What is Energy?

Physics defines work as the product of applied force on an object and the distance it moves. If
applied force cannot move the object, the work is zero (distance being zero).
Work = Force x Distance.
Notice that because work is defined as the multiplicative product of force and distance, knowing
just the amount of work doesn't tell you whether you pushed with a little force over a long distance,
or a lot of force over a short distance --- you can accomplish the same amount of work either way.
The energy of an object, or of a system, is how much work the object or system can do on some
other object or system.
In other words, energy measures the capability of an object or system to do work on another system
or object.
Consider a ball flying through the air for example. If the ball collides with another ball, the ball
will exert a force on the second ball for a moment, which does work on the second ball and causes
it to move. The newly acquired kinetic energy of the second ball after the collision is equal to the
amount of work exerted on it by the first ball.
Discuss the various forms of energy.
There are a number of ways in which a system or object can possess energy, i.e. the capability to
do work, and each way corresponds to having a different form of energy.
Kinetic Energy
Kinetic energy is the energy which is associated with the mobility of an object or a system.
Consider a baseball flying through the air. The ball is said to have "kinetic energy" for the fact that
it is in motion relative to the ground.
1

KE = 2 mv 2
Potential Energy
The energy associated with the elevation of an object or a system is called potential energy.
Consider a book sitting on a table. The book is said to have "potential energy" because if falls off,
it will be accelerated due to the gravity, giving the book kinetic energy.
PE = mgh

Thermal, or heat energy

The collective, microscopic, kinetic and potential energy of the molecules in an object or system
(the molecules have kinetic energy because they are moving and vibrating, and they have potential
energy due to their mutual attraction for one another. Temperature is really a measure of how much
thermal energy something has. The higher the temperature, the faster the molecules are moving
around and/or vibrating inside that object or system, i.e. the more kinetic and potential energy the
molecules have.
Chemical Energy
Chemical energy is really a form of microscopic potential energy, which exists because of the
electric and magnetic forces of attraction exerted between the different parts of each molecule.
These parts get rearranged in chemical reactions, releasing or adding to this potential energy.
Consider the ability of your body to do work. The glucose (blood sugar) in your body is said to
have "chemical energy" because the glucose releases energy when chemically reacted (combusted)
with oxygen. Your muscles use this energy to generate mechanical force and also heat.
Electrical Energy
All matter is made up of atoms, and atoms are made up of smaller particles, called protons (which
have positive charge), neutrons (which have neutral charge), and electrons (which are negatively
charged). Electrons orbit around the center, or nucleus, of atoms, just like the moon orbits the
earth. The nucleus is made up of neutrons and protons. Charged particles create electric fields that
exert force on other charged particles within the field. The electric field applies the force to the
charged particle, causing it to move - in other words, do work. Electrical energy is energy that's
stored in charged particles within an electric field
Electrochemical Energy
Consider the energy stored in a battery. Like the example above involving blood sugar, the
battery also stores energy in a chemical way. But electricity is also involved, so we say that the
battery stores energy "electro-chemically".
Electromagnetic Energy
Consider the energy transmitted to the Earth from the Sun by light (or by any source of light).
Light, which is also called "electro-magnetic radiation". Light really can be thought of as
oscillating, coupled electric and magnetic fields that travel freely through space.
It turns out that light may also be thought of as little packets of energy called photons (that is, as
particles, instead of waves). The word "photon" derives from the word "photo", which means
"light". Photons are created when electrons jump to lower energy levels in atoms, and absorbed
when electrons jump to higher levels.

Each photon also has a specific frequency and wavelength associated with it, which depends on
how much energy the photon has. The lower the energy, the longer the wavelength and lower the
frequency, and vice versa.
Sound Energy
Sound waves are compression waves associated with the potential and kinetic energy of air
molecules. When an object moves quickly, for example the head of drum, it compresses the air
nearby, giving that air potential energy. That air then expands, transforming the potential energy
into kinetic energy (moving air). The moving air then pushes on and compresses other air, and so
on down the chain.
Nuclear Energy
The Sun, nuclear reactors, and the interior of the Earth, all have "nuclear reactions" as the source
of their energy, that is, reactions that involve changes in the structure of the nuclei of atoms. In the
Sun, hydrogen nuclei fuse (combine) together to make helium nuclei, in a process called fusion,
which releases energy. In a nuclear reactor, or in the interior of the Earth, Uranium nuclei split
apart, in a process called fission. The energy released by fission and fusion is not just a product of
the potential energy released by rearranging the nuclei. In fact, in both cases, fusion or fission,
some of the matter making up the nuclei is actually converted into energy. Matter itself is a form
of energy. This concept involves one of the most famous formula's in physics, the formula,
E=mc2
The energy intrinsically stored in a piece of matter at rest equals its mass times the speed of light
squared.
What should a good source of fuel have?
i) It should have a high energy output per unit mass or volume.
ii) It should be easily available.
iii) It should be easy to store and transport.
iv) It should be economical.
What are the sources of energy?
There are two main sources of energy. They are conventional and non-conventional sources of
energy.
i) Conventional sources of energy
Wood, Flowing water and Fossil fuels (coal, petroleum, natural gas).
ii) Non conventional sources of energy
Solar energy, Wind energy, Biomass energy, Ocean energy (tidal energy, wave energy, ocean
thermal energy), Geothermal energy, Nuclear energy etc.

