## Energy definition

It makes our lives easier to be able to access energy so easily. However, we can't see it. What does it do for us, and what exactly is energy?

Energy is the capacity to do work or generate heat. Energy can be transferred between different stores using energy pathways.

An object works by moving against a force over a certain distance. The units for energy are **Joules,** and the symbol **J** was named after the well-known physicist James Joule.

However, you should not be confused by definition - energy is used everywhere. For example, when you burn wood, the stored chemical energy in the wood is converted into thermal energy in the form of heat to keep you warm. This is similar to us getting heat and light from the sun. However, in the latter case, the initial energy is called **nuclear energy. **

## Energy stores

An object's or a system's energy is stored in different **energy ****stores, **and energy can be **transferred** between different types of stores. Let's have a look at the energy stores that you need to know about:

**Kinetic energy**is the energy of motion. Any moving object possesses kinetic energy, and this energy store can be transferred to other objects through collisions. Also, kinetic energy can be transformed into other energy. For instance, when an object is moving on a surface with friction, kinetic energy is converted into thermal energy in the form of heat, which is usually considered lost energy.

When energy is transformed from one form to another, a fraction of energy is dissipated into an unwanted and disordered form. This is called **wasted**** energy.**

**Gravitational potential****energy**is the energy an object possesses depending on its position in a gravitational field. For example, if you put a ball on a hill, it will have gravitational potential energy. If you roll the ball downwards, this energy will be released and transformed into kinetic energy.**Elastic potential (strain) energy**is the energy an object has when it is elastic and stretched (for example, a spring or a string). The elastic object will snap back to its original position when you stop applying force on it. This is because the object has stored elastic potential energy.

**Thermal****(internal)****energy**describes how hot an object is. Thermal energy is a type of kinetic energy as the heat of an object is determined by how fast its particles are moving around.**Chemical energy**is released (transferred to another energy store) when a chemical reaction occurs. The food you eat every day has stored chemical energy released after chemical reactions occur in your digestive system.**Nuclear energy**is the energy held up in the nucleus by strong forces. It can be released after nuclear reactions take place and used in nuclear power plants every day to provide electricity.**Magnetic energy is stored in magnets,**causing them to attract or repel each other.**Electromagnetic energy**is the energy that is being carried by electromagnetic waves. Of course, visible light is also an electromagnetic wave. It possesses this energy, but other examples can vary from radiowaves used in radios to carry information to X-rays used in medical physics.

For now, we will focus on two main types of energy, potential and kinetic.

## Kinetic energy

As you have learned before, kinetic energy is the energy an object possesses because of its movement. But what else does the kinetic energy of an object depend on? Let's look at the equation for finding kinetic energy to understand the concept better.

$\begin{array}{rcl}\mathrm{kinetic}\mathrm{energy}& =& \frac{1}{2}\times \mathrm{mass}\times {\left(\mathrm{speed}\right)}^{2}\\ {E}_{k}& =& \frac{1}{2}\times m\times {v}^{2}\end{array}$

Where kinetic energy${E}_{k}$is measured in Joules$\left(\mathrm{J}\right)$mass of the object$m$is measured in kilograms$\left(\mathrm{kg}\right)$and the speed of the object$v$is measured in metres per second$(\mathrm{m}/\mathrm{s})$.

As you can see, the kinetic energy of an object depends on both its mass and its speed. If you want to understand the concept of energy fully, you should also learn about potential energy.

## Potential energy

Unlike kinetic energy, potential energy has two main forms - gravitational and elastic.

### Gravitational potential energy

As you have learned, potential energy is the energy that an object possesses because of its position in a gravitational field. Now let's dive a little deeper and look at its equation to see what other factors affect the **gravitational potential energy** of an object.

$\begin{array}{rcl}\mathrm{Gravitational}\mathrm{Potential}\mathrm{Energy}& =& \mathrm{Mass}\times \mathrm{Gravitational}\mathrm{Field}\mathrm{Strength}\times \mathrm{Height}\end{array}$

${E}_{p}=m\times g\times h$

Where gravitational potential energy${E}_{p}$is in Joules$\left(\mathrm{J}\right)$mass$m$is in kilograms$\left(\mathrm{kg}\right)$and gravitational field strength$g$is in newtons per kilogram$(\mathrm{N}/\mathrm{kg})$and height$h$is in metres$\left(\mathrm{m}\right)$.

### Elastic potential energy

As you can see, gravitational potential energy depends on the position of the object in the gravitational field and its mass. Another form of potential energy is **elastic potential energy **which is the energy an object has when it is elastic and is stretched. Let's have a look at its equation to understand the concept better.

$\mathrm{Elastic}\mathrm{Potential}\mathrm{Energy}=\frac{1}{2}\times \mathrm{Spring}\mathrm{Constant}\times {\left(\mathrm{Extension}\right)}^{2}$

${E}_{e}=\frac{1}{2}\times k\times {e}^{2}$

Where elastic potential energy${E}_{e}$is measured in Joules$\left(\mathrm{J}\right)$spring constant$k$is measured in newtons per metre$(\mathrm{N}/\mathrm{m})$, and the extension$e$is measured in metres$\left(\mathrm{m}\right)$.

The spring constant is a value that measures the stiffness of the spring and depends on the material used in the spring. This value is also equal to the force required to stretch the spring for one meter.

Now let's have a look at how energy is transferred and transformed between energy stores to understand how they are used in our everyday lives.

## Energy pathways

When energy is transferred between different stores, there are different ways in which energy transfer can occur. They are called energy pathways.

**Mechanically**- a mechanical energy transfer occurs when work is done.**By heating**- when an object heats another object by transferring some of its thermal energy.**Electrically**- energy transferred by a charge moving through a circuit.**By radiation**- when an object radiates a wave which carries energy which can be passed onto another object.

