Physics Notes on Sources of Energy

By Rahul Chadha|Updated : December 14th, 2018

Energy refers to a kind of force or power that enables us to do work. Nature has provided us with a variety of natural sources of energy. The Sun, wind, water, coal, petroleum, etc., are all natural sources of energy. Some of these are renewable like solar energy and wind energy, while some others like coal and petroleum are non-renewable sources of energy that can get depleted over a period of time.

Energy is needed for carrying out all kinds of activities right from cooking a meal to running an industry. Heat energy, obtained by burning fuels like wood, coal, kerosene or cooking gas is used for cooking our food. The energy obtained from fuels like petrol and diesel is used to run cars buses, trucks and trains. Coal and natural gas are used to provide energy in factories. Diesel is used to providing energy to run pump-sets for irrigation in agriculture. Electrical energy is used for lighting bulbs and tubes and to run radio, television, household appliances, electric trains, and factory machines. Electricity is generated by transforming other forms of energy like the chemical energy of coal or the potential energy of water stored in a high dam built across a river. In fact, all our daily activities use energy in one form or the other. If there is no energy, all our activities will come to a halt. For example, if there is no energy, we won’t be able to cook food or run machines in factories or pump-sets in agriculture. So energy is essential for our survival in this world. We will now discuss the various sources, which can provide us with energy.

What is Good Source of Energy?

In our daily lives, we use energy from different sources for doing work. We use

  • Diesel to run our trains
  • Electricity to light our street lamps
  • Muscular energy for cycling

Our choice of a fuel for performing some work depends on many factors. Some of the factors are

  • How much heat does it liberate on burning?
  • Does it create any environment problem by emitting a lot of smoke?
  • Is it easily available?

Based on the discussion above, we can easily say what the requirements for the good source of energy are. A good source of energy would be one

  • which would do a large amount of work per unit volume or mass
  • be easily accessible
  • be easy to store and transport
  • be more economical

Conventional Sources of Energy

The given visual shows the share of the different types of production of electricity:


Fossil Fuels

The remains of plants and animals buried under the earth millions of years ago are called fossils. These fossils are excellent fuels called fossil fuels. The examples of fossils fuels are coal, petroleum, and natural gas. They were formed by the decomposition of the remains of plants and animals which got buried under the earth millions of years ago. Air particles are deadly. The byproducts that form from the burning of fossil fuels are very dangerous. These small particles can exist in the air for indefinite periods of time, up to several weeks and can travel for miles. The particles, sometimes smaller than 10 microns in diameter, can reach deep within the lungs. Particles that are smaller than this can enter the bloodstream, irritating the lungs and carrying with them toxic substances such as heavy metals and pollutants. Over a lifetime of continued exposure, a person's ability to transfer oxygen and rid pollutants is impeded. Those affected could become afflicted with fatal asthma attacks and other serious lung conditions. In our power plants, turbines are used to generate electricity. The simplest turbines have one moving part, a rotor based assembly. The moving fluid acts on the blades to spin them and impart energy to the rotor. The rotor blade, with speed would turn the shaft of the dynamo and convert the mechanical energy into electrical energy.

Thermal Power Plant

The term thermal power plant is used since fuel is burnt to produce heat energy, which is converted into electrical energy. A large number of fossil fuels can be burnt every day in power stations to heat up water to produce steam, which is used to rotate the turbine to generate electricity. All the power stations are located near the coal or oil fields. The transmission of electricity is more efficient than transporting coal or petroleum over the distance.

Hydroelectric Power Plant

How do we get electricity from water? Actually, hydroelectric power plants produce electricity. A hydroelectric plant uses falling water to turn a propeller-like piece called a turbine, which then turns a metal shaft in an electric generator, which is the motor that produces electricity.

The theory is to build a dam on a large river that has a large drop in elevation. The dam stores lots of water behind it in the reservoir. Near the bottom of the dam wall there is the water intake. Gravity causes it to fall through the penstock inside the dam. At the end of the penstock, there is a turbine propeller, which is turned by the moving water. The shaft from the turbine goes up into the generator, which produces the power. Power lines connected to the generator, carry electricity to every home. The water continues past the propeller through the tailrace into the river past the dam.


