What is Energy? Energy Stores and Transfers Explained

Energy underpins every process in the universe, so let's take a look at where it comes from, the processes involved and what energy does.

We all know that energy is what makes things happen, causing the Sun to shine, allowing plants to grow, cooking food on a stove or making a basketball bounce. Whenever something heats up, cools down, moves, grows, makes a sound or changes in any way, it uses energy. From taming fire to powering smartphones, human civilisation relies on our ability to manipulate energy. Pinning down exactly what is energy can be tricky.

Most textbooks will describe energy as 'the ability to do work'. Work in this context is defined as exerting a force on an object over a distance. Lifting a cardboard box off the ground constitutes work. Continuing to hold it there on the other hand, even though it requires effort on your part, is not work.

When work is done to an object, it gains energy. This energy is referred to as kinetic energy if it's associated with the motion of the object, as with a football speeding through the air after you kick it. When you pick up the box it is said to have gained potential energy, stored using its elevation above the ground. If you let go, the box will fall, losing potential energy as it loses height, and gaining kinetic energy as it picks up speed.

Potential energy can turn into kinetic energy and vice versa countless times. Further, mechanical, sound, heat, electromagnetic, light, chemical and nuclear energy can all be transformed from one into the other.

Types of Energy

We've discussed potential and kinetic energy, but there are many forms of energy. These are listed below with an explanation as to what they involve. The most straightforward way to classify energy is by dividing it into kinetic energy and potential energy. This difference is, however, not enough to completely describe the different ways in which an object or a system can possess energy. Hence, we have nine primary forms of energy.

Potential Energy vs Kinetic Energy

Kinetic energy is associated with movement. From an oxygen molecule through to a planet, the more mass an object has and the faster it moves, the greater its kinetic energy. The motion of different types of objects gives rise to different forms of kinetic energy.

Potential energy has its roots in the force acting between two objects and the distance between them, for example, the potential energy of a rock on top of a hill comes from the gravitational force between Earth and the rock. The more massive the rock, and the greater its height, the bigger its potential energy. Different forces give rise to potential energy. Different forces give rise to potential energy under different names.

Sound Energy

Sound energy is all about vibrations. Strum a guitar string and it vibrates. This motion propagates through the air, oscillating the molecules back and forth. When the wave reaches your ear, your eardrum vibrates your brain translates the sound energy into electrical energy. We rarely use sound waves to do work but rather as a means to communicate or entertain.

Thermal Energy

Thermal energy is a combination of the kinetic and potential energy of its constituent particles. As the water in your kettle heats up, its molecules vibrate faster and faster until it reaches boiling point. In a steam engine, heat is converted to mechanical energy from the expansion when water is turned into vapour.

Light Energy

When a piece of wire, for example, is heated to a high temperature, its atoms vibrate so violently that some are excited to a high-energy electronic state. As they fall back to a lower energy state, the excess is emitted as light (plus heat). The radiation frequency depends on the wire's temperature.

Nuclear Energy

Nuclear energy is stored in the nuclei of atoms, where protons and neutrons are bound together by a strong force. Splitting or combining nuclei can release vast amounts of energy. Nuclear fission reactors split uranium or plutonium nuclei by bombarding them with neutrons, sparking a chain reaction which gives off heat. The out Sun, meanwhile, creates heat and light thanks to the nuclear fusion in its core.

Chemical Energy

Chemical energy is stored in the chemical bonds which bind atoms to molecules and other structures. In other words, it takes energy to hold atoms together, but the total amount of energy required varies depending on their configuration. In a chemical reaction where the binding of the energy of the reactants is greater than the binding energy of the products, the excess energy is released as heat and sometimes light. burning coal in a fireplace or food in your body releases chemical energy.

Elastic Energy

Elastic energy is the potential energy stored when the shape or volume of an object is distorted - for example when you jump on a trampoline. As the trampoline returns to its original shape, it propels you into the air, converting potential energy into kinetic energy. Not all materials have the same capacity to store elastic energy; a rubber band can store more than a piece of string.

