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By Shailendra Singh
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Updated on 7 Nov 2025, 16:22 IST
Energy is fundamental to all human activities from cooking food to running industries and generating electricity. A source of energy is defined as one that can provide an adequate amount of energy in a convenient form over a long period of time.
All energy sources can be categorized into two main types:
Non-renewable sources of energy are those that have accumulated in nature over millions of years and cannot be quickly replaced when exhausted. These sources include:
Fossil fuels are natural fuels formed deep under the earth from the prehistoric remains of living organisms (plants and animals). The plants and animals that died millions of years ago were gradually buried deep in the earth and covered with sediments like mud and sand. In the absence of oxygen, the chemical effects of pressure, heat, and bacteria converted these buried remains into fossil fuels.
Important Note: The buried remains of large plants were converted into coal, whereas those of small plants and animals were converted into petroleum and natural gas.
Coal is a complex mixture of compounds of carbon, hydrogen, and oxygen, with some free carbon. Small amounts of nitrogen and sulphur compounds are also present. Coal is found in deep coal mines under the surface of the earth.
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Uses of Coal:
Advantages of Coke over Coal: Coke is 98% carbon and is a better fuel than coal because it produces more heat than an equal mass of coal and does not produce smoke while burning, thus not causing air pollution.
Petroleum is a dark-colored, viscous, and foul-smelling crude oil found under the earth's crust trapped in rocks. The name petroleum means "rock oil" (petra = rock; oleum = oil). Crude petroleum is a complex mixture of several solid, liquid, and gaseous hydrocarbons mixed with water, salt, and earth particles.
Extraction and Processing:
Petroleum occurs deep underground between two layers of impervious (non-porous) rocks, with natural gas above it. Wells are drilled into the earth where oil presence is predicted by surveys. When a well is drilled, natural gas comes out first with great pressure, followed by crude oil.
Petroleum Fractions and Their Uses:
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Through fractional distillation, petroleum yields:
Natural gas consists mainly of methane (CH₄), with small quantities of ethane and propane. In fact, natural gas contains up to 95% methane, the remaining being ethane and propane.
Formation: Natural gas is formed under the earth by the decomposition of vegetable matter lying under water. This decomposition is carried out by anaerobic bacteria in the absence of oxygen.
Uses of Natural Gas:
Advantages: Natural gas has a high calorific value of up to 50 kJ/g, burns with a smokeless flame, causes no air pollution, and produces no poisonous gases on burning. It can be supplied directly through underground pipelines from gas wells to homes and factories.
The main constituent of petroleum gas is butane, though it also contains smaller amounts of propane and ethane. LPG consists mainly of butane (along with smaller amounts of propane and ethane) which has been liquefied by applying pressure.
Features:
Advantages of LPG:
The burning of fossil fuels causes significant pollution:
The pollution can be reduced through:
1. Catalytic Converters: Fitted in vehicle exhaust systems to convert harmful gases (carbon monoxide, nitrogen oxides) into harmless gases (carbon dioxide, nitrogen).
2. Scrubbers: Tanks where smoke and waste gases are sprayed with water jets before reaching the chimney, washing away soot and dissolving acidic gases.
3. Electrostatic Precipitators: Installed in chimneys to deposit unburnt carbon and fly-ash particles on inner walls, preventing their release into the air.
| Aspect | Fossil Fuels | Renewable Sources |
| Availability | Limited, will be exhausted | Inexhaustible, continuously replenished |
| Environmental Impact | High pollution (CO₂, SO₂, NOₓ, smoke) | Minimal or no pollution |
| Formation Time | Millions of years | Continuously available |
| Cost | Extraction and processing costs | Often free (sun, wind) or low-cost |
| Climate Impact | Major contributor to greenhouse effect | Low or zero greenhouse gas emissions |
| Sustainability | Non-sustainable | Sustainable for long-term use |
Renewable sources of energy are those being produced continuously in nature and are inexhaustible. For example, wood is renewable because if trees are cut, more trees will grow in due course of time.
The renewable sources include:
These are also called:
Flowing water possesses kinetic energy. At hydro-power plants, the energy of falling water is tapped by using a water turbine to drive generators.
Working Principle:
Advantages:
Disadvantages:
Moving air is called wind. Wind possesses kinetic energy due to its high speed. Solar energy is responsible for wind—wind blows due to uneven heating of earth by the sun in different regions.
