Classification of Sources of Energy- Physics Guide for Class 8

Information about Sources of Energy


Sources of Energy


Class 8


Class 8 Physics

Topics Covered

  • Classification of Sources of Energy
  • Fossil Fuels
  • Wood as Fuel
  • Coal
  • Petroleum
  • Natural Gas
  • Cleaner Fuel

Role of Friction

Energy is the 'capacity of a body to do work'. We have learnt about different forms of energy: mechanical, heat, chemical, electrical, light, sound, nuclear and so on. These are interconvertible; energy, however, cannot be created or destroyed.
  • We need energy for all our activities, from baking chappatis or cookies, to sending astronauts into space.
  • All forms of energy, that we use everyday, are stored in different ways in various sources of energy.
  • The substances which produce heat on burning in air are called fuels

Classification of Sources of Energy

We classify the sources of energy on the basis of:
  1. Occurrence
  2. Physical State
  3. Availability


  1. Natural Sources: Natural energy sources are those which are made available to us by nature. Solar energy, wind energy, energy from water (hydro energy) are some of such natural sources of energy.
  2. Synthetic Sources: Synthetic energy sources are those that use man-made materials as sources of energy. For example, chemical energy, stored in the batteries, (used in calculators, watches, etc.) is a synthetic source of energy.

Physical State

  1. Solid: Firewood, charcoal, coal are examples of solid fuels.
  2. Liquid: Kerosene, petrol and diesel are all liquid fuels.
  3. Gas: Petroleum gas, commonly used as LPG (Liquified Petroleum Gas), and natural gas, also used as CNG (Compressed Natural Gas), are examples of gaseous fuels. 


  1. Renewable: A renewable source of energy is a natural resource that can replenish itself naturally over a short period of time. Wind, sun, biomass (from plants) and hydropower (from water) are all renewable sources of energy. These are inexhaustible natural resources.
  2. Non-renewable: Energy sources, which get used up and cannot be replaced, or replenished, in a short period of time are called non-renewable sources of energy. These are also called exhaustible natural resources. Fossil fuels, (like petroleum, natural gas and coal), are non-renewable sources of energy.

Fossil Fuels

As at present, most of our energy needs are met through non-renewable energy sources; mainly fossil fuels (petroleum, natural gas and coal).
Fossil fuels are so called because they were formed, over a period of millions of years, by the action of heat (from the earth's core) and pressure (from rocks and soil) on the remains of dead plants and animals. 

Wood as a Fuel 

  • Wood is a major renewable natural resource.
  • The combustion of wood is currently the largest source of energy.
  • Wood can be used for cooking and heating; it may occasionally be used in steam engines and steam turbines (that generate electricity).
  • Combustion of wood produces heat which can be put to a variety of uses.
  • However, it also produces other gases, like CO2 (carbon dioxide) and CO (carbon monoxide), which are undesirable, irritating or dangerous. 


Coal is a readily combustible black rock or a brownish black sedimentary rock. Coal is one of the fuels that has been used for cooking food. However, it produces a lot of smoke and foul-smelling gases which cause air pollution. Worldwide, coal (a fossil fuel) is the largest source of energy used for the generation of electricity.

Occurrence of Coal

  • Coal is found deep under the surface of the earth.
  • The major coal mines in India are located in Jharkhand, Madhya Pradesh, Odisha and West Bengal.
  • The largest coal mines are at Bokaro and Jharia (in Jharkhand) and Raniganj (in West Bengal).

Formation of Coal

  • Coal was formed from plant remains got buried deep under the earth's crust.
  • Over a large period of time, the chemical and physical properties of these plant remains were changed through geological actions that led to the creation of this solid material.
  • The process, of conversion of dead plant materials into coal, is called carbonisation

Types of Coal

Depending on the temperature and pressure conditions, and the time period for which fossils remained buried under the surface of earth, different varieties of coal have been formed.
Different types of coal are
  1. peat
  2. lignite
  3. bituminous
  4. anthracite

These different types of coal differ from each other in terms of their content of volatile material and the percentage of carbon, moisture and other elements present in them.
  • Peat is a soft brown substance that is made up of 30 per cent carbon. It is the earliest stage during the formation of coal.
  • Lignite has a carbon content of about 38 per cent; it is regarded as the lowest grade of coal. It has been used mainly for electric power generation.
  • Bituminous coal has about 65 per cent carbon; it is used for power generation and for making another type of fuel, called coke.
  • Anthracite is hard coal and contains over 90 per cent carbon. It is regarded as the highest grade of coal. 