Gravitational Energy

Gravitational energy stems from the gravitational field around our planet (and other bodies). It arises, for example, when a skier rides a ski lift on a mountain slope. The higher the skier travels, the more potential energy is stored. Once they set off down the slope, this stored energy is transferred into kinetic energy as they speed down the slope.

Electromagnetic Energy

Electrical potential energy is stored when electrical charges of opposite signs are wrenched apart, or when charges of the same sign are forced together, the electrical potential generated is experienced as a voltage. Similarly, a rotating magnet in a coil induces a voltage in the coil. When the voltage is used to generate a current, the electrical potential energy can be reconverted into heat, light, or mechanical motion.

Conservation of Energy

One of our universe's most basic principles, the law of conservation of energy states that energy can neither be created nor destroyed. That is, the amount of energy in a closed system is fixed. It can, however, be transferred from one object into another, and converted from one form to another.

Although we discuss energy production, you can't create new energy - only convert existing energy to a different, usable, form. A photovoltaic panel, for instance, taps into the Sun's radiant energy converting it to usable electrical energy. Hydroelectric plants convert the kinetic energy of water to electrical energy.

While you can't destroy energy, you can waste it through inefficiency. When you drive a car chemical energy stored in the fuel is converted into first thermal energy and then kinetic energy which turns the car's wheels. Not all the chemical energy released from the fuel goes into making the vehicle move. Some energy is converted to heat and sound, and some are used to displace the air around the car - air resistance. Once this has occurred, it's very hard to turn this wasted energy into something useful.

Throughout history, numerous inventors have tried to design and build perpetual motion machines that would give out more energy than was put in, but conservation of energy has made such inventions impossible. At least thus far.

How is Energy Transferred?

Energy transfers from one form to another occur around us all the time but manipulating energy efficiently into useful forms is fundamental to modern life. Different uses require different forms of energy. A fan, for example, requires motion energy while thermal energy is required to fry an egg.

The simple act of making a piece of toast requires mastery of a large number of energy transformations. In all likelihood, the energy that powers your toaster started off its journey as coal or gas. First, these fuels are burned, releasing the energy stored in their chemical bonds as heat (thermal energy), used to boil water. the resulting high-pressure steam spins a turbine, connected to a generator which converts the motion energy into electric energy. When you switch on your toaster, an electric current run through the toaster's filaments and the electrical energy is converted into thermal and light energy.

Energy Storage

Energy transfers also allow us to store energy for future use - for example, when charging a battery or winding up a clock. Batteries convert a chemical reaction into electrical energy. An electrolyte oxidises the anode and the cathode reacts with the oxidized electrolyte to produce electricity. We can also perform the action in reverse. by applying electrical energy to special batteries which can then convert the electrical energy into chemical energy until it is needed.

Einstein's Big Idea - Mass Energy Equivalence

In 1905, Einstein revolutionised physics with a jaw-dropping revelation - matter and energy are the same. This fact is immortalised in the world's most famous equation: E = mc². Under the right conditions, energy can be converted into matter and vice versa. This energy comes from the ultra-strong bonds holding protons and neutrons together in atomic nuclei. The c in the equation represents the speed of light, about 700 million miles per hour, so even an object with a tiny mass contains a huge amount of energy. If you could turn every atom of a paperclip into energy, you would release as much energy as the atomic bomb that obliterated Hiroshima in 1945. Doing so would, however, require extreme temperature and pressure conditions that are impossible on Earth.

Equation 41 - Special theory of relativity

Where:

• E is energy which is measured in joules.
• m represents mass, measured in kilograms.
• c The speed of light in a vacuum: 700 million miles per hour.

A consequence of the mass-energy equivalence is that if a body is stationary, it still has some internal or intrinsic energy, called its rest energy, corresponding to its rest mass. When the body is in motion, its total energy is greater than its rest energy, and equivalently its total mass (also called relativistic mass in this context) is greater than its rest mass. This rest mass is also called the intrinsic or invariant mass because it remains the same regardless of this motion, even for the extreme speeds or gravity considered in special and general relativity.