On an intuitive level, 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. But pinning down exactly what energy is can be tricky.
Grab a textbook and you'll find energy described 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, however continuing to hold it there, although requiring effort on your part, is not work.
When work is done to an object, it gains energy. This energy is called 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 by virtue of 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.
One of energy's fundamental properties is that it cannot be created nor destroyed, only transformed from one type to another.
Potential energy can turn into kinetic energy and vice versa limitless times. Further, mechanical, sound, heat, electromagnetic, light, chemical and nuclear energy can all be converted from one into the other.
But while you can't destroy energy, you can certainly waste it through inefficiency. When you drive a car, for example, chemical energy stored in the fuel is converted into first thermal energy and then kinetic energy which turns the cars wheels. But not all of the chemical energy released from the fuel goes into making the vehicle move. Some are converted to heat and sound, and some are used to displace air around the car - air resistance. Once this has occurred, it's very hard to turn this wasted energy into something useful.
Throughout history, we have relied on different means to get hold of usable energy, from early windmills to coal burning steam engines. Today we consume energy mainly as petroleum, natural gas and electricity. We tap into the energy stored in the chemical bonds of petrol or natural gas by burning these fuels inside a boiler or engine.
Electricity, on the other hand, is a handy way to transporting energy converted from various more cumbersome sources into our homes or workplaces. A wind turbine, for instance, converts kinetic energy, while nuclear reactors exploit the energy locked in atomic nuclei generating first thermal and then electrical energy. Once inside our home, electricity can be used for heating, cooking, lighting and running all of our appliances and gadgets.
Energy is measured in joules (J) with one joule being the energy needed to apply a force of one newton (N) over one metre.
In practice, a variety of different units are commonly used to measure energy in its multitude of forms. The chemical energy in food is measured in calories - the amount it takes to raise the temperature of one gram of water by one degree Celsius. Your electricity bill in comparison measured the electrical energy you have used in kilowatt hours (kWh). For some context, one-kilowatt hour is enough to run one washing machine cycle or watch seven hours of TV.
Energy in a Rollercoaster
Rollercoasters are a good way to explain different types of energy as there are several forms taking place in the adrenaline-fuelled ride.
The rollercoasters cart mass determines how much energy is required to move it, as well as how much potential energy it can store.
2. Setting off
Approaching the first slope, the rollercoaster carriage has very little kinetic or gravitational potential energy.
As the cart is pulled up the slope, the energy provided by the motor is stored away as potential energy.
4. At the top
Edging slowly past the highest point, the rollercoaster train has high potential energy but low kinetic energy.
5. On a roll
As the car whizzes down the slope, its potential energy rapidly transforms into kinetic energy.
6. Wasted energy
At each stage, some of the cars kinetic energy is lost to air resistance or dissipated as heat through friction and sound.
Einsteins Big Idea
In 1905, Einstein revolutionised physics with a jaw-dropping revelation - matter and energy are one and the same. This fact is immortalised in the worlds 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
- 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.
Conservation of Energy
One of our universes most basic principles, the law of conservation of energy states that energy can neither be created or 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.
Likewise, this energy that we use doesn't disappear, it just changes into other forms. Switch on your television and the heat, sound and light energy emanating from the set gradually leaks back into the environment.
Through 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.
Types of Energy
The simplest way to classify energy is by dividing into kinetic energy and potential energy. This distinction is, however, not enough to fully describe the different ways in which an object or a system can possess energy. Hence we have nine major forms of energy.
Potential Energy vs Kinetic Energy
Kinetic energy is associated with motion. From an oxygen molecule through to a planet, the more mass an object has and the faster it moves, the greater it's 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 it's potential energy. Different forces give rise to potential energy. Different forces give rise to potential energy under different names, as we have seen.
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 in turn and your brain interprets the sound, We rarely use sound waves to do work but rather as a means to communicate or entertain.
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 form the expansion when water is turned into vapour.
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 radiations frequency all depends on the wire's temperature.
Nuclear energy is stored in the nuclei of atoms, where protons and neutrons are bound together by the strong force. Splitting or combining nuclei can release vast amounts of energy. Nuclear fission reactors split uranium or plutonium nucleai by bombarding hem with neutrons, sparking a chain reaction which gives off heat. Out Sun, meanwhile, creates heat and light thanks to the nuclear fusion in its core.
Chemical energy is stored in the chemical bonds which bind atoms into molecules and other structures. In other words, it takes energy to hold atoms together, but the total amount of energy required varies defpending 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 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.
Gravittional energy stems from the gravitational field around our planet (and other bodies). It arises, for example, when a skier rides a ski lift up a mountain slope. The higher the skier travels, the more potential energy is stored up. Once they set off down the slope, this stored energy is transferred into kinetic energy as they speed down the slope.
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.
How is Energy Transferred?
Next time you take a hot shower, drive to work or plug in your tablet, spare a thought for the science that brings energy on demand into your home.
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 runs through the toasters filaments and the electrical energy is converted into thermal and light energy.
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 oxides the anode and the cathode reacts with the oxidized electrolyte to produce electricity.