored in reservoirs so large that extraction depletes them at a practically negligible rate.

Activity 14.9 sets two debates. Coal reserves are said to be enough for another two hundred years: need we worry about coal running out? The Sun will last another five billion years: need we worry about solar energy running out? From the debate, students sort sources as exhaustible, inexhaustible, renewable and non-renewable. Exercise 7 then asks whether the two pairs are the same. They are not quite: renewable versus non-renewable is about whether a source regenerates; exhaustible versus inexhaustible is about whether the stock can run out at all. A forest regenerates, yet it can be exhausted if it is cut faster than it grows.

The chapter's summary closes on five points: our energy needs grow with our standard of living; we meet them by using energy more efficiently and by tapping new sources; the conventional sources are in danger of running out; the choice of source depends on ease and cost of extraction, the efficiency of the technology and the environmental impact; and "Many of the sources ultimately derive their energy from the Sun."

What happened to this chapter?

This chapter was printed as Chapter 14 (pp. 242-255) in the 2020-21 edition of NCERT's Class X Science. It is absent from the current book: Reprint 2026-27 lists 13 chapters, and its imprint records a "Revised Edition October 2022". It was dropped in NCERT's 2022 rationalisation of textbooks; the front matter explains the rationalisation in general terms and gives no reason chapter by chapter.

A little of its content survives elsewhere in the current book. Chapter 4, "Carbon and its Compounds", keeps a short box on the formation of coal and petroleum: "Coal and petroleum have been formed from biomass which has been subjected to various biological and geological processes." The rest of this page is the 2020-21 text.

Source: NCERT, Science Class X, 2020-21 edition (whole-book zip, Wayback Machine capture of 9 October 2021); NCERT, Science Class X, Reprint 2026-27, front matter (Contents and imprint) and ch. 4 (p. 70).

PART 3 — UPSC Integration

Frames for Mains Answers

1. Should India keep extending its nuclear programme? Start from the chapter's physics: fission gives about 10 million times the energy of burning carbon atom for atom, with no smoke. Then its fears, in NCERT's own list: spent fuel storage and disposal, accidental leakage, high cost and limited uranium. Bring in the dated record: 24 NPCIL units with 8,780 MWe (1.58% of capacity on 31 August 2026), the PFBR's first criticality on 6 April 2026, BSMR-200 awaiting financial sanction, and the target of 100 GW by 2047. Conclude by weighing these gains against the chapter's own list of hazards and costs.

2. Will India meet 50 percent of its energy needs from renewables by 2030? Separate the two targets first. The NDC's capacity target (about 50% non-fossil capacity) was met in 2025, five years early, and stood at 54.88% in August 2026; the NDC for 2031-35 raises it to 60% of capacity by 2035. The COP26 statement speaks of energy requirements from renewables, which is a harder test, because capacity is not generation: solar runs only in daylight and wind only when it blows, exactly the limits the chapter lists. Storage, transmission and grid flexibility decide the gap.

3. Large dams: renewable but not free of cost. Hydro is renewable, says NCERT, but dams submerge farmland and homes, destroy ecosystems, release methane from rotting vegetation and displace people (Tehri, Sardar Sarovar). Use the chapter's four tests (ease, cost, efficiency, environmental damage) to weigh a large storage dam against smaller schemes and other renewables.

4. Clean cooking as an energy question. WHO's 2.1 billion people cooking on smoky fuels and 2.9 million deaths a year (2021) turn the chapter's charcoal and biogas paragraphs into a public-health argument. Biogas burns without smoke, leaves no ash, and returns the slurry as manure: energy, sanitation and soil fertility from one plant.

Exam Strategy

Prelims fact-traps:

• Biogas: up to 75% methane (NCERT); made by anaerobic micro-organisms in a sealed digester; slurry rich in nitrogen and phosphorus.
• Wind: speed above 15 km/h; about 2 ha per MW; needs storage back-up.
• Solar cell: 0.5-1 V, about 0.7 W; no moving parts; silicon cells, silver interconnections.
• OTEC: at least 20 K difference, depths up to 2 km, ammonia as working fluid.
• Tides: mainly the Moon's pull, not the Sun's heat. Waves: from winds, so ultimately the Sun.
• Not from the Sun: geothermal, nuclear (and tides, mainly). Coal, wind, hydro and bio-mass all trace back to the Sun.
• Fission splits heavy nuclei (uranium, plutonium, thorium) with low-energy neutrons; fusion joins light ones (²H + ²H → ³He + n), as in the Sun.
• 1 eV = 1.602 × 10⁻¹⁹ J; 1 u ≈ 931 MeV; E = Δm c² (Einstein, 1905).
• Solar constant: NCERT about 1.4 kW/m²; NASA about 1,361 W/m².
• Tehri is on the Bhagirathi; Sardar Sarovar on the Narmada.
• BSMR-200 is a light-water pressurised water reactor of about 220 MWe, not a PHWR.
• CEA capacity, 31 August 2026: non-fossil 54.88%, solar 30.30%, coal 40.47%, nuclear 1.58%.

