## What is energy?

**Energy **is what moves the world around us. Energy is transferred whenever a physical action (even at atomic levels) takes place or the state of a system changes.

**Energy** is a property of an object or system that quantifies its capacity to do work.

One of the fundamental laws of the universe is that **energy can neither be created nor destroyed; it can only be transferred from one form to another**. This principle is known as the **conservation of energy. **In a system** **in which neither matter nor energy can be exchanged with the surroundings, energy is always constant. But what are these different forms of energy?

## Forms of Kinetic Energy

**Kinetic energy** is a form of energy that an object has due to its motion.

Kinetic energy depends on two factors. The mass of the object and the velocity at which it's moving.

${E}_{K}=\frac{1}{2}m{v}^{2}$

$\mathrm{Kinetic}\mathrm{energy}=0.5\times \mathrm{mass}\times {\mathrm{velocity}}^{2}$

Where${E}_{k}$is the kinetic energy in joules$\left(\mathrm{J}\right)$,$m$is the mass in kilograms$\left(\mathrm{kg}\right)$and$v$is the velocity in metres per second$\left(\frac{\mathrm{m}}{\mathrm{s}}\right)$.

Recall that we can apply an external force to accelerate an object. When this happens, we say that work was done over the object.

**Work** is an energy transfer resulting from applying a force along a displacement.

Doing work on an object changes its kinetic energy. Therefore, we can calculate work as the change in kinetic energy.

$W=\Delta {E}_{K}$

or in words

$\mathrm{Workdone}=\mathrm{change}\mathrm{in}\mathrm{kinetic}\mathrm{energy}$

It takes the same amount of work to bring a body to a specific speed as to stop once it is moving at that speed. However, the work done will be negative in the latter case because the force has to act against the object's motion.

There are two types of kinetic energy:

An object moving in a straight line has **translational kinetic energy**

A spinning object has** rotational kinetic energy**.

Let's now look into heat energy which is also related to kinetic energy.

## Heat energy

**Heat or thermal energy **is produced due to the motion of atoms. In other words, it is the kinetic energy of atoms, molecules, or ions. Heat transfer always happens from a point of higher temperature to another of lower temperature. There are 3 ways of transferring heat between two bodies: **conduction, convection, and radiation.**

**Conduction** is when heat energy transfers between neighbouring atoms or molecules due to collisions.

**Convection** is the upward movement of the hotter regions of a liquid or gas while denser, cooler ones sink, causing heat transfer.

**Radiation** is a process in which electromagnetic waves or subatomic particles covey energy from one point to another.

When heating a pot of water, the metal handle is heated by conduction as heat transfers through the metal body of the pan. Radiation occurs as the hot source emits infrared waves. And finally, the water molecules at the bottom rise and transfer heat by convection.

## Other forms of Kinetic Energy

**Sound energy -**Sound waves transfer energy throughout the air molecules as vibrations or kinetic energy of the air molecules.**Radiant energy or****Light****energy**is released when**photons**dissipate energy into the surroundings. It travels as electromagnetic radiation. An energy transfer occurs when the light comes in contact with an external surface - for instance, photosynthesis in plants and light falling on solar panels.

We now have a clear idea of how an object that moves has kinetic energy. Now, let's look at how an object can store energy.

## Forms of Energy: Potential Energy

When someone says that they have *potential*, it means they have the capacity to do something thanks to their innate abilities. A similar logic applies to potential energy.

**Potential energy** is a system's capacity to do work due to its configuration or that of a body because of its position.

Potential energy is stored energy that can be released to work or increase kinetic energy. There are different types of potential energy. Let's go over a few of them.

**Gravitational Potential Energy**

You may have noticed that a rock causes a bigger splash when dropped into the water as we release it from a greater height. When we elevate an object in a gravitational field, it gains **gravitational potential energy (GPE)**. The higher we raise the rock, the more potential energy it acquires. Then, when we drop it, this potential energy converts into kinetic energy as the rock falls.

**Gravitational potential energy **is the energy gained when an object is moved against an external gravitational field.

Water stored on top of a dam has **gravitational potential energy**. When the water is released and falls, this** potential energy** becomes **kinetic energy**. The water in motion then drives the turbines to produce **electricity (electrical energy**).