But, constructions of big dams have certain problems associated with it. The dams can be constructed only in a limited number of places. Large areas of agricultural land and human habitation are to be sacrificed as they get submerged. The vegetarian, which is, submerged rots under the anaerobic conditions and emits methane gas which is also a greenhouse gas.

Improvements in the Technology for Using Conventional Sources of Energy


Anaerobic degradation is a fermentation process, which takes place in the absence of oxygen. In this process, animal and plant wastes are easily degraded by anaerobic bacteria (microorganisms) in the presence of water. These bacteria decompose (more appropriately, degrade) the carbon compounds present in the wastes and, in this process, gases such as methane, carbon dioxide, hydrogen, and hydrogen sulphide are liberated. The mixture of gases so liberated is called biogas which is an excellent fuel as it contains about 65 % methane which is combustible. Biogas can be burnt in gas stoves at homes. The residue left is rich in nitrogen and phosphorous and is used as manure.

Animal Dung as Fuel

In many villages in India, dried animal dung (in the form of cakes) is still being used as fuel for domestic purposes. However, it is unwise to burn animal dung directly to obtain heat because of the following three reasons:

  1. Animal dung contains compounds of nitrogen and phosphorus which are vital nutrients for plant growth. If the dung is burnt as such, these nutrients are destroyed and hence cannot be returned to the soil.
  2. As the dung cakes burn, a lot of smoke is produced which causes air pollution.
  3. Since dung cakes burn inefficiently, their use leads to a considerable wastage of fuel.

Therefore instead of burning animal dung directly, the dung should first be processed, so that the vital nutrients are retrieved before it is burnt. In fact, it is possible to convert animal dung into a clean fuel called biogas and the residue in which the vital nutrients remain intact can be used as manure in agriculture. The process by which this is achieved is called anaerobic degradation.

Two types of biogas plants are being used in our country. They are -

1. The fixed-dome type
2. The floating gas-holder type

The main raw material used in these plants is animal dung (cow-dung, the dung of horse, elephant, goat, etc). Plant wastes like vegetable skins, fruit pulp, and human excreta may be added to the animal dung.

Biogas Plants

  1. Fixed-Dome Type Biogas Plant

The plant consists of a dome-shaped underground tank D called digester which is made with bricks and cement. A gas outlet G is provided near the ceiling of the dome. On one side of the digester is inlet chamber I, used for introducing the slurry (which is a mixture of animal dung and water). An outlet chamber O is provided on the other side of the digester. The outlet chamber is connected to the overflow tank F.


Slurry is prepared in the mixing tank M by mixing animal dung and water. The slurry is introduced into the digester through the inlet chamber I. It is allowed to ferment for 50 to 60 days in the digester. During this period, the anaerobic bacteria present in the dung carries out the degradation, as a result of which biogas is evolved. This biogas begins to collect in the dome. As the gas keeps collecting in the dome, its pressure keeps on increasing. Due to increased pressure, the used slurry in the digester is forced out through the outlet chamber O into the overflow tank F. This slurry is taken out of the plant. It is rich in nitrogen and phosphorous compounds and is excellent manure. The gas outlet G is connected by a pipe to a stove or a burner. When the gas pressure falls, fresh slurry is added to the digester to produce more gas. Thus a continuous supply of gas is maintained.

2. Floating Gas-Holder Type Biogas Plant

This plant consists of an underground tank D called digester made of bricks and cement. A drum-shaped gas holder G floats over the slurry in the digester. The drum is made of steel and it floats in an inverted position so that gas can collect in it. A gas outlet is provided at the top of the drum.


