Energy Physics

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?

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.

• 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.

Slingshots are an application of elastic potential (strain) 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$\left(\mathrm{m}\right)$.

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.

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.

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.

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