An object's gravitational potential energy depends on its height, the strength of the gravitational field it is in, and the object's mass.

${E}_{P}\mathit{=}\mathit{}mgh$

or in words

$\mathrm{Gravitational}\mathrm{potential}\mathrm{energy}=\mathrm{mass}\times \mathrm{gravitational}\mathrm{field}\mathrm{strength}\times \mathrm{height}$where${E}_{P}$is the gravitational potential energy in$\mathrm{joules}\left(\mathrm{J}\right)$,$m$is the mass of the object in$\mathrm{kilograms}\left(\mathrm{kg}\right)$,$\mathrm{h}$is the height in$\mathrm{meters}\left(\mathrm{m}\right)$, and$g$is the gravitational field strength on Earth$(9.8\mathrm{N}/\mathrm{kg})$.

The change in the potential energy of a system defines the work done over it.

$\u2206{E}_{P}=W$

or in words

$\mathrm{Change}\mathrm{in}\mathrm{gravitational}\mathrm{potential}\mathrm{energy}=\mathrm{Work}\mathrm{done}\mathrm{to}\mathrm{lift}\mathrm{the}\mathrm{object}$

**Chemical Potential Energy**

**Chemical energy** is a type of potential energy stored in the bonds between the atoms or molecules of different compounds.

This energy is transferred when the bonds break during chemical reactions. Common examples involving the transfer of chemical energy are:

The combustion of coal converts the chemical energy into heat and light.

Batteries store chemical energy, which is converted into electric energy.

Leaves convert the sun's energy into chemical energy.

**Nuclear potential energy** is the energy that is within the nucleus of an atom. It is one of the most powerful sources of energy in the universe.

Nuclear energy can be released by:

**Fusion -**when two small nuclei combine such as the isotopes of hydrogen-deuterium and tritium combining to form helium and one free neutron.**Fission -**breaking down a**parent nucleus**into two different nuclei known as the daughters. The nucleus of an atom like Uranium can break down into smaller nuclei of equal masses with the release of energy.**Radioactive decay -**Unstable nuclei dissipate energy in the form of harmful radioactive waves (nuclear energy to radiation energy).

**Elastic potential energy**

**Elastic potential energy **is stored in elastic materials as a result of stretching or compressing.

When a force stretches a spring over a certain distance, it gains elastic potential energy. Once released, the spring moves and contracts to its original position. The energy released during the contraction equals the work done to stretch it.

**Mechanical energy**

So then what is mechanical energy?

**Mechanical energy** is the energy possessed by an object due to its motion and or its position.

It is given by the following equation:

${E}_{\text{mechanical}}={E}_{P}+{E}_{K}$

or in words

$\mathrm{Mechanical}\mathrm{energy}=\mathrm{Potential}\mathrm{energy}+\mathrm{Kinetic}\mathrm{energy}$

When the cart climbs the hill, its **potential energy increases constantly,** but** its speed decreases, so the kinetic energy keeps on reducing**. At the top of the hill, the **potential energy reaches maximum** and the **kinetic energy is Zero**. When the object travels downhill, **its potential energy starts decreasing as it loses height. **However, the **kinetic energy increases as it gains speed**. The sum of both these energies is always constant, according to the principle of conservation. Throughout the journey, the energy is transferred from one form to another.

## Examples of Energy Transfer

Whenever the state of a system changes, energy transfers from one state to another. Energy can be transferred in different ways:

**Mechanical energy transfer****Electrical energy transfer****Thermal energy transfer****Radiation energy transfer**

Let's work on a few examples that will test your understanding of the concepts we covered in this article.

Calculate the braking force required to stop a bike of mass$8\mathrm{kg}$moving at a speed of$20\mathrm{m}/\mathrm{s}$in$50\mathrm{m}$.

We know that work done to stop a moving vehicle is equal to the kinetic energy of the moving vehicle. Using this we can equate both the terms of work done and kinetic energy. From there we can calculate the force that will be required to stop this vehicle.