The digester has a partition wall, which divides it into two chambers I and O. The inlet chamber I is connected to a mixing tank M and the outlet chamber O is connected to an overflow tank F. Slurry is prepared in the mixing tank M by mixing animal dung and water. The slurry is introduced in the digester through the inlet chamber I. For about 50 - 60 days, the slurry undergoes anaerobic degradation and biogas is evolved. The gas collects in the drum G. As more gas collects in the drum, its pressure increases, as a result of which the used slurry is forced into the overflow tank F through the outlet chamber O. This slurry is taken out of the plant and is used as manure. The gas outlet at the top of the drum is connected by a pipe to a gas stove.

Advantages of Biogas as a fuel

  • Biogas burns without smoke & hence doesn't pollute the air
  • Biogas has a high calorific value.
  • The raw material (cow-dung) required for the production of biogas is inexpensive and easily available in villages.
  • The residue leftover in a biogas plant is excellent manure.

However, there is one limitation. The biogas plant has a high initial cost, which is beyond the reach of an average rural Indian. To overcome this problem, the Khadi and Village Industries Commission and other agencies actively promise the construction of big biogas plants called community plants, designed to supply biogas and manure to a number of families in a community. The government gives loans and subsidies to farmers for this purpose. More than 70% of the energy requirement in rural India can be met by these plants. Biogas plants are indeed a big boon to the farmers.

Wind Energy

Moving air is called wind. Wind has energy. It can pick up things and send them flying; it can lift heaps of sand and pile them into sand dunes; it can blow against huge trees and send them crashing. This shows that the wind has a lot of energy to smash and destroy. But man is intelligent enough to put this wind energy to constructive use. The energy possessed by the wind is due to its high speed, that is, the wind possesses kinetic energy and it is this kinetic energy of wind, which is utilized for doing work. In the earlier days, the energy of wind was utilized by man in winnowing to remove the husk from grains; in propelling sailboats in rivers and seas for transport purpose and in windmills to pump out water from the ground and grind grain to obtain flour. Engineless airplanes called gliders depend totally on wind energy for their flight. These days, however, the energy of wind is being used to produce electricity.


A windmill is a machine, which works with the energy of wind. Two types of windmills have been used in the past. One, to run the water-pumps for drawing water from the ground and the other for grinding grains to produce flour. We will discuss both types of windmills one by one.


The windmill used for running a water-pump is shown in the above figure. This windmill consists of big blades B to catch the wind. These windmill blades can rotate over the top of a pole P. The centre of the windmill blades is connected to one end of a device called crank C. (The crank is a long rod having a U-shaped bend, which can convert the rotary motion into up-and-down motion and vice-versa.) The U-shaped bend of the crank is connected to the piston rod R of the water-pump as shown in the figure. Let us see how this windmill works to draw water from the ground.

The kinetic energy of wind rotates the windmill blades continuously. When the blades rotate, the crank rotates. This causes its U-shaped bend to lift the pump rod up and down. The
rod works the pump that lifts the water from the ground. Thus, the wind blowing above the ground can be used to pump from a great depth. There is also another type of windmill, which uses the energy of wind to do another hard job that is of grinding grains like wheat or corn to make flour. The windmill, which works like a flour-mill is shown in the below figure. This windmill consists of the windmill blades B located on the top of a pole P. The blades are connected to a big wheel C having teeth by means of rod R. There is another big wheel D having teeth, which is arranged in such a way that it pushes against the teeth of wheel C. The wheel D has a long rod E in the downward direction. A stone-wheel F called mill-stone is fixed to the lower end of rod E. The mill-stone F is placed on another similar mill-stone G, which is fixed to the ground. Thus, the lower mill-stone is fixed and stands still whereas the upper mill-stone is turned by the long rod E.


Working of the Windmill to Grind Grain

The blowing wind rotates the blades of the windmill continuously. When the blades rotate, the rod R turns and rotates the wheel C attached to it. When the wheel C turns, it makes the wheel D turn. The wheel D, in turn, rotates the rod E. Since the upper mill-stone is fixed to rod E, the upper mill-stone F starts revolving over the fixed mill-stone G. Grains like wheat or corn are placed in the hole in the centre of the upper mill-stone through a hopper H. The wheat is crushed between the two mill-stones and is converted into flour, which falls through the edges of the mill-stones.