Mains: Use the chapter as the principle line of an energy answer: the four tests of a good source, the cleaner-not-clean principle, and each source's NCERT limits. Then add one dated Indian figure and one scheme. Never quote the chapter's undated boxes (hydro a quarter, wind rank fifth, nuclear under 3%) as current.

Practice Questions

Questions 1-3 are the NCERT exercise MCQs (2020-21 edition). Practice (UPSC-pattern, not past papers): questions 4-12.

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) (NCERT Answers, 2020-21 edition, p. 282).

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) (NCERT Answers, 2020-21 edition, p. 282). Coal formed from biomass long ago; nuclear energy has no biological origin.

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) (NCERT Answers, 2020-21 edition, p. 282). Geothermal energy (a), which the chapter traces to molten rock in the earth's crust, is also not derived from the Sun; see Table 9.

4. Consider the following sources of energy:
1. Wind energy
2. Ocean thermal energy
3. Geothermal energy
4. Nuclear fission
Which of the above do not ultimately derive their energy from the Sun?
(a) 1 and 2 only
(b) 3 and 4 only
(c) 2, 3 and 4 only
(d) 1, 2, 3 and 4

Answer: (b). Wind comes from unequal solar heating, and OTEC uses sun-warmed surface water; geothermal and nuclear energy come from the earth's interior and the nucleus (NCERT 2020-21, pp. 247, 251).

5. With reference to biogas plants, consider the following statements:
1. The digester is a sealed chamber in which there is no oxygen.
2. The gas is produced by aerobic bacteria that need a steady supply of air.
3. The slurry left behind is a manure rich in nitrogen and phosphorus.
Which of the statements given above are correct?
(a) 1 and 3 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3

Answer: (a). Decomposition in the digester is anaerobic (NCERT 2020-21, p. 247).

6. With reference to the Bharat Small Modular Reactor (BSMR-200), consider the following statements:
1. It is a pressurised water reactor of about 220 MWe.
2. Like most of India's operating reactors, it uses heavy water as moderator.
3. Its detailed project report has been approved, and financial sanction was awaited as of February 2026.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 1 and 3 only
(c) 2 and 3 only
(d) 1, 2 and 3

Answer: (b). BARC's design has no heavy water moderator system (BARC, DAE Conclave, January 2026; PIB, 4 February 2026).

7. Consider the following statements about India's climate commitments:
1. The Updated NDC (2022) aims at about 50 percent cumulative electric power installed capacity from non-fossil sources by 2030.
2. On 31 August 2026, non-fossil sources made up more than half of India's installed capacity.
3. The Updated NDC aims at meeting 50 percent of India's energy requirements from renewable energy by 2030.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Answer: (a). Statement 3 uses the wording of the COP26 announcement (1 November 2021), not of the NDC (PIB; CEA, 54.88%). The NDC for 2031-35, approved on 25 March 2026, raises the capacity target to 60% by 2035.

8. With reference to the solar constant, consider the following statements:
1. NCERT's Class X book of 2020-21 gave it as about 1.4 kW/m².
2. NASA gives total solar irradiance as about 1,361 W/m².
3. The value is fixed and does not change with solar activity.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 only
(d) 1, 2 and 3

Answer: (a). NASA notes that the value varies with solar activity.

9. Tidal energy arises mainly from
(a) the unequal heating of land and sea by the Sun.
(b) the gravitational pull of the Moon on the spinning earth.
(c) strong winds blowing across the sea.
(d) heat from molten rock under the sea floor.

Answer: (b) (NCERT 2020-21, p. 250). Option (c) describes wave energy.

10. With reference to the National Green Hydrogen Mission, consider the following statements:
1. Its initial outlay is Rs 19,744 crore.
2. Most of the initial outlay is for pilot projects.
3. Its likely outcomes by 2030 include a green hydrogen production capacity of at least 5 MMT per annum.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Answer: (c). Rs 17,490 crore of the outlay is for the SIGHT programme, and Rs 1,466 crore for pilot projects (PIB, 4 January 2023).

11. Consider the following pairs (NPCIL plant list):
1. Tarapur units 1 and 2 : Boiling water reactor
2. Kudankulam units 1 and 2 : Pressurised heavy water reactor
3. RAPS-7 : 700 MWe pressurised heavy water reactor
How many of the pairs given above are correctly matched?
(a) Only one
(b) Only two
(c) All three
(d) None

Answer: (b). Kudankulam 1 and 2 are 1,000 MWe pressurised water reactors, not PHWRs (NPCIL plant list).