Some sources of energy are renewable like sun, wind, flowing water, ocean, wood, biomass etc.
Some sources of energy are non-renewable like coal, petroleum and natural gas.
Fossil fuel
Fossil fuel is formed inside the earth from the remains of plants and animals after millions of years.
The fossil fuels are coal, petroleum and natural gas. Fossil fuels are non-renewable sources of
energy so they should be conserved and used judiciously.
Disadvantages
i) Burning of fossil fuels release gases and harmful particles which causes air pollution.
ii) Burning of fossil fuels release acidic oxides of sulphur and nitrogen which causes acid rain
which is harmful for living organisms, affects soil and water, causes damage to buildings,
corrosion of metals etc.
iii) Burning of fossil fuels release a large amount of carbon-di-oxide gas which increases the
temperature of the atmosphere and causes global warming (Green-house effect).
In thermal power plants the heat energy produced by burning fossil fuels like coal, petroleum or
natural gas is used to heat water and change it into steam which rotates the turbines of generators
to produce electricity.

Hydro power plants

In hydro power plants water from rivers are stored by constructing dams. The water from the dam
flows down through pipes and rotates the turbines of generators to produce electricity.

Advantage
i) Flowing water is a renewable source of energy.
ii) The electricity produced does not cause pollution.
iii) The water stored in dams can also be used to control floods and for irrigation.
Disadvantage
i) The initial cost is high.
ii) Large areas of land gets submerged and the decomposition of vegetation produces methane
gas which is a green house gas.
iii) It causes displacement of people from large areas of land.
Biogas
The waste materials and dead parts of living things are called biomass. E.g. wood, animal dung,
vegetable waste, agricultural waste, sewage etc. Biomass is decomposed by anaerobic
microorganisms to produce biogas. Biogas is a mixture of gases containing methane, carbon
dioxide, hydrogen and hydrogen sulphide.

Biogas plant
The biogas plant has a large underground tank made of bricks and cement. The lower part is the
digester and the upper part has a dome with a gas outlet. On one side of the tank above the ground

is a mixing tank and on the other side is an overflow tank. Animal dung is mixed with water in the
mixing tank and the slurry is sent into the digester. In the digester the slurry is decomposed by
anaerobic microorganisms and after a few days biogas is produced. The gas is taken out through
the gas outlet and used for heating and lighting purposes. The slurry left behind is rich in nitrogen
and phosphorus and is used as manure for crops.
Wind Energy
Wind energy is used in wind mills which converts the kinetic energy of the wind into mechanical
or electrical energy. The kinetic energy of wind can be used to do mechanical work like lifting
water from wells or grinding grains in flour mills. It can also be used to rotate the turbines of
generators to produce electricity.
A single wind mill produces only a small amount of electricity. So a large number of wind mills
in a large area are coupled together to produce more electricity in wind energy farms.
Advantages
i) It is a renewable source of energy.
ii) It does not cause pollution.
iii) The recurring cost is less.
Disadvantages
i) Wind is not available at all times.
ii) It requires a large area of land.
iii) A minimum wind speed of 15 km/h is required.
Solar energy
Solar energy is the energy obtained from the sun in the form of heat and light. The heat energy is
used in solar heating devices like solar cooker, solar water heater, solar furnaces etc. The light
energy is used in solar cells.
i) Solar cooker
The box type solar cooker has an insulated box painted black inside. It is covered by a glass
plate which allows heat to enter inside but does not allow heat to escape out. It has a mirror to
reflect more sunlight into the box. The food to be cooked is kept in containers inside the box It
can produce a temperature of 100 to 140.
ii) Solar water heater
A solar water heater has an insulated box painted black inside with a system of copper tubes. It
is covered with a glass plate which allows heat to enter inside but does not allow heat to escape
out. When water flows through the copper tube it absorbs heat and becomes hot.
Solar cell is a device which converts solar energy into electrical energy. Solar cells are made from
semiconductors like silicon, germanium, gallium etc. A single solar cell produces a voltage of