Energy From the Sea

Tidal Energy

Tidal power is classified as a renewable energy source because tides are caused by the orbital mechanics of the solar system. The root source of the energy is the orbital kinetic energy of the earth-moon system, and also the earth-sun system. Tidal power has great potential for future power and electricity generation because of the essentially inexhaustible amount of energy contained in these rotational systems. However, there has been a controversy over whether this is a perpetual motion machine because the moon does not lose energy just because energy is extracted from tides. In fact, even if all kinetic energy in the oceans were depleted until not a single molecule is moving (which is impossible), the moon will continue to orbit the earth. This is not only because the moon is extremely large and has a vast potential energy, but also that it can orbit a large mass (in this case the Earth) forever, as long as there is no friction or change in the mass that it is orbiting. Tidal power is reliably predictable (unlike wind energy and solar power). Tidal energy is harnessed by constructing a dam across a narrow opening to the sea. A turbine is fixed at the opening of the dam to convert tidal energy into electricity. The efficiency of tidal power generation in ocean dams largely depends on the height of the rise and fall of the tidal swell, which can be up to 10 m.

Wave Energy

The kinetic energy possessed by huge waves near the sea-shore can be trapped to produce electricity. These waves are generated by strong winds blowing across the sea. This can be more economical where the waves are very strong.

Ocean Thermal Energy

It is the conversion of energy arising from the temperature difference between warm surface water of oceans and cold deep ocean current into electrical energy or other useful forms of energy. It is abbreviated as OTEC. OTEC utilizes the temperature difference that exists between deep and shallow waters — within 20° of the equator in the tropics — to run a heat engine. Because the oceans are continually heated by the sun and cover nearly 70% of the Earth's surface, this temperature difference contains a vast amount of solar energy, which could potentially be tapped for human use. If this extraction could be done profitably on a large scale, it could be a solution to some of the human population's energy problems. The total energy available is one or two orders of magnitude higher than other ocean energy options such as wave power, but the small size of the temperature difference makes energy extraction difficult and expensive. Hence, existing OTEC systems have an overall efficiency of only 1 to 3%.

Geothermal Energy

Geothermal Energy has been around for as long as the Earth has existed. "Geo" means earth, and "thermal" means heat. So, geothermal means earth-heat. Deep under the surface, water
sometimes makes its way close to the hot rock and turns into boiling hot water or into steam. The hot water can reach temperatures of more than 300 degrees Fahrenheit (148 degrees
Celsius). This is hotter than boiling water (212 degrees F / 100 degrees C). It doesn't turn into steam because it is not in contact with the air. When this hot water comes up through a crack in the earth, we call it a hot spring. The steam trapped in rocks is routed through a pipe to a turbine and used to generate electricity.

Nuclear Energy

Another major form of energy is nuclear energy, the energy that is trapped inside each atom. One of the laws of the universe is that matter and energy can't be created nor destroyed. But they can be changed in form. Matter can be changed into energy. An atom's nucleus can be split apart when this is done a tremendous amount of energy is released. The energy is both heat and light energy. Einstein said that a very small amount of matter contains a very Large amount of energy. This energy, when let out slowly, can be harnessed to generate electricity. When it is let out all at once, it can make a tremendous explosion as in an atomic bomb.

A nuclear power plant uses uranium as a "fuel." The word fission means to split apart. Inside the reactor of an atomic power plant, uranium atoms are split apart in a controlled chain reaction. In a chain reaction, particles released by the splitting of the atom go off and strike other uranium atoms splitting those. Those particles given off split still other atoms in a chain reaction. In nuclear power plants, control rods are used to keep the splitting regulated so that it doesn't go too fast. If the reaction is not controlled, you could have an atomic bomb. The reaction also creates radioactive material. This material could hurt people if released, so it is kept in a solid form.



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