Wind Generator Setup:
When fast-moving wind strikes the blades of a wind turbine, the turbine rotates continuously. The turbine shaft is connected to a generator. When the turbine rotates, its shaft drives the generator, producing electricity.
Wind Energy Farms:
To generate large amounts of electricity, many wind turbines are erected over a big area—called a wind energy farm. The electricity from each generator is combined to obtain large-scale power.
Advantages:
Limitations:
The sun is the source of all energy, providing heat and light energy free of cost. Solar energy is the energy obtained from the sun through nuclear fusion reactions taking place inside it, liberating enormous amounts of heat and light energy.
Solar Constant: The amount of solar energy received per second by one square meter of near-earth space (perpendicular to sun's rays) at an average distance between sun and earth is called the solar constant—about 1.4 kilojoules per second per square meter (1.4 kJ/s/m² or 1.4 kW/m²).
A solar cooker consists of an insulated metal or wooden box painted black inside, with a thick glass sheet cover and a plane mirror reflector. Food is placed in black-painted metal containers inside the box.
Working: The reflector focuses sunlight onto the cooker box. Sun's heat rays pass through the glass and are absorbed by the black interior. The glass prevents heat from escaping (greenhouse effect), raising the temperature to 100-140°C in 2-3 hours, cooking the food.
Advantages:
Limitations:
A coil of copper tube painted black is placed in a box similar to a solar cooker. When water passes through the copper coil, it absorbs sun's heat rays and becomes hot. Solar water heaters are used in hotels and hospitals but cannot work at night or on cloudy days.
Solar cells convert solar energy (sunlight) directly into electricity.
Construction: A solar cell is usually made from silicon, consisting of a sandwich of a silicon-boron layer and a silicon-arsenic layer. Small amounts of boron and arsenic are present. Wires are soldered to the top and bottom layers, and the cell is covered with glass or transparent plastic for protection.
Working: When sunlight falls on the solar cell surface, it makes loosely held electrons in silicon atoms move, generating current in the connected wires. A single cell produces about 0.5 V.
Solar Cell Panels: Multiple solar cells joined in series form a solar cell panel, providing much more power. Best-designed solar cells can generate 240 W/m² in bright sunlight with maximum efficiency of about 25%. Cells are connected using silver wires (best conductor) for higher efficiency.
Advantages:
Disadvantages:
Uses:
Biomass is the dead parts of plants and trees and waste material of animals organic matter used as fuel to produce energy. Biomass includes wood, agricultural wastes (crop residues), and cow-dung. Biomass contains chemical energy in the form of carbon compounds. Biomass is another form of solar energy, as all plants and trees grow using sun's energy.
Wood: Wood has been used as fuel for a long time. It is renewable if enough trees are planted. However, wood has disadvantages: produces smoke (pollutes air) and has low calorific value (only 17 kJ/g).
Charcoal: When wood is burnt in limited air supply, water and volatile materials are removed, leaving behind charcoal mainly carbon.
Advantages of Charcoal:
Traditional Use Problems: Directly burning cow-dung cakes has disadvantages:
Biogas Solution: Converting cow-dung into biogas allows smokeless fuel use, while spent dung retains nutrients for use as manure. Only organic matter decomposes into biogas; nitrogen and phosphorus remain intact.
Biogas Composition: Biogas is a mixture of methane, carbon dioxide, hydrogen, and hydrogen sulphide. The major constituent is methane up to 75% making it an excellent fuel.
Production Process: Biogas is produced by anaerobic degradation of animal wastes like cow-dung (or plant wastes) in the presence of water. This degradation is carried out by anaerobic bacteria (which do not require oxygen). These bacteria decompose complex carbon compounds (carbohydrates, proteins, fats) to form methane gas, the main constituent of biogas.
Biogas Plant (Gobar Gas Plant):
Construction:
Working:
Uses of Biogas:
Energy from the sea can be obtained in three forms: Tidal energy, Wave energy, and Ocean thermal energy.
The rise of sea water due to gravitational pull of the moon is called "high tide"; the fall is called "low tide." Tidal waves build up and recede twice daily, providing large energy in coastal areas.