about 0.5 to 1 V and produces about 0.7W electricity. So several solar cells are arranged in a solar
panel to produce more electricity.
Advantages
i) It uses renewable source of energy.
ii) It produces electricity which does not cause pollution.
iii) It can be used in remote areas where there is no power supply.
Disadvantages
i) It uses a special grade of silicon which is expensive.
ii) Since silver is used for connecting the cells together it is more expensive.
iii) The current produced is DC and to convert it to AC increases the cost.
Energy from the sea is obtained in three different forms. They are Tidal energy, Sea wave energy
and Ocean thermal energy.
i) Tidal energy
The periodic rise and fall of sea level due to gravitational attraction of the moon causes tides. A
dam is constructed at a narrow opening between the land and sea. The movement of water during
high tide and low tide can be used to rotate the turbines of generators to produce electricity.

ii) Sea wave energy

When strong wind blows over the sea it produces huge waves. The kinetic energy of the moving
waves can be used to rotate the turbines of generators to produce electricity.
iii) Ocean thermal energy
There is a temperature difference between the warm surface water and the cold water at the bottom
of the oceans. This difference is about 20C. The warm surface water is used to boil liquid
ammonia and the vapour is used to rotate the turbines of generators to produce electricity. The
cold water from the bottom is then pumped up to cool the vapour back to liquid.
Nuclear Energy
Nuclear energy is the energy is the energy released during nuclear reactions. During nuclear
reactions some mass is converted into energy and so a very large amount of energy is produced

during nuclear reactions. Nuclear reactions are of two types. They are Nuclear fission and nuclear
fusion.
i) Nuclear fission: It is a nuclear reaction in which the nucleus of a heavy atom like uranium,
plutonium, etc. splits into smaller nuclei with the release of a large amount of energy. It is used to
make atom bombs and to produce electricity.
In a nuclear power plant the heat energy produced by a controlled nuclear fission chain reaction
is used to produce steam which rotates the turbines of generators to produce electricity.
ii) Nuclear fusion: It is a nuclear reaction in which small nuclei fuse together to form a heavier
nucleus with the release of a very large amount of energy. The energy of sun is produced by the
fusion of hydrogen nuclei to form helium nucleus. It is also used to make the hydrogen bomb.
Advantages of nuclear energy
i) It produces a very large amount of energy per unit mass than any other source of energy.
ii) If safety measures are taken, it is more environment friendly than fossil fuels.
Disadvantages of nuclear energy
i) The cost of a nuclear reactor is very high.
ii) The availability of nuclear fuel is limited.
iii) Nuclear reactors produce harmful nuclear wastes which is difficult to dispose.
How is energy converted?
Energy can be converted from one form into another in three basic ways.
1. Through the action of forces.
i.
Gravitational Forces - when gravity accelerates a falling object, its converts its
potential energy to kinetic energy. Likewise, when an object is lifted the
gravitational field stores the energy exerted by the lifter as potential energy.
ii.
Electric and Magnetic Force Fields - Charged particles, upon which electrical
fields exert forces, possess potential energy in the presence of an electric field in a
way similar to that of an object in a gravitational field. These force fields can
accelerate particles, converting a particle's potential energy into kinetic energy.
Likewise, charged particles can interact via the electric and magnetic fields they
create, transferring energy between them, and in the case of an electrical current in
a conductor, cause molecules to vibrate, i.e. converting electrical potential energy
into heat.
iii. Frictional Forces - The macroscopic (large-scale) energy of an object, that is, the
potential and kinetic energy associated with the position, orientation, or motion of
the entire object, not counting the thermal or heat energy of the system, can be
converted into thermal energy (heat), whenever the object slides against another
object. The sliding causes the molecules on the surfaces of contact to interact via
electromagnetic fields with one another and start vibrating.

2. When atoms absorb or emit photons of light. When light falls on an object, an incident
photon may either pass through the object, be reflected by the object, or be absorbed by
the atoms making up the object. If most of the photons pass through, the object is said to
be transparent. Depending on the smoothness of the surface on the scale of the photon's
wavelength, the reflection may be either diffuse (rough surface) or coherent (smooth
surface).
If the photon is absorbed, the photon's energy may also be split up and converted in the
following ways:
i.

Photothermal effect: the energy absorbed may simply produce thermal energy, or
heat in the object. In this case the photon's energy is converted into vibrations of
the molecules called phonons, which is actually heat energy.
ii.
Photoelectric effect: the energy absorbed may be converted into the kinetic energy
of conduction electrons, and hence electrical energy.
iii. Photochemical effect: the energy may bring about chemical changes which
effectively store the energy.
3. When nuclear reactions occur, that is, when there are rearrangements of the subatomic
particles that make up the nuclei of atoms. There are two basic types: Fission - when
nuclei combine, and Fusion - when nuclei split apart.