Destructive Distillation and its Products

Strong heating of coal, in absence of air, is called destructive distillation. We obtain many products, like coke, coal tar and coal gas, through the destructive distillation of coal. 

Coke: It is a hard and porous substance. Its main uses are listed below:
  • It is used as a domestic as well as an industrial fuel in stoves and furnaces. It gives little (or no) smoke.
  • It is used for extraction of metals. 
  • It can be used to make fuel gases (like water gas, CO + H2).

Coal Tar: It is a black coloured viscous liquid. It is used in the manufacture of:
  • synthetic dyes
  • drugs
  • explosives
  • perfumes
  • paints
  • photographic materials
  • roofing materials.

Coal Gas: It is an inflammable gas. It is used as:
  • a domestic fuel
  • an industrial fuel (usually only for those industries which are located near coal processing units). 


Petroleum is a fossil fuel. It is a dark coloured, viscous and foul-smelling liquid, commonly called crude oil.
It is found in rock formations in the earth.
Its name is derived from the Latin word petra (meaning 'rock') and oleum (meaning 'oil'); literally, it means rock oil.

Occurrence of Petroleum

  • Petroleum is found at moderate depths (500 m - 2,000 m) between two layers of impervious rocks. 
  • Petroleum deposits are usually found mixed with salt water.
  • Petroleum, being lighter than salt water, floats over it.
  • Natural gas is found above petroleum; it is trapped between the rock cap and the petroleum layer.

Refining of Petroleum

  • Petroleum is a mixture of various materials which are separated from one another and are used for different purposes. 
  • The process, of separating different components of petroleum into their various useful forms, is called refining of petroleum. 

Petroleum Products and their uses

Due to its great commercial importance, petroleum has often been called liquid gold. We give below the list of its main products, and the main use(s) of each of them:
  1. Residual Oil: It is further separated into following products:
    (i) Asphalt: It is a black and sticky solid. It is used for making roads and coating the underside of electric poles (to prevent their rusting).
    (ii) Paraffin wax: It is a white semi-solid, which is often used as grease. It is also used for making vaseline, ointments and candles.
    (iii) Lubricating oil: It is a viscous oil used for lubricating machines. 
  2. Fuel Oil: It is used as a fuel in boilers.
  3. Diesel: It is used to run generators and heavy vehicles (such as buses, tractors, etc.). 
  4. Kerosene: It has been used as a domestic fuel as well as in lanterns.
  5. Gasoline, or Petrol: It is mainly used as a fuel in cars, scooters and motor cycles. It is also used for 'dry cleaning' of clothes. Highly refined petrol (aviation fuel) is used as a fuel in aeroplanes.
  6. Petroleum Gas: It is liquified under pressure and is then known as Liquified Petroleum Gas (LPG). LPG is used as a domestic fuel.

Natural Gas

Natural gas is another fossil fuel that is found along with petroleum in oil wells. Natural gas, when compressed and stored under pressure, is called Compressed Natural Gas (CNG)
In India, gas fields have been discovered in Godavari-Krishna basin, Mumbai High and Tripura.

Uses of Natural Gas

  • It burns readily and has a high calorific value.
  • It is used as a fuel in homes and industries.
  • It is also used as a better, and greener, automobile fuel and as 'base ingredient' in the manufacture of fertilisers and chemicals.

Cleaner Fuels 

Economic development, and rapidly growing population, are putting a strain on the environment, infrastructure and different natural resources. 
Pollution, soil erosion, deforestation, rapid industrialisation, urbanisation and land degradation are all worsening the problem.
  • Burning of fossil fuels, such as coal and petroleum products, is a major source of pollution. 
  • The present day strategy, to address these environmental issues, is to develop technology that uses renewable natural resources (such as biomass, water, wind and solar energy).
  • India is blessed with an abundance of sunlight, water and biomass.
  • Fuels, like LPG, CNG and biomass are cleaner and better fuels.
  • When burnt, biomass and LPG release carbon dioxide but in relatively smaller amounts.
  • When biomass crops are grown, a significant amount of carbon dioxide is consumed through photosynthesis.
  • Natural gas is a cleaner fuel as it provides relatively low amounts of pollutants. It has fewer emissions than coal and petroleum products; also, it leaves (virtually) no ash particles after burning.
  • For our future and present day energy needs, we need to look for alternative (cleaner) sources, such as solar energy, tidal energy, geothermal energy, hydel energy and wind energy.
  • This will help to protect the environment and reduce the risk of global warming.
  • Further fossil fuels are then more likely to be available for future generations. 
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