Harnessing Method: A tidal barrage (dam) is built across a narrow opening to the sea. During high tide, sea water flows into the reservoir and turns turbines. During low tide, stored water flows out into the sea, also turning turbines. Both processes generate electricity.
Limitations:
Due to wind blowing on the sea surface, very fast sea-waves move with high speed. These waves possess significant kinetic energy that can be used to generate electricity.
Methods (Experimental):
Note: These are experimental; full-size wave-energy generators require many years of development. Viable only where sea-waves are very strong.
Water at the ocean surface gets heated by the sun and attains higher temperature than colder water at deeper levels. There is always a temperature difference between surface water and deeper water.
OTEC (Ocean Thermal Energy Conversion) Power Plants: A temperature difference of 20°C or more between surface water and deeper water is needed. In one type, warm surface water boils a liquid like ammonia or CFC. High-pressure vapors turn the turbine to generate electricity. Colder deep water is pumped up to cool and condense the vapors back into liquid. This process repeats continuously.
Advantages:
'Geo' means 'earth'; 'thermal' means 'heat.' Geothermal energy is heat energy from hot rocks inside the earth, used to produce electricity. This is one of the few sources that do not come from solar energy. At some places ("hot spots"), rocks at some depth below the earth's surface are very hot due to fission of radioactive materials naturally present in these rocks.
Extraction Method: Extremely hot rocks heat underground water, turning it into steam. As steam builds up between rocks under high pressure, a hole is drilled and a pipe inserted. High-pressure steam comes up through the pipe and turns a turbine-generator to produce electricity. Sometimes two holes are drilled—cold water pumped in one, heated by hot rocks, and steam extracted from the other.
Advantages:
Disadvantages:
In India: Very limited commercial exploitation sites two locations in Madhya Pradesh and Himachal Pradesh.
Nuclear energy is released during nuclear reactions involving changes in the nucleus of an atom. It can be obtained through nuclear fission or nuclear fusion.
Source: The source of nuclear energy is the mass of the nucleus. A small amount of nucleus mass is destroyed during nuclear reactions, converting into tremendous energy.
The process in which the heavy nucleus of a radioactive atom (uranium, plutonium, thorium) splits into smaller nuclei when bombarded with low-energy neutrons is called nuclear fission. A tremendous amount of energy is produced. The sum of masses of smaller nuclei is slightly less than the original heavy nucleus mass this mass loss appears as tremendous energy.
Example Reaction: When uranium-235 atoms are bombarded with slow-moving neutrons, the uranium nucleus breaks up to produce barium-139, krypton-94, and 3 neutrons, along with tremendous energy:
²³⁵U + ¹n → ¹³⁹Ba + ⁹⁴Kr + 3¹n + Tremendous energy
The fission of 1 atom of uranium-235 produces 10 million times more energy than burning 1 atom of carbon from coal.
Nuclear Power Plant:
Construction:
Working:
Nuclear Wastes: Radioactive waste materials are extremely harmful to all living beings.
Nuclear Power Plants in India: Six locations:
Currently, only about 3% of India's total electrical power comes from nuclear plants. In industrialized countries like France, Germany, and Japan, over 30% comes from nuclear plants.
Advantages:
Disadvantages:
The process in which two nuclei of light elements (like hydrogen) combine to form a heavy nucleus (like helium) is called nuclear fusion. Tremendous energy is produced. Nuclei are positively charged and repel each other so millions of degrees of temperature and millions of pascals of pressure are required to force lighter nuclei to fuse. Nuclear fusion is carried out by heating lighter atoms to extremely high temperatures under extremely high pressure.
Example Reaction: When deuterium atoms (heavy hydrogen, mass number 2) are heated to extremely high temperature under extremely high pressure, two deuterium nuclei combine to form a helium nucleus and a neutron, with tremendous energy:
²H + ²H → ³He + ¹n + Tremendous energy
Hydrogen Bomb: Thermonuclear reactions (fusion at very high temperature) produce the hydrogen bomb a weapon of mass destruction. The H-bomb contains deuterium (²H) and tritium (³H) with lithium-6 (⁶Li). An atom bomb (fission-based) is used to detonate the H-bomb. Atom bomb explosion creates temperatures that trigger fusion reactions of deuterium and tritium, producing enormous energy and causing destruction. Lithium-6 produces more tritium when hit by neutrons. H-bombs are much more powerful than atom bombs.