$Fs=\frac{1}{2}m{v}^{2}\phantom{\rule{0ex}{0ex}}F=\frac{1}{2}\times \frac{8\mathrm{kg}\times 20\mathrm{m}/\mathrm{s}\times 20\mathrm{m}/\mathrm{s}}{50\mathrm{m}}\phantom{\rule{0ex}{0ex}}F=32\mathrm{N}$

So the force required to stop this bike is$32\mathrm{N}$. Let's simplify this problem and try to understand how energy is being transferred from one form to another.

The bike has kinetic energy due to its mass and the velocity at which it moves. As the rider does work against the bike by pressing the brake lever, decreasing the kinetic energy. Furthermore, the brake's friction disperses part of the kinetic energy as heat and sound as well. In the above image, you can see how the brake lever pushes the brake shoes onto the wheels, ultimately slowing it down.

What about the energy conversions when an object is falling? let's find that out using our next example.

If a$500\mathrm{g}$apple is dropped from a height of$100\mathrm{m}$above the ground, at what speed will it hit the ground? Ignore any effects of air resistance.

The gravitational potential energy of the apple is converted into kinetic energy as it falls and increases in velocity. Therefore the potential energy at the top is equal to the kinetic energy at the bottom at the time of impact. using the equation for **mechanical energy.**

The total mechanical energy of the apple at all times is given by:

${E}_{total}={E}_{P}+{E}_{K}$

When the apple is at a height of$100\mathrm{m}$, the velocity is zero hence${E}_{KE}=0$. Then the total energy is:

${E}_{\mathrm{total}}={E}_{P}$

When the apple is about to hit the ground the potential energy is zero, hence the total energy is now:

${E}_{\mathrm{total}}={E}_{K}$

Velocity during impact can be found by equating the${E}_{P}$to${E}_{K}$. At the moment of impact, the kinetic energy of the object will be equal to the potential energy of the apple when it was dropped.

$\begin{array}{rcl}mgh& =& \frac{1}{2}m{v}^{2}\\ gh& =& \frac{1}{2}{v}^{2}\\ v& =& \sqrt{2gh}\\ v& =& \sqrt{2\times 9.8\mathrm{N}/\mathrm{kg}\times 100\mathrm{m}}\\ v& =& 44.27\mathrm{m}/\mathrm{s}\end{array}\phantom{\rule{0ex}{0ex}}$

The apple has a velocity of$44.27\mathrm{m}/\mathrm{s}$when it hits the ground. **The potential energy of the apple is converted into kinetic energy as it falls. **

## Forms of Energy - Key takeaways

**Energy**is a property of an object or system that quantifies its capacity to do work.Energy can neither be created nor destroyed it can only be transferred from one form to another.

Whenever a system changes its state, energy is transferred from one form to another.

The energy of an isolated system is always constant.

Energy can be transferred in the form of mechanical, light, electric potential, nuclear, gravitational potential, chemical, and heat.

**Kinetic energy**is due to the motion of an object.**Potential energy**can be of many types depending on the nature of the forces acting on the body.**Gravitational potential energy**is the energy gained when an object is moved against an external gravitational field.**Chemical energy**is a type of potential energy stored in the bonds between the atoms or molecules of different compounds.**Mechanical energy**is the sum of kinetic and potential energy.Transfer in energy can be in the form of heat, radiation, electrical, and mechanical.

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##### Frequently Asked Questions about Forms of Energy

What is light energy with examples?

Light energy is released when **photons** carry energy into the surroundings; this is what we see as visible light. Energy transfer occurs when the light comes in contact with an external surface. An example of light energy is photosynthesis in plants, light falling on solar panels. The sun is the largest source of light energy.

What are the causes of heat energy?

Heat or thermal energy is produced due to the rapid motion of atoms. It can be said that this is due to the kinetic energy of the atoms, molecules, or ions. Heat transfer happens from high to low temperatures due to conduction, convection and radiation.

How is gravitational energy generated?

Gravitational energy is generated when an object is lifted to a certain height in the earth's gravitation field. it can also be defined as the work done to lift an object to a certain height **h** in the field

What is chemical energy?

Chemical energy is a type of potential energy stored in the bonds between the atoms or molecules of different compounds. This energy is transferred when the bonds are broken during chemical reactions.

What are some examples of mechanical energy?

Examples of mechanical energy are a moving car (kinetic energy), a hydrostatic turbine rotated by the flow of water, and A bullet fired from a gun.

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