NCERT Exemplar · Class 10 Science Sources of Energy

Nuclear fission — a heavy nucleus (\(\displaystyle ^{235}_{92}\mathrm{U}\)) splits on absorbing a slow neutron, releasing more neutrons that split further nuclei: a chain reaction. \[{}^{235}_{92}\mathrm{U} + {}^{1}_{0}\mathrm{n} \rightarrow {}^{141}_{56}\mathrm{Ba} + {}^{92}_{36}\mathrm{Kr} + 3\,{}^{1}_{0}\mathrm{n} + \text{energy} \] The energy comes from the small mass lost: \[E = \Delta m\,c^2 \] In a reactor the chain is controlled: a moderator slows the neutrons, control rods (boron/cadmium) absorb the excess, and the coolant carries heat away to raise steam for a turbine-generator. Caution: spent fuel stays radioactive for a very long time, so it must be stored shielded and disposed of safely. Answer: Controlled nuclear fission chain reaction in a nuclear reactor.

Two main devices: a solar cell (photovoltaic) converts sunlight directly to electricity; a solar cooker/heater collects and traps heat using blackened absorbing surfaces and reflectors. \(\displaystyle \text{Sunlight} \xrightarrow{\text{Si photovoltaic cell}} \text{Electricity (DC)}\) Limitations: (i) the energy is dilute and unavailable at night or on cloudy days; (ii) a single cell/collector gives only a small, intermittent output. Overcome: many cells are wired into panels/arrays and a large reflecting or concentrating area is used to collect more energy; a storage battery holds charge for use after sunset. Caution: a box-type cooker's glass cover must stay sealed, else the trapped heat escapes by convection. Answer: Solar cells and solar cookers/collectors, in arrays/with concentrators and storage to offset their dilute, intermittent supply.

Conventional: fossil fuels (coal, petroleum, natural gas), thermal power, hydro power, biomass (biogas), wind. Non-conventional: solar, sea (tidal, wave, ocean thermal), geothermal, nuclear. Geothermal energy — molten rock trapped near the surface forms a hot spot; underground water reaching it turns to steam, which is piped to a turbine. \(\displaystyle \text{Heat of hot spot} \xrightarrow{\text{underground water}} \text{Steam} \xrightarrow{\text{turbine}} \text{Mechanical energy} \xrightarrow{\text{generator}} \text{Electrical energy}\) Caution: only a few sites have a hot spot close enough to the surface, so a plant can be set up only there. Answer: Conventional: fossil fuels, thermal, hydro, biomass, wind; non-conventional: solar, sea, geothermal, nuclear. Geothermal steam from a hot spot drives a turbine-generator.

Need: energy demand keeps rising, while fossil fuels are limited and their burning pollutes the air and adds to global warming. Tidal — a dam across a narrow opening to the sea; water flows in at high tide and out at low tide through turbines. \(\displaystyle \text{Tidal water flow} \xrightarrow{\text{dam turbine}} \text{Electricity}\) Wave — devices trap the kinetic energy of waves to turn a turbine. \(\displaystyle \text{Wave KE} \xrightarrow{\text{turbine}} \text{Electricity}\) Ocean thermal (OTEC) — warm surface water boils ammonia; its vapour drives the turbine and cold deep water condenses it. \(\displaystyle \text{Warm surface water} \xrightarrow{\text{boils ammonia}} \text{Vapour} \xrightarrow{\text{turbine}} \text{Electricity}\ \ (\Delta T \ge 20\,^{\circ}\mathrm{C})\) Caution: a tidal dam needs a narrow bay with a large tidal range, so few coastal sites qualify. Answer: Demand is rising and fossil fuels are finite and polluting; sea energy comes from tidal dams, wave devices and OTEC.

Burning fossil fuels releases gases that pollute the air and add to global warming: \[\mathrm{C} + \mathrm{O_2} \rightarrow \mathrm{CO_2}\ (\text{greenhouse gas}) \] \[\mathrm{S} + \mathrm{O_2} \rightarrow \mathrm{SO_2}, \qquad \mathrm{N_2} + \mathrm{O_2} \rightarrow 2\,\mathrm{NO} \] \(\displaystyle \mathrm{SO_2}, \mathrm{NO_x} + \mathrm{H_2O} \Rightarrow\) acid rain, harming soil, water bodies and monuments; unburnt particles cause respiratory disease. Steps: use energy judiciously and conserve it, with efficient smokeless appliances; burn fuel completely; fit control devices (scrubbers, electrostatic precipitators, catalytic converters) to chimneys and vehicles. Non-conventional sources: site dams to spare forests and settlements; dispose of nuclear waste and spent solar-cell chemicals safely. Answer: Greenhouse gases, acid rain and particulate pollution; minimised by conserving energy, complete combustion, control devices and careful siting and waste disposal for non-conventional sources.

Largely agree — most usable energy traces back to the sun: \(\displaystyle \text{Sun} \xrightarrow{\text{photosynthesis}} \text{Plants} \xrightarrow{\text{burial, heat, pressure}} \text{Coal/petroleum}\) \(\displaystyle \text{Sun} \xrightarrow{\text{uneven heating of air}} \text{Wind}\) \(\displaystyle \text{Sun} \xrightarrow{\text{evaporation}} \text{Rain} \xrightarrow{} \text{River flow (hydro energy)}\) \(\displaystyle \text{Sun} \xrightarrow{\text{photosynthesis}} \text{Biomass growth} \xrightarrow{} \text{Biogas/wood}\) Exceptions: nuclear energy comes from fission of uranium nuclei, geothermal energy from the Earth's own hot interior, and tidal energy from the gravitational pull of the moon and sun on ocean water — none of these three is solar in origin. Answer: Agree, except for nuclear, geothermal and tidal energy.

Biomass is plant and animal waste, such as cattle dung and crop residue, that stores chemical energy and serves as fuel. Principle: anaerobic digestion — bacteria break down wet organic matter without air, releasing methane-rich gas. \(\displaystyle \text{Dung + water (slurry)} \xrightarrow[\text{15--30 days, no O}_2]{\text{anaerobic bacteria}} \mathrm{CH_4} + \mathrm{CO_2} + \text{spent slurry (manure)}\) Working: slurry from the mixing tank enters the sealed digester through the inlet; gas collects in the dome above the slurry and leaves by the outlet pipe and valve; spent slurry is pushed into the outlet tank as manure. Caution: keep air out of the digester, or the anaerobic bacteria stop producing gas. Answer: Biomass is organic waste; a biogas plant digests it anaerobically in a sealed digester, and methane-rich biogas collects in the dome.