Source of Sun's Energy: The sun is a huge mass of hydrogen gas at extremely high temperature. The sun is a big thermonuclear furnace where hydrogen atoms continuously fuse into helium atoms. Mass is lost during these fusion reactions, producing energy. The main fusion reaction in the sun:
2²H → ⁴He + 2⁰e + Tremendous energy
This energy is released as heat and light, making the sun shine and provide us with energy. Thus, nuclear fusion reactions of hydrogen are the source of sun's energy.
Fusion vs. Fission: An advantage of fusion over fission is that fusion releases much more energy. Also, fusion products are not radioactive—they're harmless and can be disposed of easily without environmental contamination.
Challenge: The biggest disadvantage: It has not been possible to have a controlled fusion reaction or safely use the enormous heat for electricity production.
While the PDF document focuses on educational content for CBSE Class 10 students and doesn't provide specific global statistics, we can outline the general understanding:
Fossil Fuels Dominate:
Nuclear Energy:
Renewable Energy:
Scientists are showing renewed interest in exploring alternative sources of energy for two reasons:
The document emphasizes the environmental problems associated with conventional energy sources and highlights the importance of developing cleaner alternatives.
Hydrogen is listed among renewable sources of energy.
Why Hydrogen is Promising:
Hydrogen gas has the highest calorific value of 150 kJ/g among all fuels. Because of its high calorific value, hydrogen is an extremely good fuel. Most common fuels are hydrocarbons compounds of hydrogen and carbon. Since hydrogen has the highest calorific value, a fuel containing higher percentage of hydrogen will have higher calorific value than another fuel with lower hydrogen percentage. For example, LPG has higher hydrogen percentage than coal, so LPG has higher calorific value than coal.
Advantages of Hydrogen:
Current Challenges:
The document discusses solar cookers, solar water heaters, and solar cells. Emerging improvements include:
Traditional geothermal is limited to areas with hot rocks near the surface. Emerging technology allows drilling deeper to access geothermal energy in more locations.
Wave energy devices and ocean thermal energy conversion (OTEC) plants are being developed. These are experimental but show promise for coastal regions with strong wave action or significant temperature gradients.
Beyond traditional biomass:
While fission is currently used in power plants, fusion promises much more energy with non-radioactive products. The challenge is achieving controlled fusion research continues worldwide (ITER project, etc.)
Critical for renewable energy integration:
While choosing the most appropriate fuel, consider these characteristics of an ideal/good fuel:
| Concept | Formula/Value | Explanation |
| Calorific Value | Heat produced per gram of fuel completely burned | Expressed in kJ/g (kilojoules per gram) |
| Solar Constant | 1.4 kJ/s/m² or 1.4 kW/m² | Amount of solar energy received per second by 1 m² of near-earth space perpendicular to sun's rays |
| Einstein's Mass-Energy Relation | E = mc² | Energy (E) produced from mass (m) destroyed; c = speed of light (3 × 10⁸ m/s) |
| Atomic Mass Unit | 1 u = 1.66 × 10⁻²⁷ kg = 931 MeV | Energy equivalent of 1 atomic mass unit |
| Energy Units | 1 eV = 1.602 × 10⁻¹⁹ J<br>1 MeV = 1.602 × 10⁻¹³ J | Electron volt and million electron volt for nuclear energy |
| 1 kg Mass Energy | E = 1 × (3 × 10⁸)² = 9 × 10¹⁶ Joules | If 1 kg mass completely converts to energy |
| Fuel | Calorific Value (kJ/g) |
| Dung cakes | 6-8 |
| Wood | 17 |
| Coal | 25-30 |
| Charcoal | 33 |
| Alcohol (Ethanol) | 30 |
| Diesel (Fuel oil) | 45 |
| Kerosene | 48 |
| Petrol | 50 |
| Biogas | 35-40 |
| Natural gas | 33-50 |
| LPG (Cooking gas) | 50 |
| Methane | 55 |
| Hydrogen | 150 |
| Aspect | Thermal Power Plant | Hydro Power Plant | Nuclear Power Plant |
| Energy Source | Fossil fuels (coal, oil, gas) | Flowing/falling water | Nuclear fission (uranium-235) |
| Fuel Requirement | Continuous fuel supply needed | No fuel—uses water kinetic/potential energy | Can run 2-3 years on same fuel load |
| Pollution | High—CO₂, SO₂, NOₓ, smoke, ash | Very low—no air pollution | No air pollution but radioactive waste |
| Renewable | Non-renewable fuels | Renewable—water cycle continuous | Non-renewable uranium fuel |
| Location | Near fuel source or transport routes | Hilly areas with rivers | Anywhere—not location-dependent |
| Environmental Impact | Greenhouse gases, acid rain | Dam construction affects ecosystems | Nuclear waste disposal challenges |
| Efficiency | Moderate | High | Very high energy density |
| Cost | Moderate | High initial construction cost | Very high installation cost |
Non-Renewable (Exhaustible):
Renewable (Inexhaustible):
The world is gradually shifting from fossil fuel dependence to cleaner, renewable alternatives due to:
Hydrogen, with the highest calorific value (150 kJ/g), represents a promising clean fuel for the future. Combined with advanced solar, wind, and fusion technologies, humanity can transition to a sustainable, low-carbon energy future.