12. With reference to the Prototype Fast Breeder Reactor (PFBR), consider the following statements:
1. It is a 500 MWe reactor at Kalpakkam, Tamil Nadu.
2. It attained first criticality on 6 April 2026.
3. First criticality means the reactor began supplying power to the grid.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Answer: (a). DAE describes first criticality as the start of a controlled fission chain reaction (DAE, 7 April 2026).

📦 Revision Capsule

Sources

• NCERT, Science, Textbook for Class X, 2020-21 edition, ch. 14 "Sources of Energy" (pp. 242-255) and Answers (p. 282), whole-book zip as archived on 9 October 2021 — Wayback Machine.
• NCERT, Science, Textbook for Class X, Reprint 2026-27, front matter (Contents and imprint) — ncert.nic.in PDF.
• NCERT, Science, Textbook for Class X, Reprint 2026-27, ch. 4 "Carbon and its Compounds" (p. 70) — ncert.nic.in PDF.
• NCERT, India: Physical Environment, Textbook for Class XI, ch. 3 "Drainage System", Reprint 2026-27 (p. 21) — ncert.nic.in PDF.
• Central Electricity Authority, "Installed Capacity (in MW) of the country as on 31.08.2026" — cea.nic.in PDF.
• PIB (Prime Minister's Office), National Statement at COP26, Glasgow, 1 November 2021, PRID 1768712 — pib.gov.in.
• PIB (Ministry of Environment, Forest and Climate Change), Cabinet approves India's updated Nationally Determined Contribution, 3 August 2022, PRID 1847813 — pib.gov.in.
• PIB (Cabinet), Cabinet approves India's Nationally Determined Contribution (2031-2035), 25 March 2026, PRID 2245209 — pib.gov.in.
• UNFCCC, NDC Registry, entry "India NDC (2031 - 2035)" dated 24 April 2026 (accessed 4 October 2026) — unfccc.int.
• PIB (MNRE), 50% non-fossil installed capacity achieved, 14 July 2025, PRID 2144627 — pib.gov.in.
• NPCIL, "NPCIL Nuclear Power Plants Details and Installed Capacity" (accessed 7 October 2026) — npcil.nic.in.
• Department of Atomic Energy, "Prototype Fast Breeder Reactor at Kalpakkam, Tamil Nadu attains First Criticality", 7 April 2026 — dae.gov.in.
• PIB (DAE), Lok Sabha reply on small modular reactors, 4 February 2026, PRID 2223310 — pib.gov.in.
• Neha Sharma and Dharmanshu Mittal (BARC), "Bharat Small Modular Reactor (BSMR)", 2nd DAE Conclave, 14-18 January 2026 — scitalks.tifr.res.in PDF.
• NPCIL, Request for Proposal, 220 MWe PHWR Bharat Small Reactors (BSR-220), December 2024 — npcil.nic.in PDF.
• NASA Goddard Earth Sciences Division, "Solar Irradiance Science" (accessed October 2026) — earth.gsfc.nasa.gov.
• THDC India Limited, "Tehri Hydro Power Project (1000 MW)" — thdc.co.in.
• MNRE, "Offshore Wind" — mnre.gov.in.
• PIB (Cabinet), Viability Gap Funding for offshore wind, 19 June 2024, PRID 2026699 — pib.gov.in.
• MNRE, "Pradhan Mantri Kisan Urja Suraksha evam Utthaan Mahabhiyaan (PM KUSUM)" — mnre.gov.in.
• PIB (MNRE), PM-KUSUM amendments, 13 November 2020, PRID 1672580 — pib.gov.in.
• PIB (MNRE), Cabinet approves PM-Surya Ghar: Muft Bijli Yojana, 29 February 2024, PRID 2010133 — pib.gov.in.
• PIB (MNRE), Cabinet approves National Green Hydrogen Mission, 4 January 2023, PRID 1888547 — pib.gov.in.
• PIB (Ministry of Petroleum and Natural Gas), ethanol blending, 10 July 2026, PRID 2283118 — pib.gov.in.
• PIB (MNRE), International Solar Alliance to become a treaty-based organisation, 5 December 2017, PRID 1512459 — pib.gov.in.
• United Nations Treaty Collection, Framework Agreement on the establishment of the International Solar Alliance, as amended on 3 October 2018, registration I-56484 — treaties.un.org PDF.
• International Solar Alliance, home page (accessed October 2026) — isa.int.
• World Health Organization, "Household air pollution" fact sheet, 16 December 2025 — who.int.
• Ministry of Coal, "Coal & Lignite Resource" — coal.gov.in.