A better understanding of how energy transfers between different stores can be gained is by considering what is happening to the object or what the object is doing.

Describe the energy transfers that occur when a person jumps into the air and falls back down again.

**Solution**

A person can jump into the air due to the chemical reactions in their muscles, causing the muscles to move, so there is initially a transfer from chemical to kinetic energy.

As the person moves higher up into the air, they begin to slow down due to the earth's gravitational field. Their energy is transferred from a kinetic energy store to a gravitational potential energy store. After reaching their maximum height, the person begins to accelerate back down towards the earth.

Their gravitational potential energy is converted back into kinetic energy. Or in other words, energy is transferred from a gravitational potential energy store to a kinetic energy store.

Energy transfer is important because you can't store energy in every useful form. For example, light is one of the most useful things in our lives, but it can't be stored. If you want to have light, you can carry a torch which has stored chemical energy in its battery. This chemical energy then causes an electric current which then causes the bulb to heat up. As a result of this heating process (which happens too quickly for the human eye to observe), the bulb emits light and allows us to see in the dark.

## Conservation of energy

The principle of conservation of energy states that energy **cannot be created or destroyed**. The amount of energy that existed back when the universe was formed is the same as the amount of energy that exists at the moment that you are reading this explanation. Energy can only be **transferred** between the different stores. For example, a battery doesn't just create energy from nothing. It contains chemical energy, which then is transferred into the circuit to create a current.

When energy transfers occur, not all energy is transferred into the desired store. Some of it is transferred to unwanted stores and is known as **lost ****energy**.

For example, when a bat hits a ball, there is a loud sound which means that some of the kinetic energy from the bat is transferred to sound waves. Another example is an object moving along a rough surface. It has to do work against friction, and some energy is lost since it transforms into thermal and sound energy.

But as you can see, even when we talk about lost energy, it isn't really lost but transformed into an unwanted disordered form of energy. So we can conclude that the initial total energy a system has is equal to the final total energy. We can mathematically express this conclusion as shown below.

$\mathrm{Initial}\mathrm{total}\mathrm{energy}\mathrm{of}\mathrm{a}\mathrm{system}=\mathrm{Final}\mathrm{total}\mathrm{energy}\mathrm{of}\mathrm{a}\mathrm{system}$

${E}_{i}={E}_{f}$

## Efficiency

When you or a machine wastes energy while trying to do work, it is called **inefficient**. The " wasted " energy is what we previously called **lost energy**. In practice, you can't do work without lost energy, but you can reduce it to the desired minimum.

In physics, as the lost energy reduces, the efficiency becomes higher. Mathematically, it is the ratio of useful output energy to total energy input and can be expressed using the following equation.

$\mathrm{Efficiency}=\frac{\mathrm{Useful}\mathrm{energy}\mathrm{output}}{\mathrm{Total}\mathrm{energy}\mathrm{input}}$

In order to calculate efficiency as a percentage, we should multiply the result we get from the previous equation by a hundred.

$\mathrm{Efficiency}\%=\frac{\mathrm{Useful}\mathrm{energy}\mathrm{output}}{\mathrm{Total}\mathrm{energy}\mathrm{input}}\times 100$

Note that efficiency can't be equal to or higher than 100% in practice simply because there is always lost energy in some form which means that "useful energy output" is always going to be less than "total energy input"

A kettle has$50\mathrm{J}$of energy and converts$38\mathrm{J}$into thermal energy. The rest of the energy is lost to the surroundings in the form of sound. Calculate the efficiency of the kettle.

**Solution**

Since the goal of the kettle is to provide thermal energy, the given value of$38\mathrm{J}$is the fraction of energy that we have named "useful energy output"

And it is said that the kettle is supplied with$50\mathrm{J}$which is the fraction that we have named "total energy input."

We know that the formula for efficiency is:

$\mathrm{Efficiency}\%=\frac{\mathrm{Useful}\mathrm{energy}\mathrm{output}}{\mathrm{Total}\mathrm{energy}\mathrm{input}}\times 100$

If we put the given variables into the formula, we can find the kettle's efficiency.

$\mathrm{Efficiency}\%=\frac{38}{50}\times 100=76\%$

## Energy Physics - Key takeaways

- Energy is the capacity to do work. It can exist in various forms and even be transferred between systems or bodies.
- Kinetic energy is the energy of motion. Any moving object possesses kinetic energy, and this energy store can be transferred to other objects through collisions.
- Gravitational potential energy is the energy an object possesses depending on its position in a gravitational field.
- Elastic potential (strain) energy is the energy an object has when it is elastic and is stretched (for example, a spring or a string).
- Energy is conserved. This means that it cannot be created or destroyed.
- An object's or a system's energy is stored in different energy stores, and energy can be transferred between different types of stores.
- In physics, as the lost energy reduces, the efficiency increases. Mathematically, it is the ratio of useful output energy to total energy input.

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##### Frequently Asked Questions about Energy Physics

What is energy in physics?

**Energy **is the capacity to do work. It can exist in various forms and even be transferred between systems or bodies.

How is energy measured in physics?

According to the International System of Units (SI), energy is measured in Joules.

What is potential energy in physics?

There are two types of potential energy. These can be defined as:

- Gravitational potential energy is the energy an object possesses depending on its position in a gravitational field.
- Elastic potential (strain) energy is the energy an object has when it is elastic and is stretched (for example, a spring or a string).

What is chemical energy in physics?

Chemical energy is the energy released (transferred to another energy store) when a chemical reaction occurs.

What is electrical energy in physics?

Electrical energy is the type of energy that results from the movement of charged particles.

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