Energy is the foundation of modern civilization from cooking meals to powering industries and illuminating cities. Understanding sources of energy, their characteristics, environmental impacts, and sustainable alternatives is crucial for informed decision-making about our energy future.
Point of Lesson:
Infinity Learn's Note: This comprehensive guide covers all essential concepts about Sources of Energy for CBSE Class 10. Understanding energy sources, their impacts, and sustainable alternatives is crucial not just for exams, but for becoming informed citizens who can contribute to solving global energy and environmental challenges.
For the latest updates and more educational content, visit: Infinity Learn.
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Hydrogen has the highest calorific value (150 kJ/g) among all fuels, making it extremely efficient. It burns cleanly, producing only water as a byproduct no pollution. However, challenges in production, storage, and infrastructure currently limit widespread use.
Calorific value is the amount of heat produced by burning 1 gram of fuel completely (measured in kJ/g) it tells us how much energy the fuel provides. Ignition temperature is the minimum temperature to which a fuel must be heated so it catches fire and starts burning.
Solar cookers cannot be used at night because sunshine is not available. They cannot be used on cloudy days either. Box-type solar cookers cannot be used for baking or frying because these require stronger heating, and box-type cookers only reach 100-140°C. For high-temperature cooking, spherical reflector-type solar cookers with concave mirrors are needed.
Biogas is superior because:
Nuclear fusion releases much more energy than fission, and fusion products are not radioactive they're harmless and easy to dispose of without environmental contamination. However, the biggest disadvantage is that it has not been possible to achieve a controlled fusion reaction or safely use the enormous heat produced for electricity generation. Fusion requires extremely high temperatures and pressures that are difficult to maintain and control.
The five main sources of energy that power our modern world represent a fundamental mix of both renewable and non-renewable resources. Solar energy stands as the primary energy source for Earth, providing light and heat that sustains ecosystems through the sun's radiation, which can be harnessed using photovoltaic cells and solar thermal technologies for electricity generation and heating applications.
Fossil fuels, including coal, oil, and natural gas, currently dominate global energy consumption, serving as the backbone of electricity generation and transportation systems, though they are non-renewable and contribute significantly to carbon emissions. Wind energy harnesses the kinetic energy of moving air through turbine technology, converting it into electricity with minimal environmental impact, making it one of the fastest-growing renewable sources worldwide.
Hydroelectric power generates electricity by utilizing the potential energy of flowing water, typically through dam infrastructure, and represents one of the most established forms of renewable energy with decades of reliable operation. Nuclear energy, produced through atomic fission reactions, provides a low-carbon electricity source capable of generating massive amounts of power from relatively small amounts of uranium fuel, though it requires careful safety management and waste disposal protocols. These five sources collectively account for the vast majority of global energy needs, with each offering distinct advantages in terms of availability, cost-effectiveness, environmental impact, and scalability for different applications ranging from residential heating to industrial manufacturing processes.
The selection and utilization of these energy sources depend on geographical factors, technological infrastructure, economic considerations, and environmental policies that vary significantly across regions and nations. Understanding these primary energy sources helps consumers, policymakers, and businesses make informed decisions about energy consumption patterns, investment priorities, and sustainability strategies that balance immediate energy needs with long-term environmental stewardship and climate change mitigation goals