Just as some things in nature remain unchanged for a long time, similarly too many things are constantly changing. During each change, each object exchanges some energy with its surrounding material. The various changes we see in various substances are due to this exchange of energy. Since people understood how energy changes occur from one form to another, modern civilization has become possible.

In everyday life we use energy in a variety of ways - we use energy for walking and cycling, driving on roads and boats through the water, cooking food in the oven, lighting our homes and offices, making products, and sending astronauts into space, etc. In all of these activities, energy is being continuously exchanging from one object to another.

# What is energy?

In physics, energy is defined as the ability to do work. Where the work is said to be done on an object when a force makes changes in the position of the objects. And the force is something that can change the shape and position of the objects.

According to classical physics, there are two entities of nature - matter and energy. They are both immortal. That is, matter or energy is not destroyed, only one form of energy can be converted into another form of energy, similarly, it is applicable to matter. From all these things came the law of conservation of mass and the law of conservation of energy.

However, in modern physics, according to Einstein's special theory of relativity, it is possible to convert matter into energy and energy into matter. From which the law of conservation of mass energy is obtained.

## Units of Energy

As energy is defined by work, the unit of energy is the same as the unit of work. The SI unit of work is the joule (J), so Joule (J) is also the SI unit of energy, which is named in honor of James Prescott Joule. This 1 joule (J) is equal to 1 newton meter (Nm). The CGS unit of energy is erg. Where 1 erg is equal to 10^−7 joules.

Other units of energy are like that - Kilojoule (KJ), Megajoule (MJ), Gigajoule (GJ), Calorie (Cal), BTU (British Thermal Unit), Watthour (Wh), Kilowatt-hour (kWh), Electronvolts (eV). And the relation of all energy units to the SI unit of energy is as follows -

1 Kilojoule (kJ) = 10^3 joules (J)
1 Megajoule (MJ) = 10^6 joules (J)
1 Gigajoule (MJ) = 10^9 joules (J)
1 Calorie (Cal) = 4.2 joules (J)
1 BTU (British Thermal Unit) = 1055 joules (J)
1 Watt (W) = 1 joule/second (J/sec) ⦍ unit of power ⦎
1 Megawatt (MW) = 10^6 joules/second (J/sec) ⦍ unit of power ⦎
1 Watthour (Wh) = 3600 joules (J)
1 Kilowatt (kW) = 10^3 joules/second (J/sec) ⦍ unit of power ⦎
1 Kilowatt-hour (kWh) = 3.6×10^6 joules (J)
1 Electronvolts (eV) = 1.6×10^-19 Jjoules (J)

## Types of Energy

Energy resides in different objects and takes different forms. These different forms of energy are just like that - mechanical energy, heat energy, light energy, sound energy, magnetic energy, electrical energy, chemical energy, and atomic energy.

All these energies can be converted from one form to another. For example, all foods contain chemical energy, now when a person eats these foods these chemical energy accumulates in the person's body. This chemical energy then takes the form of kinetic energy from a person's body during work or play.

The chemical energy in the coal and the kinetic energy in the water flow can be converted into electrical energy, which can then be converted into light and heat energy. Similarly, other forms of energy can also be transformed.

Mechanical Energy

Mechanical energy is the sum of potential energy and kinetic energy. Where potential energy is stored energy, which is held by an object due to the distortion of various parts of a system. For example - when a spring is compressed or stretched the potential energy become stored in the spring. There are also different types of potential energies such as - gravitational potential energy, elastic potential energy, electric potential energy.

And kinetic energy is a type of energy which possesses due to the motion of objects. For example - a running person contains kinetic energy.

Mechanical Energy (E) = Potential Energy (P) + Kinetic Energy (K)

According to the principle of conservation of mechanical energy in an isolated system in the absence of non-conservative forces (such as friction, air resistance), the total amount of mechanical energy remains constant.

Heat Energy

Heat is a type of energy that is transferred from one body or system to another and creates different temperatures in the body or system. This heat energy always flows from the hot body to the cold body during contact. And this flow continues until the two bodies reach the same temperature. As a result of this energy exchange, different states of matter can be observed.

Light Energy

Light is a type of electromagnetic radiation. It is a form of energy that is produced when an electron in an atom jumps from one energy level to another. Actually, when the atoms of an object are heated, the electrons move from one energy level to another and as a result, it releases photons in the form of light.

Sound Energy

Sound is the kind of energy that is produced when an object vibrates. It is a form of energy that can be heard by living beings. There are different types of sounds that exist in nature, some of which we are able to hear, and some of which we never hear.

The sound we hear is called audible sound. The frequency range of this sound is 20Hz to 20kHz. We cannot hear the sound that is below or above the range of this audible sound.

Magnetic Energy

Each magnetic field has energy, also called magnetic energy. The magnetic field is generated by an electric current, that is why the magnetic energy is an energy form of moving charged particles (like moving electrons).

Electrical Energy

Electrical energy is a type of energy that is stored in charged particles in an electric field. The electric fields are simply the regions around the charged particles. In other words, charged particles create electric fields that force other charged particles into the field.

Electrical energy is the most advantageous form of energy for most humans to use. This energy is very easy to use and can movable from one place to another. It is used to run computers, home appliances, even transportation, and used to do a variety of things.

Chemical Energy

The energy stored in the bonds of chemical compounds (such as atoms and molecules) is called chemical energy. This energy is released when a chemical reaction occurs. Generally, when once chemical energy is released from a substance, the substance is converted into a completely new substance.

Atomic Energy

The atom caries a lot of energy which is called atomic energy. You can learn more about atomic energy from Nuclear energy - radioactivity ( radioactive decay ), nuclear reactions such as - nuclear fission, and nuclear fusion.

## Energy sources

Now if we talk about the source of energy, then there are various sources of energy, but they can be divided into two basic categories - Renewable sources of energy ( also known as non-conventional sources of energy) and Nonrenewable sources of energy ( also known as conventional sources of energy).

Renewable sources of energy:

The energy sources that are collected from renewable resources or that can be easily replenished are called Renewable energy sources. For example - Solar energy, Wind energy, Hydro energy, Tidal energy, Geothermal energy, Biomass energy.

Nonrenewable sources of energy:

Non-renewable energy sources are such energy sources that cannot be rapidly replaced by natural means to maintain the ease of use, for example - coal, natural gas, oil, and nuclear energy.

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