What Is Energy ?

Energy lights our cities, powers our vehicles, and runs machinery in factories. Energy is defined as the ability or the capacity to do work.

We use energy to do work and make all movements. When we eat, our bodies transform the food into energy to do work. When we run or walk or do some work, we ‘burn’ energy in our bodies. Cars, planes, trolleys, boats, and machinery also transform energy into work. There are many sources of energy that help to run the various machines invented by man.

The discovery of fire by man led to the possibility of burning wood for cooking and heating thereby using energy. For several thousand years human energy demands were met only by renewable energy sources—sun,biomass (wood, leaves, twigs), hydel (water) and wind power.

As early as 4000–3500 BC, the first sailing ships and windmills were developed harnessing wind energy. Fuelwood and dung cakes are even today a major source of energy in rural India. Solar energy is used for drying and heating.

World population grew 3.2 times between 1850 and 1970, per capita use of industrial energy increased about twentyfold, and total world use of industrial and traditional energy forms combined increased more than twelvefold.

Due to the problems associated with the use of fossil fuels, alternative sources of energy have become important and relevant in today’s world. Also known as non-conventional sources of energy, they cause less emission and are available locally. Most of the renewable sources of energy are fairly non-polluting and considered clean. Renewable energy sources include the sun, wind, water, agricultural residue, fuelwood, and animal dung. Fossil fuels are non-renewable sources. Energy generated from the sun is known as solar energy. Hydel is the energy derived from water. Biomass – firewood, animal dung, and biodegradable waste from cities and crop residues – is a source of energy when it is burnt. Geothermal energy is derived from hot dry rocks, magma, hot water springs, natural geysers, etc. Ocean thermal is energy derived from waves and also from tidal waves. 

Through the method of co-generation a cleaner and less polluting form of energy is being generated. Fuel cells are also being used as cleaner energy source.

Work and heat are two categories of processes or mechanisms that can transfer a given amount of energy. 

The second law of thermodynamicslimits the amount of work that can be performed by energy that is obtained via a heating process—some energy is always lost as waste heat. The maximum amount that can go into work is called the available energy. Systems such as machines and living things often require available energy, not just any energy. Mechanical and other forms of energy can be transformed in the other direction into thermal energywithout such limitations.

There are many forms of energy, but all these types must meet certain conditions such as being convertible to other kinds of energy, obeying conservation of energy, and causing a proportional change in mass in objects that possess it. Common energy forms include the kinetic energy of a moving object, the radiant energy carried by light and other electromagnetic radiation, the potential energy stored by virtue of the position of an object in a force field such as a gravitational, electric or magnetic field, and the thermal energy comprising the microscopic kinetic and potential energies of the disordered motions of the particles making up matter. 

Some specific forms of potential energy include elastic energy due to the stretching or deformation of solid objects and chemical energy such as is released when a fuel burns. Any object that has mass when stationary, such as a piece of ordinary matter, is said to have rest mass, or anequivalent amount of energy whose form is called rest energy, though this isn't immediately apparent in everyday phenomena described by classical physics.

According to mass–energy equivalence, all forms of energy (not just rest energy) exhibit mass. For example, adding 25 kilowatt-hours (90 megajoules) of energy to an object in the form of heat (or any other form) increases its mass by 1 microgram; if you had a sensitive enough mass balance or scale, this mass increase could be measured.

Our Sun transforms nuclear potential energy to other forms of energy; its total mass does not decrease due to that in itself (since it still contains the same total energy even if in different forms), but its mass does decrease when the energy escapes out to its surroundings, largely as radiant energy.

Although any energy in any single form can be transformed into another form, the law of conservation of energy states that the total energy of a system can only change if energy is transferred into or out of the system. This means that it is impossible to create or destroy energy. The total energy of a system can be calculated by adding up all forms of energy in the system. Examples of energy transfer and transformation include generating or making use of electric energy, performingchemical reactions, or lifting an object. Lifting against gravity performs work on the object and stores gravitational potential energy; if it falls, gravity does work on the object which transforms the potential energy to the kinetic energy associated with its speed.

More broadly, living organisms require available energy to stay alive; humans get such energy from food along with the oxygen needed to metabolize it. Civilisation requires a supply of energy to function; energy resources such as fossil fuelsare a vital topic in economics and politics. Earth's climate and ecosystem are driven by the radiant energy Earth receives from the sun (as well as the geothermal energy contained within the earth), and are sensitive to changes in the amount received. For example, while energy is always conserved (in the sense that the total energy does not change despite energy transformations), energy can be converted into a form, e.g., thermal energy, that cannot be utilized to perform work. When one talks about "conserving energy by driving less", one talks about conserving fossil fuels and preventing useful energy from being lost as heat.

This usage of "conserve" differs from that of the law of conservation of energy

Total commercial energy consumption has been growing tremendously since the last decade. Per capita commercial energy consumption in low-income countries have more than doubled. About 15% of the world's population living in the wealthy industrialized nations consume over half the energy used in the world. In some respects, the global energy system has evolved in a cleaner direction in the last 25 years. The share of world primary energy derived from natural gas – the cleanest fossil fuel – has increased by more than 25%. So has the use and generation of renewable energy sources.

Still, the overall efficiency of energy production remains extremely low: on average, more than 90% of energy consumed is lost or wasted in the process of conversion from raw materials such as coal to the final energy service such as the light to read a book. The main problem isn’t that we use energy, but how we produce and consume energy resources. What we really need are energy sources that will last forever and can be used without polluting the environment.

How energy is measured

One of the basic measuring blocks for energy is called a Btu or British thermal unit. Energy can also be measured in joules (pronounced the same way as ‘ jewels’). One joule is the amount of energy needed to lift 1 pound about 9 inches. It takes 1000 joules to equal a Btu. Joule is named after an English physicist named James Prescott Joule who lived from 1818 to 1889. He discovered that heat is a type of energy.

Around the world, scientists measure energy in joules rather than Btu. Physicists, who are scientists who study force, motion and energy, say that energy is the ability to do work, and work is moving something against a force, like gravity. There are a lot of different kinds of energy in the universe, and that energy can do different things.

Energy can be found in many things, and takes many forms. There is a kind of energy called kinetic energy in objects that are moving. There is something that scientists call potential energy in objects at rest that will make them move if resistance is removed.

Energy causes things to happen around us. During the day, the sun gives out light and heat energy. At night, street lamps use electrical energy to light our way.

When a car drives by, it is being powered by gasoline, a type of stored energy.

The food we eat contains energy. We use that energy to work and play.

We learned the definition of energy in the introduction:

"Energy Is the Ability to Do Work."

Energy can be found in a number of different forms. It can be chemical energy, electrical energy, heat (thermal energy), light (radiant energy), mechanical energy, and nuclear energy.

Energy makes everything happen and can be divided into two types:

Stored energy is called  potential energy.

Moving energy is called  kinetic energy.

With a pencil, try this example to know the two types of energy.

The moving pencil uses kinetic energy.

You used your own energy to lift and move the pencil. Moving it higher than the floor adds energy to it. As it rests on the desk, the pencil has potential energy. That means the pencil has more potential energy.